SURROUND-SOUND SYSTEM

SYSTEME DE SONORISATION D'AMBIANCE

English Abstract

Abstract of the Disclosure
Spectral imbalance (alteration in timbre) when
playing home video versions of motion pictures is
overcome by re-equalization according to a unique
correction response curve which compensates for the
equalization for playback in large theater-sized
auditoriums inherent in motion picture soundtracks.
Surround-sound home playback of motion pictures is
enhanced by employing main channel loudspeakers that
produce generally direct sound fields and surround
channel loudspeakers that produce generally diffuse
sound fields. Preferably, further equalization is
applied to the reproduced surround channel to
compensate for the differences in perceived timbre
between direct and diffuse sound fields. In addition,
the reproduced surround-sound channel is further
enhanced by decreasing the interaural cross-correlation
of the surround-sound channel sound field at listening
positions within the room, preferably by introducing
slight pitch shifting in the signals applied to
multiple surround loudspeakers.

Claims

28
28045-8
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A surround-sound system for reproducing program
material recorded on multiple sound channels, including left,
right, and surround-sound channels, in a relatively small room,
such as in a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,
comprising
loudspeaker means for generating, when located in its
or their operating positions with respect to the room, in response
to first and second input signals, first and second sound fields
at listening positions within the room,
means for coupling said left and right sound channels,
as said first and second input signals, to said loudspeaker means,
said means for coupling said left and right sound channels to
said loudspeaker means including frequency response correcting
means for correcting the frequency response of said left and
right sound channels to compensate for errors that result from
reproducing said large auditorium equalized program material in
said small room,
additional loudspeaker means for generating, when
located in its or their operating positions with respect to the
room, in response to a third input signal, a third sound field at
listening positions within the room, and
surround coupling means for coupling said surround-
sound channel, as said third input signal, to said additional
loudspeaker means.

29
28045-8
2. The system of claim 1 wherein said surround-sound
system is also for reproducing a center sound channel,
said loudspeaker means generating, when located in its or their
operating positions with respect to the room, in response to a fourth
input signal a fourth sound field at listening positions within the room,
and
said means for coupling also coupling said center sound
channel, as said fourth input signal, to said loudspeaker means, said
means for coupling including frequency response correcting means for
correcting the frequency response of said center sound channel to
compensate for errors that result from reproducing said large
auditorium equalized program material in said small room.
3. The system of claim 1 wherein
said additional loudspeaker means includes first and second
additional loudspeaker means, and
said surround coupling means
further includes decorrelating means for deriving from
said surround-sound channel two sound channels which, when
reproduced by said first and second additional loudspeaker
means located in their operating positions with respect to the
room, generate first and second surround-sound fields having
low-interaural cross-correlation with respect to each other at
listening positions within the room, and
couples said two sound channels to said first and second
additional loudspeaker means.
4. The system of claim 3 wherein said decorrelating
means includes means for shifting the pitch of said two sound channels with
respect to each other.

28045-8
5. The system of claims 1, 2, or 3 wherein said frequency
response correcting means comprises a circuit having a transfer characteristic
of a filter with a shelving response such that its characteristic response is
relatively flat up to about 4 to 5 kHz, rolls off between about 4 to 5 kHz and
about 10 kHz, and is relatively flat above about 10 kHz.
6. The system of claim 5 wherein said characteristic
response, subject to a tolerance of about +1 dB up to about 10 kHz and
about +2 dB from about 10 kHz to 20 kHz, is:
<IMG>

31 28045-8
7. A surround-sound system for reproducing program
material recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relatively small room, such as in a home,
wherein said program material is equalized for reproduction in a large
auditorium whose room-loudspeaker system is aligned to a response curve
having a high-frequency roll off, comprising
loudspeaker means for generating, when located in its or their
operating positions with respect to the room, in response to first and
second input signals, first and second sound fields in which the direct
sound field component of each sound field is predominant over the
diffuse sound field component at listening positions within the room,
means for coupling said left and right sound channels, as said
first and second input signals, to said loudspeaker means, said means
for coupling said left and right sound channels to said loudspeaker
means including frequency response correcting means for correcting
the frequency response of said left and right sound channels to
compensate for errors that result from reproducing said large
auditorium equalized program material in said small room,
additional loudspeaker means for generating, when located in
its or their operating positions with respect to the room, in response to
a third input signal, a third sound field in which the diffuse sound
field component is predominant over the direct sound field component
at listening positions within the room, and
surround coupling means for coupling said surround-sound
channel, as said third input signal, to said additional loudspeaker
means.

