Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02827686 2013-09-18
-1-
SOUND SYSTEM AND METHOD OF SOUND REPRODUCTION
This application is divided from Canadian Application Serial No. 2,449,410
filed on February 8, 2002.
BACKGROUND OF THE INVENTION
1) Field of the Invention
[0001] The field of the present invention relates to sound
reproduction and,
more specifically, to a speaker configuration and related sound processing for
use in
a sound system.
2) Background
[0002] Attaining optimal sound quality in surround sound or multi-
channel
sound systems, over the largest possible listening area, can be quite
challenging.
Some of the difficulties in achieving optimal sound quality in such systems
result from
the fact that a wide variety of different surround sound and multi-channel
audio
formats and speaker configurations exist, so that a particular sound system
may have
reasonably acceptable sound with respect to one or perhaps two audio formats
yet
sub-optimal sound with respect to other audio formats. Therefore, where a
consumer
desires, for example, to use a single sound system to play sound recordings in
a
variety of different formats, different levels of sound quality, some of which
are poor
or impaired, are likely to be experienced. While the user can adjust speaker
CA 02827686 2013-09-18
-2-
positioning or relative balances to try to improve sound quality, such
techniques may
involve significant manual effort or inconvenience, may be hard to reproduce
consistently, and may benefit only one or perhaps a few listeners in a
relatively small
portion of the listening area.
[0003] Existing surround sound recording formats include those referred to
as
5.1, 6.1 and 7.1. The 5.1 surround format comprises a compressed data stream
containing five channels, generally designated left, center, right, surround
left, and
surround right, named for the speaker positions for which the channel
information is
intended. A low frequency effects channel is formed by a combination of the
five
other channels, and may be provided to a sub-woofer. The 6.1 surround format
includes the same five channels as the 5.1 surround format, but adds a
surround
back channel, which may be fed to one or more back speakers in a surround
sound
system. The 7.1 surround format is similar to the 6.1 surround format, but has
two
surround back channels (surround back left and surround back right) rather
than a
single back channel, for a total of seven channels. Thus, the 5.1 surround
format has
two surround channels (surround left and right), the 6.1 surround format has
three
surround channels (surround left, right and back), and the 7.1 surround format
has
four surround channels (surround left and right, and surround back left and
right).
[0004] Basic surround system speaker configurations generally
include from
six to eight speakers placed at conventionally well-established locations,
according to
the type of surround format they are intended to play. A six-speaker surround
system
typically includes left, right and center speakers (with the right and left
speakers
spaced widely apart), a sub-woofer, and surround left and right speakers
(which may
CA 02827686 2013-09-18
-3-
be monopolar or dipolar in nature). A seven-speaker surround system typically
includes the same speaker arrangement as the six-speaker surround system, but
adds a back surround speaker. An eight-speaker surround system typically
includes
the same speaker arrangement as the six-speaker surround system, but adds a
back
left surround speaker and a back right surround speaker.
[0005] The enjoyment experienced by a listener in a surround sound
system
can be affected by a number of factors, including the listener's physical
position
relative to the various speakers, as well as by the particular format of the
audio track
being played on the system. For example, when a 5.1 surround format soundtrack
is
played on an eight-speaker (7.1) surround system, certain anomalies may occur.
An
example is that, if the 5.1 surround left and surround right audio signals are
monaural, then the left and right surround effects can disappear, being
replaced by a
single central "phantom" sound image at the rear. Another phenomenon is that
if the
listener is positioned in the middle of the surround left and surround right
speakers,
he or she may perceive the surround left and right sound (if monaural) to be
higher in
volume that it otherwise would be, primarily due to the additive effect of the
sound
waves intersecting at the listener's position (known as a "lift" effect). If
the sound
pans from one side to the other (e.g., from left to right), the sound volume
may
appear to increase as left/right balance is achieved, and then appear to
decrease as
the sound continues to pan, even though the audio output level remains
constant,
due to the same "lift" effect. The sound quality may also seem to be
"unstable," in
the sense that if the listener moves from the center position, the sound might
seem to
"flip" from one side to the other.
CA 02827686 2013-09-18
-4-
[0006] Some of these effects can be mitigated in 5.1 surround sound
systems
by the use of adaptive de-correlation with respect to the surround left and
right
speakers, which derives two substantially de-correlated signals when the
surround
left and right signals are monaural, in order to provide an improved
enveloping
surround effect.
[0007] When a 6.1 surround format soundtrack is played on an eight-
speaker
(7.1) surround system, certain other anomalies may be experienced. Since the
two
rear surround speakers (left and right) are each fed with an identical
monaural signal
(that is, the same surround back signal), a centrally located "phantom" image
may
result when the listener is positioned approximately equidistant from the
speakers.
Reported side effects of this arrangement include "coloration" associated with
the
phantom image (for example, the sound may seem "unnatural"), a narrow "sweet
spot" due to lack of sound image stability when the listener moves off center,
and a
comb filter effect (in other words, nulls may be produced due to sound wave
cancellation effects).
[0008] Besides surround systems, a variety of multi-channel
recording and
playback systems also exist. Examples of some common multi-channel sound
systems are Dolby AC-3, DTS, and DVD-Audio, each of which has its own specific
digital encoding format. Unlike cinema sound, there is generally no single
adopted
standard of either loudspeaker type (e.g., full range, satellite plus sub-
woofer, dipole,
monopole) or speaker layout for most multi-channel audio formats. Any user
therefore desiring to listen to multi-channel soundtracks, and/or any of the
surround
formats (5.1., 6.1 and 7.1), is required either to accept one speaker layout
optimized
CA 02827686 2013-09-18
-5-
for a particular audio format and experience a compromised performance for all
others, or to reconnect various speakers to suit the audio format a particular
soundtrack.
[0009] Beyond the surround sound environment, other sound systems
also
face similar challenges, such as attaining a suitably wide "sweet spot" in
which the
perceived area and stability of a stereo sound image is maximized. In most
traditional sound systems, the convention has been to place left and right
speakers
far apart physically, under the theory that the human ear is thereby better
able to
perceive the richness of the audio subject matter. However, under many
left/right
speaker configurations, the sound at off-axis listening positions may be sub-
optimal.
The quality of sound at a given off-axis listening position may be affected
not only by
the difference between left and right volumes resulting from the different
distances to
the left and right speakers, but also by the slight difference in time it
takes the aural
information to reach the listener.
[0010] Accordingly, it would be advantageous to provide an improved sound
system which overcomes one or more of the foregoing problems or shortcomings.
SUMMARY OF THE INVENTION
[0011] The present invention is generally directed to improved sound
reproduction systems and methods involving a speaker configuration and/or
placement, and related sound processing, for enlarging the perceived area and
stability of a sound image generated from right and left source signals.
CA 02827686 2013-09-18
-6-
[0012] In one aspect, a sound reproduction system comprises a pair
of
speakers (left and right) located in close proximity, and a sound processor
which
provides audio signals to the pair of speakers. According to a preferred
embodiment,
the sound processor acts to "spread" the sound image produced by the two
closely
spaced speakers by employing a cross-cancellation technique wherein a
cancellation
signal is derived, for example, from the difference between the left and right
channels. The resulting difference signal is scaled, delayed (if necessary)
and
spectrally modified before being added to the left channel and, in opposite
polarity, to
the right channel. The spectral modification to the difference channel
preferably
takes the form of a low-frequency boost over a specified frequency range, in
order to
restore the correct timbral balance after the differencing process which
causes a loss
of bass when the low-frequency signals in each channel are similar. Additional
phase-compensating all-pass networks may be inserted in the difference channel
to
correct for any extra phase shift contributed by the usually minimum-phase-
shift
spectral modifying circuit so that the correct phase relationship between the
canceling signal and the direct signal is maintained over the desired
frequency range.
[0013] Alternatively, a linear-phase network may be employed to
provide the
spectral modification to the difference channel, in which case compensation
can be
provided by application of an appropriate, and substantially identical,
frequency-
independent delay to both left and right channels.
[0014] The various speaker configuration and sound processing
embodiments
as described herein may be employed in connection with a surround sound system
to
achieve improved sound reproduction. A sound reproduction system for a
surround
CA 02827686 2013-09-18
-7-
sound stereophonic system may comprise a set of speakers (e.g., front, left,
center,
surround left, and surround right), including a pair of surround back speakers
located
in close proximity, and a sound processor. The sound processor receives left
and
right surround channel signals (either side or rear surround signals), and
generates a
difference signal therefrom. The resulting difference signal may be processed
as
described above ¨ i.e., scaled, delayed (if necessary) and spectrally modified
before
being added to the left channel and, in opposite polarity, to the right
channel.
Additional phase-compensating all-pass networks may, as noted above, be
inserted
in the difference channel to correct for any extra phase shift contributed by
the
usually minimum-phase-shift spectral modifying circuit so that the correct
phase
relationship between the canceling signal and the direct signal is maintained
over the
desired frequency range.
[0015] In the automobile or vehicle context, the pair of central
speakers may
be placed in a common enclosure with a central dividing partition that is
inserted into
or else integral with the front console or dashboard of the automobile. In
certain
embodiments, the center speakers may be placed with their diaphragms facing
down
and in close proximity to a rigid reflecting surface such that substantially
all of the
sound energy is directed forward, towards the listener, via an arrow slot in
the
enclosure. The resultant radiating system provides the dual benefit of
occupying less
dashboard area, where space is always at a premium, and possessing a very wide
directional characteristics due to the slot having dimensions that can be made
very
small with respect to the wavelength the radiated sound.
CA 02827686 2013-09-18
-8-
[0015a] Accordingly, the present invention provides a narrow profile
sound
system, comprising: a drive unit disposed on a mounting surface, said mounting
surface forming a barrier acoustically isolating the drive unit's forward
radiation from
its rearward radiation; a sound reflecting surface facing the drive unit; and
a narrow
sound duct terminating in an elongate output slot, the sound duct being
defined by
the sound reflecting surface, the mounting surface, and an interior sidewall
disposed
between the sound reflecting surface and the mounting surface that follows a
rear
contoured edge of the drive unit opposite the sound duct, such that the sound
duct
provides a substantially straight path from the drive unit to the output slot;
whereby
forward radiation from the drive unit is turned at a substantially right angle
and
channeled through the sound duct directly towards the output slot.
[0015b] There is also provided a narrow-profile audio speaker system
comprising: a first drive unit mounted on a first baffle member whereby the
first drive
unit's forward radiation is isolated from its rearward radiation; a second
drive unit
mounted on a second baffle member whereby the second drive unit's forward
radiation is isolated from its rearward radiation; a first sound reflecting
surface facing
said first drive unit; a second sound reflecting surface facing said second
drive unit;
a first narrow sound duct terminating in a first sound output aperture, the
first narrow
sound duct being defined by the first sound reflecting surface, the first
baffle member,
and a first interior sidewall disposed between the sound reflecting surface
and the
first baffle member; whereby forward radiation from said first drive unit is
turned at a
substantially right angle and travels along a substantially straight path to
exit the first
sound output aperture; and a second narrow sound duct terminating in a second
CA 02827686 2013-09-18
-9-
sound output aperture, the second narrow sound duct being defined by the first
sound reflecting surface, the second baffle member, and a second interior
sidewall
disposed between the sound reflecting surface and the second baffle member;
whereby forward radiation from said second drive unit is turned at a
substantially right
angle and travels along a substantially straight path to exit the second sound
output
aperture.
[0015c] There is also provided a narrow-profile audio speaker system
comprising: a first drive unit and a second drive unit mounted on a common
baffle
whereby each drive unit's forward radiation is isolated from its rearward
radiation; a
first sound reflecting surface facing said first drive unit; a second sound
reflecting
surface facing said second drive unit; a first sound duct terminating in a
first sound
output aperture, the first sound duct being defined by the first sound
reflecting
surface, the baffle, and a first interior sidewall disposed between the sound
reflecting
surface and the baffle; whereby forward radiation from said first drive unit
is turned at
a substantially right angle and travels along a substantially straight path to
exit the
first sound output aperture; a second sound duct parallel to said first sound
duct and
terminating in a second sound output aperture, the second sound duct being
defined
by the first sound reflecting surface, the baffle, and a second interior
sidewall
disposed between the sound reflecting surface and the baffle; whereby forward
radiation from said second drive unit is turned at a substantially right angle
and
travels along a substantially straight path to exit the second sound output
aperture;
and a common center wall disposed between said first sound duct and said
second
CA 02827686 2013-09-18
-10-
sound duct, said common center wall forming part of the first interior
sidewall and the
second interior sidewall.
[001514 There is also provided a narrow profile sound system,
comprising: a
drive unit disposed on a mounting surface, said mounting surface forming a
barrier
acoustically isolating the drive unit's forward radiation from its rearward
radiation; a
sound reflecting surface facing the drive unit; and a narrow sound duct
terminating in
an elongate output slot, the sound duct being defined by the sound reflecting
surface,
the mounting surface, and an interior sidewall disposed between the sound
reflecting
surface and the mounting surface that follows a rear contoured edge of the
drive unit
opposite the sound duct, such that the sound duct provides a substantially
straight
path from the drive unit to the output slot; whereby forward radiation from
the drive
unit is turned and channeled through the sound duct in a linear path directly
towards
the output slot.
[0015b] There is also provided a narrow-profile audio speaker system
comprising: a first drive unit mounted on a first baffle member whereby the
first drive
unit's forward radiation is isolated from its rearward radiation; a second
drive unit
mounted on a second baffle member whereby the second drive unit's forward
radiation is isolated from its rearward radiation; a first sound reflecting
surface facing
said first drive unit; a second sound reflecting surface facing said second
drive unit; a
first narrow sound duct terminating in a first sound output aperture, the
first narrow
sound duct being defined by the first sound reflecting surface, the first
baffle member,
and a first interior sidewall disposed between the sound reflecting surface
and the
first baffle member; whereby forward radiation from said first drive unit is
turned and
CA 02827686 2013-09-18
-11-
channeled through the first narrow sound duct in a linear path directly
towards the
first sound output aperture; and a second narrow sound duct terminating in a
second
sound output aperture, the second narrow sound duct being defined by the first
sound reflecting surface, the second baffle member, and a second interior
sidewall
disposed between the sound reflecting surface and the second baffle member;
whereby forward radiation from said second drive unit is turned and channeled
through the second narrow sound duct in a linear path directly towards the
second
sound output aperture.
[0016] Further embodiments, variations and enhancements are also
disclosed
herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram illustrating playback of a soundtrack in
a 5.1
surround system.
[0018] FIG. 2 is a diagram illustrating playback of a 5.1 surround format
soundtrack in a 7.1 surround sound system.
[0019] FIG. 3 is a diagram illustrating playback of a 6.1 surround
format
soundtrack in a 7.1 surround sound system.
[0020] FIG. 4 is a diagram illustrating the concept of a "sweet
spot" in the
context of 6.1 surround format playback in a 7.1 surround sound system.
[0021] FIG. 5 is a diagram illustrating movement of the phantom
image in
conjunction with the listener's movement.
CA 02827686 2013-09-18
-12-
[0022] FIG. 6 is a diagram of a speaker configuration for a
surround sound
system, in accordance with a preferred embodiment as described herein.
[0023] FIG. 7 is a diagram illustrating 6.1 surround format
playback in the
surround sound system shown in FIG. 6.
[0024] FIG. 8 is a simplified block diagram of a sound processing system in
accordance with one or more embodiments as disclosed herein, as may be used,
for
example, in connection with the speaker configuration illustrated in FIG. 6.
[0025] FIG. 9-1 is a more detailed diagram of a sound processing
system as
may be used, for example, in connection with the system illustrated in FIG. 6
[0026] FIG. 9-2 is a diagram of a sound processing system in general
accordance with the layout illustrated in FIG. 9-1, further showing examples
of
possible transfer function characteristics for certain processing blocks.
[0027] FIG. 10 is a diagram of a sound processing system
illustrating
representative transfer functions.
[0028] FIG. 11 is a diagram of a sound system in accordance with the
general
principles of the systems illustrated in FIGS. 8 and 9, as applied in the
context of a
surround sound system.
[0029] FIG. 12 is a conceptual diagram illustrating
processing/operation for 5.1
surround format playback in the context of a surround sound system such as
shown,
for example, in FIG. 601 11.
[0030] FIGS. 13 and 14 are graphs illustrating examples of
frequency
response and phase transfer functions for a sound processing system having
particular spectral weighting and other characteristics.
CA 02827686 2013-09-18
-13-
[0031] FIGS. 15-1, 15-2, and 15-3 are graphs illustrating examples
of gain
and/or phase transfer functions for a sound processing system in accordance
with
FIG. 9-2.
[0032] FIG. 16 is a diagram of a sound processor employing a linear
spectral
weighting filter.
[0033] FIG. 17 is a diagram of a preferred automobile sound system
in
accordance with one or more embodiments as disclosed herein.
[0034] FIG. 18 is a diagram of a surround sound system for an
automobile or
other vehicle.
[0035] FIGS. 19-1, 19-2 and 19-3 are diagrams illustrating possible
placement
of a pair of center speakers.
[0036] FIG. 20-1 is a front cut-away view of a preferred speaker
enclosure for
a pair of stereo speakers.
[0037] FIG. 20-2 is a top cross-sectional view of the speaker
enclosure shown
in FIG. 20-1.
[0038] FIG. 20-3 is an oblique front view of the speaker enclosure
shown in
FIGS. 20-1 and 20-2.
[0039] FIG. 20-4 is a diagram illustrating sound reflection from a
downward
oriented speaker, such as a speaker in the speaker enclosure of FIGS. 20-1
through
20-3.
[0040] FIG. 21 is a block diagram illustrating an example of an
automobile
sound system for providing potentially improved extreme right/left sound, in
connection with the pair of closely spaced center speakers.
CA 02827686 2013-09-18
-14-
[0041] FIG. 22 is a graph illustrating a relationship between
speaker
separation in various embodiments as disclosed herein and difference channel
gain.
[0042] FIG. 23 is a diagram of another embodiment of a surround
sound
system for an automobile or other vehicle.
[0043] FIGS. 24-1 and 24-2 are diagrams comparing the audio effect of
speaker placement and sound processing between the prior art and various
embodiments as disclosed herein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0044] According to various embodiments as disclosed herein, a preferred
sound reproduction system comprises, in one aspect, a pair of speakers located
in
close proximity, and a sound processor which provides audio signals to the
pair of
speakers. The sound processor preferably acts to "spread" the sound image
produced by the two closely spaced speakers by employing a cross-cancellation
technique wherein a cancellation signal is derived, for example, from the
difference
between the left and right channels. The resulting difference signal is
scaled,
delayed (if necessary) and spectrally modified before being added to the left
channel
and, in opposite polarity, to the right channel, thereby enlarging the
perceived area
and stability of the stereo sound image. Further details of preferred sound
processing techniques are described later herein.
[0045] Some advantages of various embodiments disclosed herein can
be
appreciated by way of contrast and comparison with conventional surround/multi-
channel sound systems. FIG. 1, for example, is a diagram illustrating playback
of a
CA 02827686 2013-09-18
-15-
surround-encoded soundtrack in a 5.1 surround system 100. As shown in FIG. 1,
the
5.1 surround system 100 includes a front left speaker 104, a front right
speaker 105,
a center speaker 102, a sub-woofer 109, a surround left speaker 114, and a
surround
right speaker 115. In the example shown in FIG. 1, the surround left and right
speakers 114, 115 are both dipolar speakers, which distribute sound in
multiple
(typically opposite) directions and are thereby provide improved ambient
sound. The
surround left and right speakers 114, 115 are typically widely spaced on
opposite
sides of a room (or other listening space), at positions which are above and
slightly to
the rear of the desired listening position.