32
28045-8
8. The system of claim 7 wherein said surround-sound
system is also for reproducing a center sound channel,
said loudspeaker means generating, when located in its or their
operating positions with respect to the room, in response to a fourth
input signal a fourth sound field in which the direct sound field
component of the sound field is predominant over the diffuse sound
field component at listening positions within the room, and
said means for coupling also coupling said center sound
channel, as said fourth input signal, to said loudspeaker means, said
means for coupling including frequency response correcting means for
correcting the frequency response of said center sound channel to
compensate for errors that result from reproducing said large
auditorium equalized program material in said small room.
9. The system of claim 7 wherein
said additional loudspeaker means includes first and second
additional loudspeaker means, and
said surround coupling means
further includes decorrelating means for deriving from
said surround-sound channel two sound channels which, when
reproduced by said first and second additional loudspeaker
means located in their operating positions with respect to the
room, generate first and second surround-sound fields having
low-interaural cross-correlation with respect to each other at
listening positions within the room, and
couples said two sound channels to said first and second
additional loudspeaker means.
10. The system of claim 9 wherein said decorrelating
means includes means for shifting the pitch of said two sound channels with
respect to each other.

33 28045-8
11. The system of claims 7, 8, 9 or 10 wherein said
frequency response correcting means comprises a circuit having a transfer
characteristic of a filter with a shelving response such that its characteristic
response is relatively flat up to about 4 to 5 kHz, rolls off between about 4 to
5 kHz and about 10 kHz, and is relatively flat above about 10 kHz.
12. The system of claim 11 wherein said characteristic
response, subject to a tolerance of about +1 dB up to about 10 kHz and
about +2 dB from about 10 kHz to 20 kHz, is:
<IMG>
13. The system of claim 11 wherein said means for
coupling said surround channel to said additional loudspeaker means includes
surround channel frequency response correcting means for correcting the
frequency response of the surround channel to compensate for the listener
perceived difference in timbre between the surround sound channel and the
other sound channels.
14. The system of claim 13 wherein said surround
channel frequency response correcting means comprises a circuit having a
transfer characteristic substantially implementing the inverse of the response
curve defining the amount by which the sound pressure level in a direct
sound field exceeds that in a diffuse sound field for equal loudness.
15. The system of claim 13 wherein said response is
defined by the international standard of ISO 454-1975(E).

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28045-8
16. The system of claims 7, 8, 9, or 10 wherein said
surround coupling means includes surround channel frequency response
correcting means for correcting the frequency response of the surround
channel to compensate for the listener perceived difference in timbre between
the surround sound channel and the other sound channels.
17. The system of claim 16 wherein said surround
channel frequency response correcting means comprises a circuit having a
transfer characteristic substantially implementing the inverse of the response
curve defining the amount by which the sound pressure level in a direct
sound field exceeds that in a diffuse sound field for equal loudness.
18. The system of claim 16 wherein said response is
defined by the international standard of ISO454-1975(E).

28045-8
19. A surround-sound system for reproducing program
material recorded on multiple sound channels, including left, right, and
surround-sound channels, in a room, comprising
loudspeaker means for generating, when located in its or their
operating positions with respect to the room, in response to first and
second input signals, first and second sound fields in which the direct
sound field component of each sound field is predominant over the
diffuse sound field component at listening positions within the room,
means for coupling said left and right sound channels, as said
first and second input signals, to said loudspeaker means,
additional loudspeaker means for generating, when located in
its or their operating positions with respect to the room, in response to
a third input signal, a third sound field in which the diffuse sound
field component is predominant over the direct sound field component
at listening positions within the room, and
surround coupling means for coupling said surround-sound
channel, as said third input signal, to said additional loudspeaker
means, said surround coupling means including surround channel
frequency response correcting means for correcting the frequency
response of the surround channel to compensate for the listener
perceived difference in timbre between the surround sound channel
and the other sound channels.
20. The system of claim 19 wherein said surround
channel frequency response correcting means comprises a circuit having a
transfer characteristic substantially implementing the inverse of the response
curve defining the amount by which the sound pressure level in a direct
sound field exceeds that om a diffuse sound field for equal loudness.
21. The system of claim 19 wherein said response is
defined by the international standard of ISO 454-1975(E).

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28045-8
22. The system of claims 19, 20, or 21 wherein
the system is for reproducing said program material in a
relatively small room, such as in a home,
said program material being equalized for reproduction
in a large auditorium whose room-loudspeaker system is
aligned to a response curve having a high-frequency roll off,
and
said means for coupling said left and right sound channels to
said loudspeaker means includes frequency response correcting means
for correcting the frequency response of said left and right sound
channels to compensate for errors that result from reproducing said
large auditorium equalized program material in said small room.
23. The system of claim 22 wherein said frequency
response correcting means comprises a circuit having a transfer characteristic
of a filter with a shelving response such that its characteristic response is
relatively flat up to about 4 to 5 kHz, rolls off between about 4 to 5 kHz and
about 10 kHz, and is relatively flat above about 10 kHz.
24. The system of claim 23 wherein said characteristic
response, subject to a tolerance of about +1 dB up to about 10 kHz and
about +2 dB from about 10 kHz to 20 kHz, is:
<IMG>

37
28045-8
25. The system of claim 22 wherein
said additional loudspeaker means includes first and second
additional loudspeaker means, and
said surround coupling means
further includes decorrelating means for deriving from
said surround-sound channel two sound channels which, when
reproduced by said first and second additional loudspeaker
means located in their operating positions with respect to the
room, generate first and second surround-sound fields having
low-interaural cross-correlation with respect to each other at
listening positions within the room, and
couples said two sound channels to said first and second
additional loudspeaker means.
26. The system of claim 25 wherein said decorrelating
means includes means for shifting the pitch of said two sound channels with
respect to each other.
27. The system of claims 19, 20, or 21 wherein
said additional loudspeaker means includes first and second
additional loudspeaker means, and
said surround coupling means
further includes decorrelating means for deriving from
said surround-sound channel two sound channels which, when
reproduced by said first and second additional loudspeaker
means located in their operating positions with respect to the
room, generate first and second surround-sound fields having
low-interaural cross-correlation with respect to each other at
listening positions within the room, and
couples said two sound channels to said first and second
additional loudspeaker means.

38
28045-8
28. The system of claim 27 wherein said decorrelating
means includes means for shifting the pitch of said two sound channels with
respect to each other.
29. The system of claim 19 wherein said surround-sound
system is also for reproducing a center sound channel, said loudspeaker
means generating, when located in its or their operating positions with
respect to the room, in response to a fourth input signal a fourth sound field
at listening positions within the room, and said means for coupling also
coupling said center sound channel, as said fourth input signal, to said
loudspeaker means.
30. The system of claim 29 wherein
the system is for reproducing said program material in a
relatively small room, such as in a home, said program material being
equalized for reproduction in a large auditorium whose room-
loudspeaker system is aligned to a response curve having a high-
frequency roll off,
said means for coupling said left, center, and right sound
channels to said loudspeaker means including frequency response
correcting means for correcting the frequency response of said left,
center, and right sound channels to compensate for errors that result
from reproducing said large auditorium equalized program material in
said small room.

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28045-8
31. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a relatively small room, such as in a home, wherein said program
material is equalized for reproduction in a large auditorium whose room-
loudspeaker system is aligned to a response curve having a high-frequency
roll off, comprising
correcting the frequency response of said left and right sound
channels to compensate for errors that result from reproducing said
large auditorium equalized program material in said small room,
generating, in response to the frequency response corrected left
and right sound channels, first and second sound fields at listening
positions within the room, and
generating, in response to said surround-sound channel, a third
sound field at listening positions within the room.
32. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a relatively small room, such as in a home, wherein said program
material is equalized for reproduction in a large auditorium whose room-
loudspeaker system is aligned to a response curve having a high-frequency
roll off, comprising
correcting the frequency response of said left and right sound
channels to compensate for errors that result from reproducing said
large auditorium equalized program material in said small room,
generating, in response to the frequency response corrected left
and right sound channels, first and second sound fields at listening
positions within the room, and
generating, in response to said surround-sound channel, third
and fourth sound fields having low-interaural cross-correlation with
respect to each other at listening positions within the room.

28045-8
33. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a relatively small room, such as in a home, wherein said program
material is equalized for reproduction in a large auditorium whose room-
loudspeaker system is aligned to a response curve having a high-frequency
roll off, comprising
correcting the frequency response of said left and right sound
channels to compensate for errors that result from reproducing said
large auditorium equalized program material in said small room,
generating, in response to the frequency response corrected left
and right sound channels, first and second sound fields in which the
direct sound field component of each sound field is predominant over
the diffuse sound field component at listening positions within the
room, and
generating, in response to said surround-sound channel, third
and fourth sound fields in which the diffuse sound field component of
each sound field is predominant over the direct sound field component
at listening positions within the room.
34. The method of claim 33, the method further
comprising correcting the frequency response of the surround channel to
compensate for the listener perceived difference in timbre between the
surround sound channel and the other sound channels.

41 28045-8
35. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a relatively small room, such as in a home, wherein said program
material is equalized for reproduction in a large auditorium whose room-
loudspeaker system is aligned to a response curve having a high-frequency
roll off, comprising
correcting the frequency response of said center sound channel
to compensate for errors that result from reproducing said large
auditorium equalized program material in said small room,
generating, in response to the frequency response corrected left
and right sound channels, first and second sound fields in which the
direct sound field component of each sound field is predominant over
the diffuse sound field component at listening positions within the
room, and
generating, in response to said surround-sound channel, third
and fourth sound fields in which
the diffuse sound field component of each sound field is
predominant over the direct sound field component at listening
positions within the room, and
the sound fields have low-interaural cross-correlation
with respect to each other at listening positions within the
room.
36. The method of claim 35, the method further
comprising correcting the frequency response of the surround channel to
compensate for the listener perceived difference in timbre between the
surround sound channel and the other sound channels.