[0046] The speakers 102, 104, 105, 109, 114, and 115 in the 5.1 surround
system 100 are generally arranged to provide optimum sound for a listener 107
positioned in the approximate center of the speaker arrangement. However, a
5.1
surround system lacks an effective directional component to the immediate left
and
right sides and to the rear of the listener 107. Therefore, a 6.1 or 7.1
surround
system, both of which have a rear speaker component, is generally capable of
providing superior sound and audio effects in certain contexts. A 6.1 surround
system, as previously indicated, adds a single rear surround speaker, while a
7.1
surround system adds two rear surround speakers typically spaced relatively
far
apart from one another.
[0047] FIG. 2 is a diagram of a 7.1 surround system 200, illustrating
playback
of a 5.1 surround-encoded soundtrack. As shown in FIG. 2, the 7.1 surround
system
200 includes front left and right speakers 204, 205, a center speaker 202, a
sub-
woofer 209, a surround left speaker 214, a surround right speaker 215, a
surround
CA 02827686 2013-09-18
-16-
back left speaker 224, and a surround back right speaker 225. In the
particular
example of FIG. 2, as with FIG. 1, the surround left and right speakers 214,
215 are
dipolar in nature. The surround back left and right speakers 224, 225 are
typically
spaced relatively far apart behind the listener 207. When a 5.1 encoded
soundtrack
is played on a 7.1 surround system 200 such as shown in FIG. 2, the surround
left
and right speakers 214, 215 receive the left and right surround channel
information,
and the surround back left and right speakers 224, 225 may or may not receive
the
left and right surround channel information, depending upon how the user has
programmed the system 200. In either case, certain anomalies can occur. For
example, if the left and right surround channels are monaural, the left/right
surround
effect can seem to disappear and be replaced by a single central "phantom"
sound
image 230 at the rear of the listener 207. This effect can be mitigated by the
use of
adaptive de-correlation, which involves derivation of two substantially de-
correlated
signals from the single monaural channel in order to provide an improved
enveloping
surround effect.
[0048] FIG. 3 is a diagram illustrating 6.1 surround format playback
in a 7.1
surround system. In FIG. 3, the speakers labeled 3xx generally correspond to
the
same speakers labeled 2xx in FIG. 2. When a soundtrack in a 6.1 surround
format is
played on a 7.1 surround system 300 such as shown in FIG. 3, the surround back
speakers 324, 325 are fed with identical monaural signals (derived from the
single
surround back channel in the 6.1 encoding format), which may or may not be
delayed
with respect to each other to compensate for unequal distances from the
optimum
listening position. As illustrated in FIG. 3, the identical monaural signals
being played
CA 02827686 2013-09-18
-17-
through the surround back speakers 324, 325 produces a central "phantom" sound
image 330 when the listener is positioned approximately equidistant from them.
Reported side effects include "coloration" associated with the phantom sound
image
330, which can lead to listener confusion or an unnatural sound, a narrow
"sweet
spot" (see FIG. 4) due to lack of sound image stability when the listener
moves off
center from the axis which is equidistant from both surround back speakers
324, 325
(see FIG. 5), and suppression of certain frequency ranges due to cancellations
(i.e.,
nulls) caused by a "comb filter" effect as the sound waves interfere with one
another.
As a result, the sound quality of a 6.1 surround format soundtrack, when
played back
in a 7.1 surround system 300, can suffer significantly, particularly for
listeners that are
not positioned in an optimum listening position.
[0049] As previously indicated in the Background section hereof,
replay of
soundtracks in other multi-channel formats (such as Dolby AC-3, DTS or DVD-
Audio)
can also suffer from similar effects, depending upon the nature of the signals
fed to
the different left/right and back surround speakers.
[0050] FIG. 6 is a diagram showing a speaker configuration for a
surround
sound system 600 in accordance with a preferred embodiment as described
herein.
The sound system 600 of FIG. 6 includes, similar to the systems 200 and 300
shown
in FIGS. 2 and 3, respectively, front left and right speakers 604, 605, a
front center
speaker 602, a sub-woofer 609, a surround left speaker 614, and a surround
right
speaker 615. The sound system 600 further includes a surround back left
speaker
624 and a surround back right speaker 625, which are preferably positioned in
close
proximity to one another, possibly even within the same speaker enclosure. The
CA 02827686 2013-09-18
-18-
surround back left and right speakers 624, 625 are preferably identical and
may be
either dipolar or monopolar in nature, but are shown in FIG. 6 as monopolar.
The
speaker configuration of the sound system 600 illustrated in FIG. 6, coupled
with a
preferred sound processing technique, can provide improved sound quality when,
for
example, playing audio tracks recorded in any of the surround sound or multi-
channel
formats.
[0051] When the sound system 600 of FIG. 6 is used to play a
soundtrack
recorded in 7.1 surround format, the various left, right, center, and surround
left/right
channel audio signals are fed to the appropriate individual speakers, as would
normally be done with a typical 7.1 surround speaker configuration. However,
the
surround back left and right speakers 624, 625 preferably receive the surround
back
right channel audio signal and surround back left channel audio signal after
sound
processing as further described in more detail later herein.
[0052] When, on the other hand, the sound system 600 of FIG. 6 is
used to
play a soundtrack recorded in 6.1 surround format, the various left, right,
center, and
surround left/right channel audio signals are again fed to the appropriate
individual
speakers, as would normally be done with a typical 7.1 surround speaker
configuration. Typically, assuming that Surround EX playback is properly
selected
(e.g., a Surround EX flag is present), the surround back left and right
speakers 624,
625 both receive and respond directly to the surround rear channel audio
signal. The
central rear sound image produced by the closely spaced surround back left and
right
speakers 624, 625 from the monaural signal (i.e., the surround rear channel
audio
signal) is stable over a much wider area, as compared to widely spread
surround
CA 02827686 2013-09-18
-19-
back left and right speakers, and has significantly less "coloration" or
unnaturalness
than the audio sound produced by such widely spaced rear surround speakers.
[0053] In some instances, such as, for example, where the 6.1
Surround
soundtrack is matrix-encoded, or where Surround EX processing is not invoked
for
whatever reason, a somewhat different type of playback may be experienced. In
such a case, the sound system may effectively treat the soundtrack as a 5.1
soundtrack, and may send to the surround back left and right speakers 624, 625
the
surround left and right channel audio signals, which may be mixed with at
least some
portion of the monaural channel information (if the soundtrack is matrix
encoded).
According to a preferred sound system as disclosed herein, the surround back
left
and right speakers 624, 625 both receive and respond directly to the surround
rear
channel audio signal, if such information is present, and, after suitable
sound
processing, as further described herein, to the surround left/right channel
audio
signals. FIG. 7 illustrates the playback of a 6.1 surround-encoded soundtrack
in the
sound system 600 of FIG. 6 in such a situation. As shown in FIG. 7, a wide
monaural
sound image is projected from the surround back left and right speakers 624,
625.
The surround left and right channel audio signals are fed to both the surround
left
and right speakers 614, 615, and to the surround back left and right speakers
624,
625 after sound processing as further described later herein.
[0054] When the sound system 600 of FIG. 6 is used to play a soundtrack
recorded in 5.1 surround format, the various left, right and center channel
audio
signals are fed to the appropriate individual speakers, as would normally be
done
with a typical 7.1 surround speaker configuration. Preferred operation with
respect to
CA 02827686 2013-09-18
-20-
the surround left and right speakers 614, 615 and surround back left and right
speakers 624, 625 depends in part upon the nature of the surround left/right
channel
audio signals. When the surround left/right channel audio signals are monaural
in
nature, the sound system 600 preferably uses adaptive de-correlation to
provide a
de-correlated signal for the side surround speakers 614, 615, and provides a
direct
feed to the surround back left and right speakers 624, 625 to produce a
superior rear
central image. However, when the surround left/right channel audio signals are
stereo in nature, the surround left/right channel audio signals are fed
directly to the
surround left and right speakers 614, 615 without adaptive de-correlation,
and, if
desired, after suitable sound processing as further described herein, to the
surround
back left and right speakers 624, 625. The surround left and right channel
audio
signals are processed such that the apparent rear sound image size is
increased,
and its stability is improved at off-axis listening positions. The
appropriately
apportioned and summed output of the two side surround speakers 614, 615 and
the
two surround back speakers 624, 625 creates a near-continuous rear-half sound
field, thereby improving the sound experience for listeners over a wider area.
[0055] FIG. 12 is a simplified diagram conceptually illustrating
playback of a
5.1 surround format soundtrack in the sound system 600 of FIG. 6, when the
sound
system 600 is configured to apply the surround left and right channel audio
signals
1211, 1212 to the rear surround speakers 1224, 1125. As illustrated in FIG.
12,
when the surround left and right channel audio signals 1211, 1212 are
monaural,
adaptive de-correlation processing (as represented by blocks 1271 and 1272) is
CA 02827686 2013-09-18
-21-
activated, and when they are stereo in nature, adaptive sound processing for
the rear
surround speakers 1224, 1225 (as represented by block 1201) is activated.
[0056] More generally, the techniques described herein are capable
of
producing potentially improved sound for a stereo signal in connection with a
speaker
configuration that includes two speakers placed in close proximity. Whenever a
stereo signal from any encoded program (e.g., surround sound or multi-channel
soundtrack), or any audio product or source, is fed directly to the
appropriate right
and left speakers (e.g., left and right surround speakers) and, after suitable
sound
processing as further described herein, to the pair of speakers placed in
close
proximity (e.g., surround back speakers). The pair of closely spaced speakers
is
thereby capable of generating a sound image of improved stability and quality
over a
wider area, thus enlarging the optimum listening area and providing greater
satisfaction to the listeners.
[0057] Further details regarding preferred sound processing for
closely spaced
speakers (such as rear surround speakers 624, 625 in FIG. 6) will now be
described.
FIG. 8 is a generalized block diagram of a sound processing system 800 in
accordance with on embodiment as disclosed herein, as may be used, for
example,
in connection with the speaker configuration illustrated in FIG. 6, or more
generally, in
any sound system which utilizes multiple audio channels to provide stereo
source
signals. As shown in FIG. 8, a left audio signal 811 and right audio signal
812 are
provided to a sound processor 810, and then to a pair of closely spaced
speakers
824, 825. The left audio signal 811 and right audio signal 812 may also be
provided
to left and right side (surround or non-surround) speakers, not shown in FIG.