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28045-8
37. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a room, comprising
generating, in response to said left and right sound channels,
first and second sound fields in which the direct sound field
component of each sound field is predominant over the diffuse sound
field component at listening positions within the room,
correcting the frequency response of the surround channel to
compensate for the listener perceived difference in timbre between the
surround-sound channel and the other sound channels, and
generating, in response to the frequency response corrected
surround-sound channel, a third sound field in which the diffuse
sound field component is predominant over the direct sound field
component at listening positions within the room.
38. The method of claim 37 wherein the method is for
reproducing said program material recorded on multiple sound channels in a
relatively small room, such as in a home, said program material being
equalized for reproduction in a large auditorium whose room-loudspeaker
system is aligned to a response curve having a high-frequency roll off,
the method further comprising correcting the frequency
response of said left and right sound channels to compensate for errors
that result from reproducing said large auditorium equalized program
material in said small room.

43
28045-8
39. A method for reproducing program material recorded
on multiple sound channels, including left, right, and surround-sound
channels, in a room, comprising
generating, in response to said left and right sound channels,
first and second sound fields in which the direct sound field
component of each sound field is predominant over the diffuse sound
field component at listening positions within the room,
correcting the frequency response of the surround channel to
compensate for the listener perceived difference in timbre between the
surround-sound channel and the other sound channels, and
generating, in response to said frequency response corrected
surround-sound channel, third and fourth sound fields in which
the diffuse sound field component of each sound field is
predominant over the direct sound field component at listening
positions within the room, and
the sound fields have low-interaural cross-correlation
with respect to each other at listening positions within the
room.
40. The method of claim 39 wherein the method is for
reproducing said program material in a relatively small room, such as in a
home, said program material being equalized for reproduction in a large
auditorium whose room-loudspeaker system is aligned to a response curve
having a high-frequency roll off,
the method further comprising correcting the frequency
response of said center sound channel to compensate for errors that
result from reproducing said large auditorium equalized program
material in said small room.

Description

131~9~i9
IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
APPLICATION FOR PATENT
SURROUND-SOUND SYSTE~I
Inventor: Tomlinson Bolman
Attorneys' Docket No.: HOLM-0100
Backqround of the Invention
The invention relates generally to sound
reproduction. More specifically, the invention relates
to multiple channel sound reproduction systems having
improved listener perceived characteristics.
Multiple channél sound reproduction systems which
include a surround-sound channel (often referred to in
the past as an "ambience" or "special-effects" channel)
in addition to left and right (and optimally, center)
sound channels are now relatively common in motion
picture theaters and are becoming more and more common
in the homes of consumers. A driving force behind the
proliferation of such systems in consumers' homes is
the widespread availability of surround-sound home
video software, mainly surround-sound motion pictures
(movies) made for theatrical release and subsequently
transferred to home video formats (e.g., videocassettes
and videodiscs).
Although home video software formats have two-
channel stereophonic soundtracks, those two channels
carry, by means of amplitude and phase matrix encoding,
four channels of sound information--left, center,
: . ,

1 3 1 09 1 9
right, and surround, usually identical to the two-
channel stereophonic motion-picture soundtracks from
which the home video soundtracks are derived. As is
also done in the motion picture theater, the left,
center, right, and surround channels are decoded and
recovered by consumers with a matrix decoder, usually
referred to as a "surround-sound" decoder. In the home
environment, the decoder is usually incorporated in or
is an accessory to a videocassette player, videodisc
player, or television set/video monitor. Although
nearly universal in motion picture theater
environments, the center channel playback is often
omitted in home systems. A phantom-image center
channel is then fed to left and right loudspeakers to
make up for the lack of a center channel speaker.
Motion picture theaters equipped for surround
sound typically have at least three sets of
loudspeakers, located appropriately for reproduction of
the left, center, and right channels, at the front of
the theater auditorium, behind the screen. The
surround channel is usually applied to a multiplicity
of speakers located other than at the front of the
theater auditorium.
It is the recommended and common practice in the
industry to align the sound system of large
auditoriums, particularly a motion picture theater's
loudspeaker-room response, to a standardized frequency
response curve or "house curve." The current
standardized house curve for movie theaters is a
recommendation of the International Standards
Organization designated as curve X of ISO 2969-1977(E).
The use of a standardized response curve is significant
hecause in the final steps of creating motion picture

1310919
soundtracks, the soundtracks are almost always
monitored in large (theater-sized) auditoriums
("mixing" and ~dubbing" theaters) whose loudspeaker-
room responses have been aligned to the standardized
response curve. This is done, of course, with the
expectation that such motion picture films will be
played in large (theater-sized) auditoriums that have
been aligned to the same standardized response curve.
Consequently, motion picture soundtracks inherently
c-arry a built-in equalization that takes into account
or compensates for playback in large (theater-sized)
auditoriums whose loudspeaker-room responses are
aligned to the standardized curve.
The current standardized curve, curve X of ISO
2969, is a curve having a significant high-frequency
rolloff. The curve is the result of subjective
listening tests conducted in large (theater-sized)
auditGriums. A basic rationale for such a curve is
given by Robert B. Schulein in his article "In Situ
Measurement and Equalization of Sound Reproduction
Systems," J. Audio Eng. Soc., April 1975, Vol. 23, No.
3, pp. 178-186. Schulein explains that the re-
quirement for high-/frequency rolloff is apparently
due to the free fo~ (i.e., direct) to diffuse
(i.e., reflected or reverberant) sound field
diffraction effects of the human head and ears. A
distant loudspeaker in a large listening room is
perceived by listeners as having greater high frequency
output than a closer loudspeaker, if aligned to measure
the same response. This appears to be a result of the
substantial diffuse field to free field ratio generated
by the distant loudspeaker; a loudspeaker close to a
listener generates such a small diffuse to direct sound