8. In a
CA 02827686 2013-09-18
-22-
preferred embodiment, the sound processor 810 acts to "spread" the sound image
produced by the two closely spaced speakers 824, 825 by employing a cross-
cancellation technique wherein a cancellation signal is derived, for example,
from the
difference between the left and right audio signals 811, 812. The resulting
difference
signal is scaled, delayed (if necessary) and spectrally modified before being
added to
the left channel and, in opposite polarity, to the right channel. The spectral
modification to the difference channel preferably takes the form of a low-
frequency
boost over a specified frequency range, in order to restore the correct
timbral balance
after the differencing process which causes a loss of bass when the low-
frequency
signals in each channel are similar. The effect of the sound processor 810 is
to
enlarge the perceived area and stability of the sound image produced by the
speakers 324, 325, and provide an effect of stereo sound despite the close
proximity
of the speakers 324, 325.
[0058] FIG. 9-1 is a more detailed diagram of a sound processing
system 900
in accordance with various principles as disclosed herein, and as may be used,
for
example, in connection with the sound system 600 illustrated in FIG. 6, or
more
generally, in any sound system which utilizes multiple audio channels to
provide
stereo source signals. In the sound processing system 900 of FIG. 9-1, a left
audio
signal 911 and right audio signal 912 are provided from an audio source, and
may be
fed to other speakers as well (not shown in FIG. 9-1). A difference between
the left
audio signal 911 and right audio signal 912 is obtained by, e.g., a subtractor
940, and
the difference signal 941 is fed to a spectral weighting filter 942, which
applies a
spectral weighting (and possibly a gain factor) to the difference signal 941.
The
CA 02827686 2013-09-18
-23-
characteristics of the spectral weighting filter 942 may vary depending upon a
number of factors including the desired aural effect, the spacing of the
speakers 924,
925 with respect to one another, the taste of the listener, and so on. The
output of
the spectral weighting filter 942 may be provided to a phase equalizer 945,
which
compensates for the phase shifting caused by the spectral weighting filter 942
(if non-
linear).
[0059] In FIG. 9-1, the output of the phase equalizer 945 is
provided to a
cross-cancellation circuit 947. The cross-cancellation circuit 947 also
receives the
left audio signal 911 and right audio signal 912, as adjusted by phase
compensation
circuits 955 and 956, respectively. The phase compensation circuits 955, 956,
which
may be embodied as, e.g., all-pass filters, preferably shift the phase of
their
respective input signals (i.e., left and right audio signals 911, 912) in a
complementary manner to the phase shifting performed by the phase equalizer
945
(in combination with the phase distortion caused by the spectral weighting
filter 924),
such that the phase characteristic of the central channel is substantially
1800 degrees
out-of-phase with the phase characteristic of the left and right channels over
the
frequency band of interest. The cross-cancellation circuit 947, which may
include a
pair of summing circuits (one for each channel), then mixes the spectrally-
weighted,
phase-equalized difference signal, after adjusting for appropriate polarity,
with each
of the phase-compensated left audio signal 911 and right audio signal 912. The
perceived width of the soundstage produced by the pair of speakers 924, 925
can be
adjusted by varying the gain of the difference signal path, and/or by
modifying the
shape of the spectral weighting filter 942.
CA 02827686 2013-09-18
-24-
[0060] FIG. 9-2 is a diagram of a sound processing system 900' in
general
accordance with the principles and layout illustrated in FIG. 9-1, further
showing
typical examples of possible transfer function characteristics for certain
processing
blocks. As with FIG. 9-1, in the sound processing system 900' a left audio
signal 911'
and a right audio signal 912' are provided from an audio source (not shown),
and a
difference signal 941' is obtained representing the difference between the
left audio
signal 911' and the right audio signal 912'. The difference signal 941' is fed
to a
spectral weighting filter 942', which, in the instant example, applies a
spectral
weighting to the difference signal 941', the characteristics of which are
graphically
illustrated in the diagram of FIG. 9-2. A more detailed graph of the transfer
function
characteristics (both gain and phase) of the spectral weighting filter 942' in
this
example appears in FIG. 15-1. As shown therein, the spectral weighting filter
942' is
embodied as a first-order shelf filter with a gain of 0 dB at low frequencies,
and turn-
over frequencies at approximately 200 Hz and 2000 Hz. If desired, the gain
applied
by gain/ amplifier block 946' can be integrated with the spectral weighting
filter 942',
or the gain can be applied downstream as illustrated in FIG. 9-2. In any
event, as
previously noted, the turnover frequencies, amount of gain, slope, and other
transfer
function characteristics may vary depending upon the desired application
and/or
overall system characteristics.
[0061] A phase equalizer 945' is provided in the center processing channel,
and addition phase compensation circuits 955' and 956' in the right and left
channels,
to ensure that the desired phase relationship is maintained, over the band of
interest,
between the center channel and the right and left channels. As shown
graphically in
CA 02827686 2013-09-18
-25-
both FIG. 9-2 and in more detail in FIG. 15-1, the spectral weighting filter
942' in the
instant example causes a phase distortion over at least the 200 Hz to 2000 Hz
range.
The phase equalizer 945' provides no gain, but modifies the overall frequency
characteristic of the center channel. The phase compensation circuits 955' and
956'
likewise modify the phase characteristics of the left and right channels,
respectively.
The phase compensation is preferably selected, in the instant example, such
that the
phase characteristic of the center channel (that is, the combined phase effect
of the
spectral weighting filter 942' and the phase equalizer 945') is approximately
1800 out-
of-phase with the phase characteristic of the left and right channels, over
the
frequency band of interest (in this example, over the 200 Hz to 2000 Hz
frequency
band). At the same time, the phase characteristic of the left and right
channels are
preferably remains the same, so that, among other things, monaural signals
being
played over the left and right channels will have identical phase processing
on both
channels (and thus maintain proper sound characteristics). Therefore, the
phase
compensation circuits 955' and 956' preferably are configured to apply
identical
phase processing to the left and right channels.
[0062] More detailed graphical examples of gain and phase transfer
functions
(with the gain being zero in this case when the components are embodied as all-
pass
filters) are illustrated for the center channel phase equalizer 945' in FIG.
15-2 and for
the left and right channel phase compensation circuits 955', 956' in FIG. 15-
3. In
these examples, the phase equalizer 945' is embodied as a second-order all-
pass
filter (with F = 125 Hz and Q = 0.12), and the phase compensators 955', 956'
are
each embodied as second-order all-pass filters (with F = 3200 Hz and Q =
0.12). A
CA 02827686 2013-09-18
-26-
higher Q value may be used to increase the steepness of the phase drop-off,
reducing the extent to which the center channel is out-of-phase with the left
and right
channels at low frequencies (thus minimizing the burden imposed upon the
speakers
924', 925').
[0063] FIG. 11 illustrates another implementation of the sound system 900
shown in FIG. 9-1, where all-pass filters 1157, 1158 are used in phase
compensation
blocks 1155 and 1156, respectively, to provide phase equalization and/or
compensation. In FIG. 11, elements labeled with reference numerals "11xx"
generally correspond to their counterparts labeled "9xx" in FIG. 9-1.
[0064] FIG. 10 is another diagram of a sound processing system 1000, in
accordance with the general principles explained with respect to FIGS. 3 and
9,
illustrating representative transfer functions according to an exemplary
embodiment
as described herein. In the sound processing system 1000 shown in FIG. 10,
input
audio signals X1 and X2 (e.g., left and right audio signals) are processed
along two
parallel paths, and the resultants individually summed together and provided
as
output signals Y1 and Y2, respectively (which may be fed to a pair of
speakers, e.g.,
left and right speakers located in close proximity). A difference between the
input
audio signals X1 and X2 is obtained from a subtractor 1040, which provides the
resulting difference signal 1040 to a processing block 1060 having a transfer
function
¨B. The first input audio signal X1 is also fed to a processing block 1055
having a
transfer function A, and the output of processing block 1055 is added together
with
the output of processing block 1060 and fed as the first output signal Y1.
Likewise,
the second input audio signal X2 is fed to a processing block 1056 having a
transfer
CA 02827686 2013-09-18
-27-
function -A (i.e., the inverse of the transfer function A of processing block
1055), and
the output of processing block 1056 is inverted and added together with the
inverted
output of processing block 1060, then fed as the second output signal Y2. The
overall relationship between the inputs and the outputs of the FIG. 10 sound
processing system 1000 can be expressed as:
4[1 01+ Br 1 11)[x, 1 [y1 ]
0 1 1 -1 x2 Y2
In a preferred embodiment, the transfer function -B of processing block 1060
represents the combined transfer functions of a spectral weighting filter of
desired
characteristics and a phase equalizer, such as illustrated by the difference
path in the
sound processing system 400 of FIG. 4. Also in a preferred embodiment, the
transfer
functions A and -A of processing blocks 1055 and 1056, respectively, each
represent the transfer function of a phase compensation network that performs
a
complementary phase shifting to compensate for the phase effects caused by the
processing block 1060. The polarities in FIG. 10 are selected so that
appropriate
cross-cancellation will be attained.
[0066] In a preferred embodiment, input signals X1 and X2 represent
the Z-
transforms of the left and right audio channel inputs, and Y1 and Y2 represent
the
corresponding Z-transforms of the left and right channel outputs which feed
the pair
of speakers (e.g., left and right speakers) located in close proximity. The
transfer
functions A, -A, and B may be represented in terms of z, and are determined in
part
CA 02827686 2013-09-18
-28-
by the sampling frequency Fs associated with processing in the digital domain.
According to a particular embodiment, blocks 1055 and 1056 are each second-
order
all-pass filters with f = 3200 Hertz, Q = 0.12, and may, in one example,
possess the
following transfer function characteristics based upon representative examples
of the
sampling frequency Fs:
[0066] For Fs = 48 KHz,
A(z) = ¨ 0.2578123 ¨ 0.6780222z-' + Z-2
1 - 0.6780222z-' ¨ 0.2578123z-2
[0067] For Fs = 44.1 KHz,
A(z)= ¨ 0.2944196 ¨ 0.633509z-' + Z-2
1 - 0.633509z-1 ¨0.2944196z2
[0068] For Fs = 32 KHz,
A(z)= ¨ 0.4201395 ¨ 0.469117z-' +z2
1 ¨ 0.469117z-1 ¨ 0.4201395z-2
In this particular embodiment, block 1060 may be a first-order shelf having a
gain of
0 dB at low frequencies and turn-over frequencies of 200 Hertz and 2 KHz in
cascade
with a second-order all pass filter, with f = 125 Hz, Q = 0.12, and may, in
one
CA 02827686 2013-09-18
-29-
example, possess the following transfer function characteristics based upon
representative examples of the sampling frequency Fs:
[0069] For Fs = 48 KHz,
0.1116288 - 0.0857871z-1 0.8723543-1.872104z' +z2
B(z). G x X
1- 0.9741583z-1
1-1.872104z-1 + 0.8723543z-2
[0070] For Fs = 44.1 KHz,
0.1126427 - 0.0845478z-1 O.8618468-1.861552z1 +z2
B(z) = G x ____________________ X ________________________
1- 0.9719051z-1
1-1.861552z-1 + 0.8618468z-2
[0071] For Fs = 32 KHz,
0.1173312 - 0.0788175z-1 O.814462-1.813915z' +z2
B(z)=G x , x
1 - 0.9614863z' 1-1.813915z-1 + 0.814462z-2
A gain factor may also be included in block 1060, or else may be provided in
the
same path but as a different block or element. The gain may be determined for
a
particular application by experimentation, but is generally expected to be
optimal in
the range of 10-15 dB. In one embodiment, for example, the gain factor is 12
dB.
CA 02827686 2013-09-18
-30-
[0072] FIGS. 13 and 14 are graphs illustrating examples of frequency
response and phase transfer functions for a sound processing system in
accordance
with FIG. 10 and having particular spectral weighting, equalization and phase
compensation characteristics. FIG. 13 illustrates a frequency response
transfer
function 1302 and phase transfer function 1305 for ¨B/A, which represents the
transfer function of the difference channel (¨B) and the first input channel
(X1) with
+12 dB of gain added. As shown in FIG. 13, the frequency response transfer
function 1302 exhibits a relatively flat gain in a first region 1320 of bass
frequencies
(in this example, up to about 200 Hertz), a decreasing gain in a second region
1321
of mid-range frequencies (in this example, from about 200 Hertz to about 2
KHz), and
then a relatively flat gain again in a third region 1322 of high frequencies
(in this
example, above 2 KHz). The phase response transfer function 1305 indicates
that in
the second region 1321 of mid-range frequencies (i.e., between about 200 Hertz
and
2 KHz) the output signal remains substantially in phase.
[0073] FIG. 14 illustrates a frequency response transfer function 1402 and
phase transfer function 1405 for ¨B/¨A, which represents the transfer function
of
the difference channel (¨B) and the first input channel (X2) with +12 dB of
gain
added. In FIG. 14, as with FIG. 13, the frequency response transfer function
1402
exhibits a relatively flat gain in a first region 1420 of bass frequencies (in
this
example, up to about 200 Hertz), a decreasing gain in a second region 1421 of
mid-
range frequencies (in this example, from about 200 Hertz to about 2 KHz), and
then a
relatively flat gain again in a third region 1422 of high frequencies (in this
example,
above 2 KHz). The phase response transfer function 1405 indicates that in the
CA 02827686 2013-09-18
-31-
second region 1421 of mid-range frequencies (i.e., between about 200 Hertz and
2
KHz) the output signal is substantially inverted in phase (i.e., at 180
degrees).
[0074] As noted, the output signals Y1, Y2 are preferably provided
to a pair of
speakers located in close proximity. The transfer functions A, ¨A, and B are
examples selected for the situation where the speakers are located
substantially
adjacent to one another. However, benefits may be attained in the system 1000
of
FIG. 10, or other embodiments described herein, where the pair of speakers are
not
immediately adjacent, but are nevertheless in close proximity with one
another.
[0075] FIG. 16 is a diagram of a sound processing system 1600 in
accordance
with an alternative embodiment as described herein, employing a linear
spectral
weighting filter. In the sound processing system 1600 of FIG. 16, a left audio
signal
1611 and right audio signal 1612 are processed to derive a pair of processed
audio
signals 1648, 1649 which are applied to a pair of closely spaced speakers
1624,
1625 (e.g., left and right speakers). The left and right audio signals 1611,
1612 are
operated upon by a subtractor 1640, which outputs a difference signal 1641
representing a difference between the left and right audio signals 1611, 1612.
The
difference signal 1641 is fed to a spectral weighting filter 1642 having a
linear phase
characteristic. The spectral weighting filter 1642 may have frequency response
characteristics in general accordance, for example, with the transfer function
illustrated in FIG. 7A or 7B. Because the spectral weighting filter 1642 has a
linear
phase characteristic, phase equalization and compensation are not necessary.
Therefore, the output of the spectral weighting filter 1642 may be provided
directly to
a cross-cancellation circuit 1646, which then mixes the spectrally weighted
signal
CA 02827686 2013-09-18
-32-
with each of the left and right audio channels before applying them to the
speakers
1624, 1625. To compensate for the delay caused by the spectral weighting
filter
1642, delay components 1655 and 1656 may be added along the left and right
channel paths, respectively. The delay components 1655, 1656 preferably have a
delay characteristic equal to the latency of the linear spectral weighting
filter 1642.
[0076] The amount of cross-cancellation provided by the sound
processing in
various embodiments generally determines the amount of "spread" of the sound
image. If too much cross-cancellation is applied, then the resulting sound can
seem
clanky or echoey. If, on the other hand, too little cross-cancellation is
applied, then
the sound image may not be sufficiently widened or stabilized.
[0077] The pair of speakers (e.g., speakers 824 and 825 in FIG. 8,
or closely
spaced speakers in other embodiments described herein) which receive the sound
processed information are preferably located immediately adjacent to one
another;
however, they may also be physically separated while still providing benefits
of
enlarged sound image, increased stability, and so on. Generally, the maximum
acceptable separation of the pair of speakers can be determined by
experimentation,
but performance may gradually decline as the speakers are moved farther apart
from
one another. Preferably, the two speakers are placed no further apart than a
distance that is comparable with the wavelength of the highest frequency that
is
intended to be radiated by the speakers. For a maximum frequency of 2 kHz,
this
separation would correspond to a center-to-center spacing of about 6 inches
between the two speakers. However, ideally the two speakers are placed
CA 02827686 2013-09-18
-33-
immediately next to one another, in order to attain the maximum benefit from
the
sound processing techniques as described herein.
[0078]
Certain embodiments of the invention may find application in a variety
of contexts other than home theater or surround sound systems. For example,
implementations of the invention may, in some circumstances, be applicable to
personal computer systems (e.g., configured to play audio tracks, multi-media
presentations, or video games with "three-dimensional" or multi-channel
sound),
automobile or vehicular audio systems, portable stereos, televisions, radios,
and any
other context in which sound reproduction is desired. Certain embodiments may
find
particular utility in situations in which possible speaker locations are
limited and/or the
maximum spacing between left and right speakers is severely limited, but where
two
adjacent or closely spaced speakers could be achieved. For example, the pair
of
closely spaced left and right speakers may be part of an integrated portable
stereo
unit, or else may be located atop or beneath a computer monitor, etc.
[0079] Automobile or vehicular audio systems, in particular, may benefit
from
application of the inventive concepts disclosed herein.
Audio systems are
commonplace in automobiles and certain other vehicles. Such systems generally
utilize program sources ranging from simple radios to relatively elaborate
stereo or
multi-channel systems with CD and cassette players together with multiple
equalizers, pre-amplifiers, power amplifiers etc.
[0080]
While there is a great variety in the configuration and components of
conventional automotive audio systems, most of them suffer to varying degrees
from
a number of persistent problems in providing the highest sound quality. These
CA 02827686 2013-09-18
-34-
problems partially result from the unique sound environment of the automobile
when
compared with a good listening room. Among the disadvantages are:
= Much smaller internal volume resulting in a reduced reverberation time
and lower modal density at low frequencies resulting in a lack of ambience
and an uneven bass response.
= The proximity of highly reflective surfaces (such as the windows) to
highly
absorbtive areas such as the upholstery or the occupants clothing leads to
a great variability with frequency and head position of the direct to indirect
sound arriving at the listener. Consequently even small changes in head
or seating position can cause significant and undesirable changes in the
timbral quality of the music.
= The listening positions are necessarily restricted to the seating
positions
provided (usually 4 or 5) and all of these are very asymmetrically placed
with respect to the speaker positions. Space is always at a premium within
a car interior and as a result the speakers are often placed in physically
convenient positions, that are nevertheless very poor from an acoustic
point of view, such as the foot wells and the bottom of the front and rear
side doors. As a result the listener's head is always much closer to either
the left or right speaker leading directly large inter-channel time
differences and different sound levels due to the 1/r law.
CA 02827686 2013-09-18
-35-
Additionally, the angles between the axes from the listeners ears to the
axes of symmetry of the left and right speakers is quite different for each
occupant. The perceived spectral balance is different for each channel
due to the directional characteristics of the drive units. Masking of one or
more speakers by the occupants clothes or legs can often result in the
attenuation of the mid- and high- frequencies by as much as 10dB.
All of the above adversely impact the ability to produce high quality stereo
reproduction, which ideally has the following attributes:
= A believable and stable image or soundstage resulting from the listener
being
nearly equidistant from the speakers reproducing the left and right channels
and a sufficiently high ratio of direct-to-indirect sound at the listener's
ears.
= A smooth timbral balance at all the listening positions.
= A sense of ambience resulting from a uniform soundfield.