131091q
ratio as ~o ~e insi~l-iflca~
More recently ttle rationale has been carried
Eurttler by Guntller Theile t-'On tlle Standardization oE
the Frequency nesponse of lligh-Quality Studio
lleadphones," J. ~udio Eng. Soc., December 1986, Vol.
34, No. 12, pp. 95G-969) who hypotllesized that
perceptions of loudlless and tone color ~timbre) are not
col~ letely ~e~ermille~ ~y SOUIl~ ~ressure and s~ectrum in
tlle auditory canal. ~rlleile relates thls l~ypothesls to
lU tl~e "source location effect" ~r "soul~d level lou~ness
~ivergence" ("SLD") wllicl~ occurs whenever auditory
events with differing locations are compared: a nearer
loudspeaker cequires more sound level (sound pressure)
at the ear drums to cause the sanle perceived sound
loudness as a more distant loudspeaker and the effect
is frequency dependellt.
It has also been recognized that the sound
pressure level in a free direct field exceeds
that in a dif~use field ~or equal loudness.
2~ stan~ard equalization, currently embodied in ISO 454-
1975 (E) oE the Interllational Standards Organization,
is intended to compel-sate for the differences in
perceived lou-lness and, by extensioll, timbre due to
frequency response challges ~etween such sound fields.
Perceived sound loudlless and timbre thus depends
no~ only on the locatioll at whictl sound fields are
~enerated Witll res~ect to tl-e listener but also on tlle
relative diEEuse (re1ected or reverberant~ Eield
component to free (direct) field component ratio
of the sound field at the listener.
One ma~or difLerence between ttle hollle listening
environ~ent and tlle motion picture tlleater listening
environment is in the relative sizes of the listening

13'0919
rooms--the typical home listening room, of course,
being much smaller. ~hile there is no established
standard curve to which home sound systems are aligned,
the high-frequency rolloff house curve applicable to
large auditoriums is not applicable to the considerably
smaller home listening room because of the above-
mentioned effects.
Unlike home video software media having
soundtracks transferred from motion picture film
soundtracks, recorded consumer software sound media
te.9., vinyl phonograph records, cassette tapes,
compact discs, etc.) have a built-in equalization that
compensates for typical home listening room
environments. This is because during their preparation
such recordings are monitored in relatively small (home
listening room sized) monitoring studios using
loudspeakers which are the same or similar to those
typically used in homes. Relative to large auditorium
theater environments, the response of a typical modern
home listening room-loudspeaker system or a small
studio listening room-loudspeaker system can be
characterized as substantially "flat," particularly in
the high-frequency region in which rolloff is applied
in the large auditorium house curve. A consequence of
these differences is that motion pictures transferred
to home video software media have too much high-
frequency sound when reproduced by a home system.
Consequently, the musical portions of motion picture
soundtracks played on home systems tend to sound
"bright." In addition, other undesirable results
occur--"Foley" sound effects, such as the rustling of
clothing, etc., which tend to have substantial high-
frequency content, are over-emphasized. Also, the

1310qlq
increased high-freq~ency sensitivity of home systems
often reveals details in the makeup of the soundtrack
that are not intended to be heard by listeners; for
example, changes in soundtrack noise level as dialog
tracks are cut in and out. These same problems, af
course, occur when a motion picture soundtrack is
played back in any small listening environment having
consumer-type loudspeakers, such as small monitoring
studios~
There is yet another difference between the home
sound systems and motion picture theater sound systems
that detracts from creating a theater-like experience
in the home. It has been the practice at least in
certain high-quality theater sound systems to employ
loudspeakers that provide a substantially directional
sound field for the left, center, and right channels
and to employ loudspeakers that provide a substantially
non-directional sound field for the surround channel.
Such an arrangement enhances the perception of sound
localization as a result of the directional front
loudspeakers while at the same time enhancing the
perception of ambience and envelopement as a result of
the non-directional surround loudspeakers.
In contrast, home systems typically employ main
channel (left and right channel) loudspeakers designed
to generate a compromise sound field that is neither
extremely directional nor extremely non-directional.
Surround channel loudspeakers in the home are usually
down-sized versions of the main channel loudspeakers
and generate similar sound fields. In the home
environment, little or no attention has been given to
the proper selection of directional characteristics for
the main channel and surround channel speakers.
.... ., . . ~:
. .

7 1310919
Also, with respect even to the above-mentioned
high~quality theater sound systems, no compensation has
been employed for the differences in listener perceived
timbre between the main channels and the surround
channel resulting from the generation of predominantly
direct sound fields by the main channel speakers and
the predominantly diffuse sound field produced by the
surround channel speakers.
In addition, with respect to home systems and to
the above-mentioned high quality theater sound systems,
a single (monophonic) surround-sound channel is applied
to multiple loudspeakers (usually two, in the case of
the home, located to the left and right rear of home
listening room and usually more than two, in the case
of a motion-picture theater, located on the side and
rear walls). Particularly in the home environment, the
result is that the surround-sound channel sounds to a
listener seated on the center line as though it were in
the middle of the head.
Summary of the Invention
Aspects of the present invention are directed
primarily to surround-sound reproduction systems in
relatively small listening rooms, particularly those in
homes. With respect to such, the invention solves the
problem of spectral imbalance (e.g., alteration in
timbre), particularly excessive high-frequency energy,
when playing pre-recorded sound material that is
equalized for playback in a large (theater-sized)
auditorium whose room-loudspeaker system is aligned to
a frequency response curve having a significant high-
frequency rolloff. In a preferred embodiment, re-
equalization according to a correction curve is

8 ~310919
provided in the playback system in order to restore to
a "flat" response the perceived spectral balance oE
recordings ~ransferred from motion plcture soundtracks
having an inherent high-frequency boost because of
their intended playback in large (theater-sized)
auditoriums aligne~ to the standard house curve. Such
re-equalization restores the spectral distribution
(timbre) lntended by the creators of the pre-recorded
sound material.
1~ With respect to small (home-sized) listening
rooms, a further aspect oE the invention is to generate
generally directional sound iields in response to the
left and right sound channels and in response to the
cente~ sound channel, if used, and to generate a
generally non-directional sound field in response to
the surround-sound channel.
A directional sound field is one in which the
free (direct3 component of the sound field is
predominant over the diffuse component at
2~ listening positions within the listening room. A
non--directional sound field is one in which the
diffuse component of the sound field is predominant
over the free (direct) component at listening
positions within tlle listening room. Directionality of
a sound field depends at least on the Q of the
loudspeaker or loudspeakers producing the sound field
~"Q" is a measure of the directional properties of a
loudspeaker), the number o~ loudspeakers, the size and
characteristics of the listening room, the manner in
3~ which the loudspeaker (or loudspeakers) is (or are)
acoustically coupled to (e.g., positioned with respect
to) the listening room, and the listening posltion
within the room. For example, multiple high Q

1310919
(directional) loudspeakers can be distributed so as to
produce a non-directional sound field within a room.
Also, the directionality of multiple loudspeakers
reproducing the same channel of sound can be affected
by their physical relationship to one another and
differences in amplitude and phase of the signal
applied to them.
This aspect of the invention is not concerned
se with specific loudspeakers nor with their acoustic
coupling to small listening rooms, but rather it is
concerned, in part, with the generation of direct and
diffuse sound fields for the main (left, right, and,
optionally, center) channels and for the surround
channel, respectively, in a small (home-sized) room
surround-sound system using whatever combinations of
available loudspeakers and techniques as may be
required to generate such sound fields. This aspect of
the invention recognizes that excellent stereophonic
imaging and detail combined with sonic envelopement of
the listeners can be achieved not only in large
(theater-sized) auditoriums but also in the small
(home-sized) listening room by generating generally
direct sound fields for the main channels and a
generally diffuse sound field for the surround channel.
In this way, the home listening experience can more
closely re-create the quality theater sound experience.
According to a further aspect of the invention,
the listening impression created by the generation of
direct sound fields for the main channels and a diffuse
sound field for the surround channel can be improved
even further, for all sizes of listening rooms, by the
addition of equalization to compensate for the
aifferences in perceived timbre between direct and

lO 131091't
diffuse sound fields. In other words, the Eull benefit
of the use of direct and diffuse sound fields for the
main and surround channels, respectively, is not
achieved unless appropriate equalization is provided in
the surround channel in order to compensate for its
reproduction of a diffuse sound field. ~lthough the
standard curve ISO 454 is intended to compensate for
the frequency dependent differences in perceived
loudness between direct and diffuse fields, it has not
lU heretofore been appreciated that such a correction is
relevant to surround-sound systems.
According to yet a further aspect of the
invention, the listener's impression of the surround-
sound channel can be improved, for all sizes of
listening rooms, by decreasing the interaural cross-
correlation of the surround-sound channel sound field
at listening positions within the room. Preferably,
this is accomplished by a technique such as slight
pitch shifting between multiple surround loudspeakers,
2~ which does not cause undesirable side effect. While
this aspect of the invention may be employed without
the aforementioned generation of generally direct
sound fields for the main channels and a generally
diffuse sound field for the surround channel, the
z5 combination of these aspects of the invention provides
an even more psychoacoustically pleasing listening
experience. Preferably, the combination further
includes the aspect oE the invention providing for
surround channel equalization to compensate for the
listener perceived diEference in timbre between a
direct sound field and a diffuse sound field.