[0081]
Some features are provided in automobile audio systems which can
partially mitigate the aforementioned problems. For example, an occupant can
manually adjust the sound balance to increase the proportional volume to the
left or
right speakers. Some automobile audio systems have a "driver mode" button
which
makes the sound optimal for the driver. However, because different listening
axes
exist for left and right occupants, an adjustment to the balance that
satisfies the
CA 02827686 2013-09-18
-36-
occupant (e.g., driver) on one side of the automobile will usually make the
sound
worse for the occupant seated on the other side of the automobile. Moreover,
balance adjustment requires manual adjustment by one of the occupants, and it
is
generally desirable in an automobile to minimize user intervention.
[0082] Another modification made to some automobile audio systems is to
provide a center speaker, which reduces the image instability that occurs when
the
listener is closer to either the left or right speaker when both are
reproducing the
same mono signal, with the intention of producing a central sound image. Other
potential approaches which might be taken in an attempt to mitigate the
foregoing
automotive sound problems include adding more speakers in a greater variety of
positions (e.g., at the seat tops). While such techniques can sometimes
provide a
more pleasing effect, they cannot provide stable imaging as the problems
associated
with asymmetry described above still remain. The considerable additional cost
of
such design approaches is usually undesirable in the highly cost sensitive and
competitive automotive industry. Moreover, as previously noted, space is
usually at a
premium in the automobile interior, and optimal speaker positions are limited.
[0083] According to one or more embodiments as disclosed herein
which are
designed to overcome one or more of the foregoing problems, drawbacks, or
disadvantages, a preferred automobile sound reproduction system comprises a
pair
of speakers placed close together and located in the front of the console or
dashboard with their geometric center on (or as near as possible to) the
central axis
of symmetry of the vehicle. The sound reproduction system further preferably
comprises a sound processor which provides audio signals to the pair of
speakers.
CA 02827686 2013-09-18
-37-
Because the left and right center speakers are effectively adjacent to one
another,
the difference in time of arrival of the sound information becomes minimal,
and the
relative volume level of both speakers remains approximately the same.
Moreover,
both the right and left occupant experience approximately the same volume
level
from the center pair of speakers and the ratio of direct to indirect sound is
minimized.
[0084] According to a preferred embodiment, the sound processor acts
to
"spread" the sound image produced by the two closely spaced speakers by
employing a cross-cancellation technique in which, for example, the
cancellation
signal is derived from the difference between the left and right channels. The
resulting difference signal is scaled, delayed (if necessary) and spectrally
modified
before being added to the left channel and, in opposite polarity, to the right
channel.
The spectral modification to the difference channel preferably takes the form
of a low-
frequency boost over a specified frequency range, in order to restore the
correct
timbral balance after the differencing process which causes a loss of bass
when the
low-frequency signals in each channel are similar. Additional phase-
compensating
all-pass networks may be inserted in the difference channel to correct for the
extra
phase shift contributed by the usually minimum-phase-shift spectral modifying
circuit
so that the correct phase relationship between the canceling signal and the
direct
signal is maintained over the desired frequency range.
[0085] Alternatively, a linear-phase network may be employed to provide the
spectral modification to the difference channel, in which case compensation
can be
provided by application of an appropriate, and substantially identical,
frequency-
independent delay to both left and right channels.
CA 02827686 2013-09-18
-38-
[0086]
In various embodiments, the pair of central speakers may be placed in
a common enclosure with a central dividing partition that is inserted into or
else
integral with the front console or dashboard of the automobile.
In certain
embodiments, the center speakers may be placed with their diaphragms facing
down
and in close proximity to a rigid reflecting surface such that substantially
all of the
sound energy is directed forward, towards the listener, via an arrow slot in
the
enclosure. The resultant radiating system provides the dual benefit of
occupying less
dashboard area, where space is always at a premium, and possessing a very wide
directional characteristics due to the slot having dimensions that can be made
very
small with respect to the wavelength the radiated sound.
[0087]
The use of a pair of central speakers in conjunction with sound
processing to provide improved sound quality may be employed in more than one
location in the automobile, to extend the foregoing concepts further. Thus,
for
example, a pair of rear central speakers with similar sound processing may be
added
in the rear of the vehicle, for example in the center above the rear seatback,
for use
in the play back of program with discretely encoded or simulated multi-channel
surround sound. Likewise, for larger vehicles (e.g., a limousine), a pair of
front
central speakers may be used in both the driver compartment and the passenger
compartment, the latter having applications for rear seat video presentations
of films
or music videos having multi-channel surround sound.
[0088]
FIG. 17 is a diagram of a preferred automobile sound system 1700 in
accordance with one or more embodiments as disclosed herein. In FIG. 17, two
speakers 1714, 1715 are positioned in close proximity to one another, and
receive
CA 02827686 2013-09-18
-39-
and respond to audio signals 1732 and 1733, respectively, from a sound
processor
1708. The speakers 1714, 1715 are preferably left and right speakers, may (but
need not) be nominally identical, may be separated by a distance AD from one
another as further described herein, and may be of any suitable size and type
provided that they fit within the size constraints of the available automotive
compartment(s) or other space. Further, the speakers 1714, 1715 may be
positioned
along or near the central axis of the interior of the automobile, such as, for
example,
in the center console, or atop the center of the dashboard, or in a central
island
between the driver and passenger seats.
[0089] The sound processor 1708 receives audio input signals 1702 and 1703
from a suitable audio signal source 1705, from any typical automotive audio
components (e.g., CD player, cassette player, radio, etc.) that may be
included
therewith. The audio input signals 1702, 1703 may be derived from any audio
product, including any prerecorded medium (such as a cassette, CD, or DVD),
any
digital audio file, or any wireless (e.g., radio) broadcast received by the
audio system.
The sound processor 1708 preferably processes the stereo sound signals 1702,
1703 according to techniques described in more detail herein, and provides the
processed signals 1732, 1733 (after any desired amplification or level
shifting) to the
pair of closely spaced speakers 1714, 1715. The stereo signals 1702, 1703 may
also optionally be fed, either directly or via the sound processor 1718 (if
certain
additional or complementary sound processing is desired) to additional
speakers, if
any, such as left speaker 1724 and right speaker 1725 shown in FIG. 17.
CA 02827686 2013-09-18
-40-
[0090] In a preferred embodiment, the sound processor 1708 acts to
effectively "spread" the sound image by, in a broad sense, taking the
difference
between the two audio channels 1702, 1703, spectrally modifying the
intermediate
difference signal, and then, after scaling, adding it in appropriate polarity
to the left
and right channels. When the speakers 1714, 1715 are placed close together,
side-
by-side, the resulting phenomenon causes an apparent expansion of the stereo
sound image despite the fact that the speakers 1714, 1715 are located in close
proximity.
[0091] The bass lifting or spectral weighting carried out by the
sound
processor 1708 may cause phase shifting, which can be compensated for using
phase equalization. Complementary phase compensation can be provided along
each of the audio channels 1702, 1703 prior to mixing (i.e., cross-
cancellation) so
that the left and right audio channels 1702, 1703 are substantially in phase
with the
spectrally modified difference signal. Where the bass lifting or spectral
weighting is
accomplished using linear phase filtering, however, no phase equalization may
be
needed or desired, although equal delays are preferably added to both the left
and
right audio channel paths in order to compensate for the additional delay
produced
by the linear-phase equalizer in the difference channel. The primary purpose
of the
speakers 1714, 1715 is not necessarily to provide only monaural information,
as with
a conventional centrally positioned speaker (although monaural information may
be
fed to the speakers 1714, 1715), but rather, when combined with suitable mid-
to
high-frequency processing and mixing (via the sound processor 1708), to
produce a
CA 02827686 2013-09-18
-41-
symmetrical spreading of stereo information, which results in a better stereo
presentation for both left and right occupants regardless of the listening
axis.
[0092] Because the two center speakers 1714, 1715 are closely
spaced with
respect to one another, the difference in time of arrival of the sound
information to a
given listener becomes minimal, and the relative volume level of both
speakers, as
perceived by a given listener, is approximately the same. Moreover, both the
right
and left occupant will generally experience approximately the same volume
level from
the center pair of speakers 1714, 1715. In the event that the closely spaced
speakers are unable to radiate potentially large out-of¨phase, low-frequency
components resulting from the cross-cancellation process, the very low
frequencies
can be isolated by means of a low-pass filter and directed to a separate sub-
woofer,
while a corresponding high-pass filter may be utilized to prevent high-level,
low-
frequency signals from overloading the smaller speakers. For any bass audio
components that might be difficult for the relatively small center speakers
1714, 1715
to handle, the left and right audio channels 1702, 1703 can be fed to left and
right
bass speakers 1721 and 1722, respectively, possibly in conjunction with
attenuation
at mid/high frequencies and/or boosting at low/bass frequencies as provided by
the
sound processor 1708 or any other suitable means. In embodiments in which
mid/high frequencies are output by the center pair of closely spaced speakers
and
bass or low frequencies are output by left and right door-mounted speakers,
advantages in amplifier efficiency may be achieved because less power will
generally
be needed to obtain higher volume levels.
CA 02827686 2013-09-18
-42-
[0093] When the speakers 1714, 1715 are placed in the front console
or
dashboard, or otherwise on or near the center axis of the automobile, they may
(but
need not be) mounted at a sufficient height so as to have a relatively
unobstructed
pathway to the listeners' ears, thus eliminating muffling or damping
associated with
obstructions such as seats and occupant bodies. In such embodiments, the
speakers 1714, 1715 are located at an ideal or at least preferably acoustical
position,
being less obstructed and less reflected, and allowing more space for the
sound to
unfold.
[0094] The pair of speakers (e.g., speakers 1714 and 1715 in FIG.
17) which
receive the sound processed information are preferably located immediately
adjacent
to one another; however, they may also be separated by some distance AD while
still
providing benefits of enlarged sound image, increased stability, and so on.
Generally, the farthest maximum separation of the speakers 1714, 1715 can be
determined by experimentation, but performance may gradually decline as the
speakers 1714, 1715 are moved farther apart from one another. Preferably, the
pair
of speakers 1714, 1715 are placed no further apart than a distance that is
comparable with the wavelength of the highest frequency that is intended to be
radiated by the speakers 1714, 1715. For a maximum frequency of 2 kHz, this
would
correspond to a center-to-center spacing of about 6 inches between speakers
1714
and 1715. However, ideally the speakers 1714, 1715 are placed immediately next
to
one another, in order to attain the maximum benefit from the sound processing
techniques as described herein.
CA 02827686 2013-09-18
-43-
(0096] When the pair of speakers 1714, 1715 are closely spaced, they
may be
placed in a common enclosure, with a central (preferably airtight) dividing
partition,
that may, for example, may be inserted into or else integral with the front
console or
dashboard of an automobile, or placed elsewhere near the central axis of the
automobile. FIGS. 20-1, 20-2, and 20-3 illustrate one example of an enclosure
2001,
particularly suited to applications where space is limited, housing a pair of
speakers
2014, 2015 which can receive and respond to sound processed signals from left
and
right audio channels in accordance with the various techniques described
herein.
FIG. 20-1 is a front cut-away view of the exemplary speaker enclosure 2001
housing
the pair of speakers 2014, 2015; FIG. 20-2 is a top cross-sectional view of
the
speaker enclosure 2001 shown in FIG. 20-1; and FIG. 20-3 is an oblique front
view of
the speaker enclosure 2001 shown in FIGS. 20-1 and 20-2. As shown perhaps best
in FIG. 20-3, the speaker enclosure 2001 in this example is preferably
substantially
rectangular in shape, and is preferably designed with dimensions so as to
slide into
or otherwise fit within a standard "DIN" slot (approximately 8" by 2 W) in the
front
console space of an automobile. The speaker enclosure 2001 may include a front
panel 2032, a pair of side panels 2030, a top panel 2035, a bottom panel 2039,
and
possibly a back panel 2031. To achieve isolation between the two speakers
2014,
2015, an interior wall 2016 such as illustrated in FIG. 20-1 and 20-2 may be
placed
between the speakers 2014, 2015, thus creating two separate speaker chambers,
one housing each of the two speakers 2014, 2015.
[0096] The pair of speakers 2014, 2015 may be pointed directly
frontwards;
however, in the instant example, the speakers 2014, 2015 are oriented
downwards,
CA 02827686 2013-09-18
-44-
as illustrated in FIG. 20-1. When so oriented, a slot 2019 may be located at
the
bottom of the speaker enclosure 2001, to allow the sound from the speakers
2014,
2015 to radiate outwards towards the direction of the listeners in the
automobile.
Effectively, then, the speakers 2014, 2015 only take up an amount of surface
space
corresponding to the size of the slot 2019. In an automobile environment,
front
console/dash space is typically extremely valuable since it is scarce, and
thus the
ability to position two speakers 2014, 2015 in the front console/dash while
minimizing
the amount of surface space consumed can be extremely advantageous. Audio
system controls/display(s) or other conventional console accouterments
(controls,
LCD or other displays, air vents, etc.) can be attached to or integral with
the front
panel 2032 of the speaker enclosure 2001, so the available surface space on
the
front panel 2032 is valuably utilized.
[0097] Moreover, when so oriented, the speakers 2014, 2015 may be
potentially larger in size (assuming console space is limited); for example,
each
speaker may be about 4" (for a total of 8" across collectively), which fits
into the 8"
DIN space, whereas the speakers would otherwise generally have to be under 2
1/2"
to fit within the DIN space, if oriented in a frontwards direction. The
ability to place
larger speakers in the center speaker unit allows better bass reproduction
and,
hence, reduces or potentially dispenses with the need for side (e.g., door-
mounted)
bass speakers to carry the bass information of the left and right channels.
[0098] The effect of orienting the speakers 2014, 2015 in a downward
direction
is conceptually illustrated in FIG. 20-4, which shows a generic speaker 2090
pointing
downwards towards a surface 2091. The sound output from the speaker 2090
CA 02827686 2013-09-18
-45-
radiates outward from the centerpoint along the surface 2091 in essentially
all
directions (i.e., a complete 360-degree circle). Thus, as shown in FIGS. 20-1
and 20-
3, a slot 2019 is preferably located at the bottom of the speaker enclosure
2001, to
allow the sound from the speakers 2014, 2015 to radiate outwards towards the
direction of the listeners in the automobile. A layer of insulation 2012
(e.g., foam)
preferably matching the outer contours of the speakers 2014, 2015, as
illustrated in
FIG. 20-2, may be placed within the speaker enclosure 2001, so that the sound
does
not reflect on the back panel 2031 (if any) of the speaker enclosure. In the
resulting
speaker enclosure configuration, sound emanating from the speakers 2014, 2015
is
cleanly projected through the slot 2019 to the listeners in the automobile.
[0099] In an alternative embodiment, the speakers 2014, 2015 might
be
directed upwards instead of downwards, with the slot 2019 being located at the
top of
the speaker enclosure 2001, to achieve a similar effect. The speakers 2014,
2015
may alternatively be positioned sideways, either facing towards are away from
each
other, with a pair of slots (one for each of the speakers 2014, 2015) being
vertical in
orientation rather than horizontal, as with slot 2019. In such an embodiment,
the
speaker enclosure may be taller but narrower in size.
[0100] In some circumstances, high frequencies (such as over 2 KHz)
might
become lost or reduced in the speaker enclosure configuration illustrated in
FIGS.
20-1 through 20-3. Therefore, one or more additional speakers 2017 of small
size
(e.g., tweeters) may be advantageously placed above the "bell" of the speakers
2014, 2015 and in the front panel 2032 of the speaker enclosure 2001, to
radiate the
higher frequencies.
CA 02827686 2013-09-18
-46-
[0101] While the speaker enclosure 2001 shown in FIGS. 20-1 through
20-3
has certain advantages for placement in a standard DIN space of an automobile,
it
should be understood that the closely spaced speakers 1714, 1715, whether or
not
contained in a speaker enclosure 2001, may be positioned in other areas of the
automobile as well, such as atop the front dashboard, above the rear seatback,
or in
a center console or island located between the front seats or between the
front and
back seats. Preferably, the closely spaced speakers 1714, 1715 are located on
or
near the center axis of the automobile, so as to provide optimal sound quality
evenly
to occupants on both sides.
[0102] Because of space constraints within an automobile, the centrally
located speakers (e.g., speakers 1714, 1715 in FIG. 17) may be of limited
size.
Smaller speakers, however, tend to suffer losses at low frequencies. To
compensate
for the loss of low frequency components where the central pair of speakers
are
small, left and right bass speakers (e.g., speakers 1724, 1725) may be
provided in a
suitable location ¨ for example, built into the automobile doors. The left and
right
audio channels fed to the left and right door speakers can be processed to
attenuate
the mid/high frequencies and/or boost the bass audio components,. Providing
bass
frequencies through the door speakers will not destroy the stereo effect of
the
mid/high frequencies provided by the central pair of speakers, since it is
well known
that frequencies below about 100 Hz are not normally localized by the human
listener.
[0103] In addition, as previously noted, a sub-woofer may be added
in a
suitable location within the automobile to further enhance very low frequency
bass
CA 02827686 2013-09-18
-47-
audio components. The sub-woofer may be located, for example, in the rear
console
of the car above the rear seatback, or in any other suitable location.
[0104] In certain situations, passengers in the back seat area of
the
automobile may actually experience even better sound quality than the front
occupants, because they are farther away from the stereo source (that is, the
pair of
central front speakers). Various modifications may be made to provide even
further
improved sound for the front occupants as well. For example, a similar pair of
closely
spaced speakers to those placed in the front console or area can also be
placed in
the rear of the automobile, for example, atop the rear seatback on or in the
rear
parcel shelf. The same signals that are used to feed the front pair of closely
spaced
speakers can be used to feed the rear pair of closely spaced speakers. If
desired, a
speaker enclosure 2001, such as shown in FIGS. 20A ¨ 20C, containing the pair
of
closely spaced speakers may be placed in the rear of the vehicle to house
these rear
speakers.
[0105] FIG. 19-1 is a simplified top view of an automobile 1900
illustrating an
example of placement of a pair of closely spaced speakers 1905 (whether or not
in a
speaker enclosure) in the front section of the automobile 1900 (e.g., in the
front
console or the front dash), with the addition of two door-mounted speakers
1907,
1908 for, e.g., providing added bass or low frequency audio components. FIG.
19-2
illustrates an example similar to FIG. 19-1, but adding a pair of closely
spaced
speakers 1930 (whether or not in a speaker enclosure) in the rear of the
automobile
1920. FIG. 19-3 illustrates an example of placement of speakers in a large
vehicle
such a limousine, with separate driver and passenger compartments. In the
driver
CA 02827686 2013-09-18
-48-
compartment 1941, the layout is similar to FIG. 19-1, with a pair of closely
spaced
speakers 1945 in the front area (e.g., console, dash, or the like) of the
vehicle 1940,
and pair of door-mounted left and right speakers 1947, 1948. In the passenger
compartment 1942, the layout is similar to FIG. 19-2, with two pairs of
closely spaced
speakers 1955, 1960, one in the front area and one in the rear area of the
passenger
compartment 1942, with a pair of right and left door-mounted speakers 1957,
1958
also. Of course, in any of these examples, any number of additional speakers
and
audio components may be added based upon individual need and preference,
subject to spatial limitations of the vehicle, cost, etc.
[0106] In certain applications, it may be desirable to provide surround
sound or
other multi-channel capability in a vehicular automotive system, in
conjunction with
the closely spaced speaker arrangement described previously herein. For
example,
a van or other large vehicle may have a DVD system which allows digital audio-
visual
media to be presented to the passengers of the vehicle, with the sound
potentially
being played through the vehicle audio system. In other cases, it may be
desirable to
allow for extreme right and left directional sound, which may originate by the
existence of left and right surround channels on the recorded medium, or
simply by
the presence of an extreme and intentional disparity in the relative volumes
of the left
and right channel.