lOa
1 3 1 0~ t 9 28045-8
According to a broad aspect of the invention there is
provided a surround-sound system for reproducing program material
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relatively small room, such as in
a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,
comprising loudspeaker means for generating, when located in its
or their operating positions with respect to the room, in response
to first and second input signals, first and second sound fields
at listening positions within the room, means for coupling said
left and right sound channels, as said first and second input
signals, to said loudspeaker means, said means for coupling said
left and right sound channels to said loudspeaker means including
frequency response correcting means for correcting the frequency
response of said left and right sound channels to compensate for
errors that result from reproducing said large auditorium
equalized program material in said small room, additional loud-
speaker means for generating, when located in its or their
operating positions with respect to the room, in response to a
third input signal, a third sound field at listening positions
within the room, and surround coupling means for coupling said
surround-sound channel, as said third input signal, to said
additional loudspeaker means.
According to another broad aspect of the invention
there is provided a surround-sound system for reproducing program
material recorded on multiple sound channels, including left,

-` lOb
1 3 1 09 1 ~ 28o45-8
right, and surround-sound channels, in a relatively small room,
such as in a ho~e, wherein said program material is equalized
for reproduction in a large auditorium whose room-loudspeaker
system is aligned to a response curve having a high-frequency
roll off, comprising loudspeaker means for generating, when
located in its or their operating positions with respect to the
room, in response to first and second input signals, first and
second sound fields in which the direct sound field component of
each sound field is predominant over the diffuse sound field
component at listening positions within the room, means for
coupling said left and right sound channels, as said first and
second input signals, to said loudspeaker means, said means for
coupling said left and right sound channels to said loudspeaker
means including frequency response correcting means for correcting
the frequency response of said left and right sound channels to
compensate for errors that result from reproducing said large
auditorium equalized program material in said small room,
additional loudspeaker means for generating, when located in its
or their operating positions with respect to the room, in response
to a third input signal, a third sound field in which ths diffuse
sound field component is predominant over the direct sound field
component at listening positions within the room, and surround
coupling means for coupling said surround-sound channel, as said
third input signal, to said additional loudspeaker means.
According to another broad aspect of the invention
there is provided a surround-sound system for reproducing program
material recorded on multiple sound channels, including left,
right, and surround-sound channels, in a room, comprising loud-

lOc
~31~q19 28045-8
speaker means for generating, when located in its or their
operating positions with respect to the room, in response to
first and second input signals, first and second sound fields in
which the direct sound field component of each sound field is
predominant over the diffuse sound field component at listening
positions within the room, means for coupling said left and
right sound channels, as said first and second input signals, to
said loudspeaker means, additional loudspeaker means for
generating, when located in its or their operating positions with
respect to the roomf in response to a third input signal, a
third sound field in which the diffuse sound field component is
predominant over the direct sound field component at listening
positions within the room, and surround coupling means for
coupling said surround-sound channel, as said third input signal,
to said additional loudspeaker means, said surround coupling
means including surround channel frequency response correcting
means for correcting the frequency response of the surround
channel to ~ompensate for the listener perceived difference in
timbre between the surround sound channel and the other sound
channels.
According to another broad aspect of the invention
there is provided a method for reproducing program material
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relatively small room, such as in
a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,

lOd 1 31 09 1 9 28045-8
comprising correcting the frequency response of said left and
right sound channels to compensate for errors that result from
reproducing said large auditorium equalized program material in
said small room, generating, in response to the frequency
response corrected left and right sound channels, first and
second sound fields at listening positions within the room, and
generating, in response to said surround-sound channel, a third
sound field at listening positions within the room.
According to another broad aspect of the invention
there is provided a method for reproducing program material
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relatively small room, such as in
a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,
comprising correcting the frequency response of said left and
right sound channels to compensate for errors that result from
reproducing said large auditorium equalized program ~aterial in
said small room, generating, in response to the frequency
response corrected left and right sound channels, first and
second sound fields at listening positions within the room, and
generating, in response to said surround-sound channel, third
and fourth sound fields having low-interaural cross-correlation
with respect to each other at listening positions within the
room.
According to another broad aspect of the invention
there is provided a method for reproducing program material

lOe ~ 31 09 t 9
28045-8
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relativel~ small room, such as in
a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,
comprising correcting the frequency response of said left and
right sound channels to compensate for errors that result from
reproducing sald large auditorium equalized program material in
said small room, generating, in response to the frequency response
corrected left and right sound channels, first and second sound
fields in which the direct sound field component of each sound
field is predominant over the diffuse sound field component at
listening positions within the room, and generating, in response
to said surround-sound channel, third and fourth sound fields in
which the diffuse sound field component of each sound field is
predominant over the direct sound field component at listening
positions within the room.
A~cording to another broad aspect of the invention
there is provided a method for reproducing program material
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a relatively small room, such as in
a home, wherein said program material is equalized for
reproduction in a large auditorium whose room-loudspeaker system
is aligned to a response curve having a high-frequency roll off,
comprising correcting the frequency response of said center
sound channel to compensate for errors that result from reproduc-
ing said large auditorium equalized program material in said