[0107] A block diagram illustrating an example of an automobile sound
system
2100 for providing potentially improved extreme right/left sound, in
connection with
the pair of closely spaced center speakers 2114, 2115, is illustrated in FIG.
21. The
system 2100 shown therein operates much as described with the FIG. 17 sound
CA 02827686 2013-09-18
-49-
system 1700 with respect to the closely spaced center speakers 2114, 2115,
producing the illusion of a widened stereo sound image for the occupants of
the
vehicle. In addition, the sound system 2100 illustrates the feed of left and
right audio
signals 2102, 2103 to left and right door-mounted speakers 2124, 2125,
optionally
through low pass filters 2181, 2182, respectively, to emphasize the bass
tones. To
produce extreme left/right sounds, which may be difficult with the closely
spaced
speakers 2114, 2115, some portion (dictated by a gain factor k) of the
difference
between the left and right audio channels 2102, 2103 is mixed in to each of
the
signals fed into the left and right door mounted speakers 2124, 2125. When the
left
and right audio channels 2102, 2103 are close in amplitude (and frequency),
the
signals mixed into the left and right channels fed to the door mounted
speakers 2124,
2125 are negligible. However, as the difference between the left and right
audio
channels 2102, 2103 grows, the signal fed into the left or right channel
(depending on
which channel is larger) grows proportionately (in a linear or non-linear
fashion,
depending upon preference). A large difference between the left and right
audio
channels 2102, 2103 indicates an extreme left or right sound, which, in the
sound
system 2100 of FIG. 21, can be successfully reproduced in the left or right
door-
mounted speakers 2124, 2125.
[0108] Another embodiment, directed to a surround or multi-channel
sound
system 1800 as may be utilized in a vehicle, is illustrated in block form in
FIG. 18. As
shown therein, the sound system 1800 may include an audio signal source 1805
which provides a source for left and right audio channels 1802, 1803, which
are fed
to a sound processor 1808 which functions in a manner similar to sound
processor
CA 02827686 2013-09-18
-50-
1708 shown in FIG. 17, or various other sound processor embodiments described
herein with respect to closely spaced left/right central speakers. The left
and right
audio signals 1802, 1803 may, in the present example, comprise front left and
front
right audio signals of a surround sound formatted medium. A center audio
signal of
the surround sound formatted medium may be mixed into the signals 1832, 1833
provided to the closely spaced speakers 1814, 1815, and may also be provided
to
additional center speakers 1817 (e.g., tweeters), if provided. The closely
spaced
speakers 1814, 1815 and additional speakers 1817 may be embodied and arranged,
for example, in the form of the speaker enclosure and arrangement illustrated
in
FIGS. 20A ¨ 20C. A surround left and surround right audio channel 1871, 1872
may
be fed into surround left and right speakers 1824, 1825, which may be dipolar
or
monopolar in nature. The surround left and right speakers 1824, 1825 may be
generally used to provide ambient sound. When the surround left and right
audio
channels 1871, 1872 are monaural in nature, adaptive decorrelation may be
employed, as well understood in the art, to prevent signal cancellation or
similar
sound deterioration.
[0109] Left and right speakers 1834, 1835, which may be, e.g., door-
mounted
speakers, may be directly fed the left and right audio channels 1802, 1803, or
else
may be fed only the bass/low frequency tones, possibly mixed with extreme
right or
left sound components, such as described previously with respect to the sound
system of FIG. 21.
[0110] In addition, the sound system 1800 of FIG. 18 may further be
provided
with an additional pair of closely spaced speakers (not shown) located at the
rear of
CA 02827686 2013-09-18
-51-
the vehicle. The additional pair of closely spaced speakers may be fed the
same
processed left and right audio channel signals 1832, 1833 as provided to the
front
closely spaced speakers 1814, 1815, or may be fed similarly processed signals
derived from the surround left and right audio channel signals 1871, 1872, or
alternatively, surround back left and back right audio channel signals (not
shown), if
the audio product is encoded in a 7.1 surround or similar multi-channel
format.
[0111] FIG. 23 is a diagram of a surround or multi-channel sound
system 2300
similar to the sound system 1800 shown in FIG. 18, but illustrating the
presence of a
pair (right and left) of closely spaced surround back speakers 2394, 2395. In
the
embodiment shown in FIG. 23, a rear surround processor 2398 receives as inputs
two surround back channels 2392, 2393 provided from the audio signal source
2305.
The rear surround processor 2398 preferably provides sound processing to the
two
surround back channels 2392, 2393 for the closely spaced rear surround
speakers
2394, 2395 in a manner similar to that for the closely spaced front right/
left speakers
2314, 2315, using any of the sound processing techniques described herein for
closely spaced speakers. The sound processing for the surround back speakers
2394, 2395 need not be identical to that of the closely spaced front
right/left speakers
2314, 2315, but may differ in terms of spectral weighting, gain, etc., to
account for the
fact that the surround back speakers 2314, 2315 may serve a different purpose
to
some degree than the front right/left speakers 2314, 2315.
[0112] The content of the surround back channels 2392, 2393 may
depend
upon the format of the encoded audio product. In 5.1 surround format, for
example,
the surround back channels 2392, 2393 may be the same as the right and left
CA 02827686 2013-09-18
-52-
surround channels 2371, 2372. In 6.1 surround format, the surround back
channels
2392, 2393 may be the same as the right and left surround channels 2371, 2372,
added or mixed with the single surround back channel. In 7.1 surround format,
the
surround back channels 2392, 2393 are preferably the independent left and
right
surround back channels encoded in the audio product.
[0113] In various embodiments as described herein, improved sound
quality
results from a stereo sound image that has stability over a larger area than
would
otherwise be experienced with, e.g., speakers spaced far apart without
comparable
sound processing. Consequently, the audio product (e.g., soundtrack) can be
enjoyed with optimal or improved sound over a larger area, and by more
listeners
who are able to experience improved sound quality even when positioned
elsewhere
than the center of the speaker arrangement. Thus, for example, a home theater
surround sound system or automobile sound system may be capable of providing
quality sound to a greater number of listeners, not all of whom need to be
positioned
in the center of the speaker arrangement in order to enjoy the playback of the
particular audio product.
[0114] In any of the foregoing embodiments, the audio product from
which the
various audio source signals are derived, before distribution to the various
speakers
or other system components, may comprise any audio work of any nature, such
as,
for example, a musical piece, a soundtrack to an audio-visual work (such as a
DVD
or other digitally recorded medium), or any other source or content having an
audio
component. The audio product may be read from a recorded medium, such as a
DVD, cassette, compact disc, CD-ROM, or else may be received wirelessly, in
any
CA 02827686 2013-09-18
-53-
available format, from a broadcast or point-to-point transmission. The audio
product
preferably has at least left channel and right channel information (whether or
not
encoded), but may also include additional channels and may, for example, be
encoded in a surround sound or other multi-channel format, such as Dolby-AC3,
DTS, DVD-Audio, etc. The audio product may also comprise digital files stored,
temporarily or permanently, in any format used for audio playback, such as,
for
example, an MP3 format or a digital multi-media format.
[0115]
The various embodiments described herein can be implemented using
either digital or analog techniques, or any combination thereof. The term
"circuit" as
used herein is meant broadly to encompass analog components, discrete digital
components, microprocessor-based or digital signal processing (DSP), or any
combination thereof. The invention is not to be limited by the particular
manner in
which the operations of the various sound processing embodiments are carried
out.
[0116]
While examples have been provided herein of certain preferred or
exemplary filter characteristics, transfer functions, and so on, it will be
understood
that the particular characteristics of any of the system components may vary
depending on the particular implementation, speaker type, relative speaker
spacing,
environmental conditions, and other such factors.
Therefore, any specific
characteristics provided herein are meant to be illustrative and not limiting.
Moreover, certain components, such as the spectral weighting filter described
herein
with respect to various embodiments, may be programmable so as to allow
tailoring
to suit individual sound taste.
CA 02827686 2013-09-18
-54-
[0117] The spectral weighting filter in the various embodiments
described
herein may provide spectral weighting over a band smaller or larger than the
200
Hertz to 2 KHz band. If the selected frequency band for spectral weighting is
too
large, then saturation may occur or clipping may result, while if the selected
frequency band is too small, then the spreading effect may be inadequate.
Also, if
cross-cancellation is not mitigated at higher frequencies, as it is in the
spectral
weighting filters illustrated in certain embodiments herein, then a comb
filter effect
might result which will cause nulls at certain frequencies. Therefore, the
spectral
weighting frequency band, and the particular spectral weighting shape, is
preferably
selected to take account of the physical limitations of the speakers and
electronic
components, as well as the overall quality and effect of the speaker output.
[0118] While certain system components are described as being
"connected"
to one another, it should be understood that such language encompasses any
type of
communication or transference of data, whether or not the components are
actually
physically connected to one another, or else whether intervening elements are
present. It will be understood that various additional circuit or system
components
may be added without departing from teachings provided herein.
[0119] In some embodiments, the pair of closely spaced speakers may
be
forced to work harder than they would without cross-cancellation, because the
cross-
mixing of left and right signals requires that the speakers reproduce out-of-
phase
sound waves. To compensate for this effect, it may, for example, be desirable
in
some embodiments to increase the size of the amplifier(s) feeding the audio
signals
to the pair of closely spaced speakers. In any of the embodiments described
herein,
,
CA 02827686 2014-07-02
-55-
the speakers utilized in the automobile sound system may be passive or active
(i.e.,
with built-in or on-board amplification capability) in nature. The various
audio
channels may be individually amplified, level-shifted, boosted, or otherwise
conditioned appropriately for each individual speaker or pair of speakers.
[0120] While preferred embodiments of the invention have been described
herein, many variations are possible. Such variations would become clear to
one of
ordinary skill in the art after inspection of the specification and the
drawings. The
scope of the claims should not be limited by the preferred embodiments set
forth in
the examples, but should be given the broadest interpretation consistent with
the
description as a whole.