lOf 1310~9
28045-8
small room, generating, in response to the frequency response
corrected left and right sound channels, first and second sound
fields in which the direct sound field component of each sound
field is predominant over the diffuse sound field component at
listening positions within the room, and generating, in response
to said surround-sound channel, third and fourth sound fields in
which the diffuse sound field component of each sound field is
predominant over the direct sound field component at listening
positions within the room, and the sound fields have low-
interaural cross-correlation with respect to each other at
listening positions within the room.
According to another broad aspect of the invention
there is provided a method for reproducing program material
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a room, comprising generating, in
response to said left and right sound channels, first and second
sound fields in which the direct sound field component of each
sound field is predominant over the diffuse sound field
component at listening positions within the room, correcting the
frequency response of the surround channel to compensate for the
listener perceived difference in timbre between the surround-
sound channel and the other sound channels, and generating, in
response to the frequency response corrected surround-sound
channel, a third sound field in which the diffuse sound field
component is predominant over the direct sound field component
at listening positions within the room.
According to another broad aspect of the invention
there is provided a method for reproducing program material

lOg t31~9t9
28045-8
recorded on multiple sound channels, including left, right, and
surround-sound channels, in a room, comprising generating, in
response to said left and right sound channels, first and second
sound fields in which the direct sound field component of each
sound field is predominant over the diffuse sound field
component at listening positions within the room, correcting the
frequency response of the surround channel to compensate for the
listener perceived difference in timbre between the surround-
sound channel and the other sound channels, and generating, in
response to said frequency response corrected surround-sound
channel, third and fourth sound fields in which the diffuse
sound field component of each sound field is predominant over
the direct sound field component at listening positions within
the room, and the sound fields have low-interaural cross-
correlation with respect to each other at listening positions
within the room.

1310919
11 ,
.
srief Descsiption_of the Drawings
Figure 1 is a block diagram of a surround-sound
reproduction system embodying aspects of the invention.
Figure 2 is a block diagram of a surround-sound
reproduction system embodying aspects of the invention.
Figure 3 is a loudspeaker-room response curve used
by theaters, curve X of the International Standard ISO
2969-1977(E), extrapolated to 20 kHz.
Figure 4 is a correction curve, according to one
aspect of this invention, to compensate for the large
room equalization inherent in motion picture
soundtracks when played back in small listening rooms.
Figure 5 is a schematic circuit diagram showing
the preferred embodiment of a filter/equalizer for
implementing the correction curve of Figure 4.
Figure 6 is a diagram in the frequency domain
showing the locations of the poles and zeros on the s-
plane of the filter/equalizer of Figure 5.
Figure 7 is a schematic circuit diagram showing
the preferred embodiment for implementing the surround
channel direct/diffuse sound field equalizer according
to another aspect of the invention.
Figure 8 is a block diagram showing an arrangement
for deriving, by means of pitch shifting, two sound
outputs from the surround-sound channel capable of
providing, according to another aspect of the
invention, sound fields having low-interaural cross-
correlation at listening positions.

1310919
12
Detailed Description of the Invention
Figures 1 and 2 show, respectively, block diagrams
of two surround sound reproduction systems embodying
aspects of the invention. Figures 1 and 2 are
generally equivalent~ although, for reasons explained
below, the arrangement of Figure 2 is preferred.
Throughout the specification and drawings, like
elements generally are assigned the same reference
numerals; similar elements are generally assigned the
same reference numerals but are distinguished by prime
(') marks.
In both Figures 1 and 2, left (L), center (C),
right (R), and surround (S) channels, matrix encoded,
according to well-known techniques, as left total (LT~
and right total (RT) signals, are applied to decoding
and equalization means 2 and 2', respectively. Both
decoding and equalization means 2 and 2' include a
matrix decoder that is intended to derive the L, C, R,
and S channels from the applied LT and RT signals.
Such matrix decoders, often referred to as "surround
sound" decoders are well-known. Several variations of
surround sound decoders are known both for professional
motion picture theater use and for consumer home use.
For example, the simplest decoders include only a
passive matrix, whereas more complex decoders also
include a delay line and/or active circuitry in order
to enhance channel separation. In addition, many
decoders include a noise reduction expander because
most matrix encoded motion picture soundtracks employ
noise reduction encoding in the surround channel. It
is intended that the matrix decoder 4 include all such
variations.
In the embodiment of Figure 1, re-equalizer means

1~10~11(3
13
6 are placed in the respective LT and RT signal input
lines to the matrix decoder 4, whereas in the
embodiment of Figure 2, the re-equalizer means 6 are
located in the L, C, and R output lines from the matrix
decoder 4. The function of the re-equalizer ~eans 6
are explained below. In both the Figure 1 and Figure 2
embodiments, an optional direct/diffuse equalizer means
8 is located in the S output line from the matrix
decoder 4. The function of the direct/diffuse
equalizer means 8 is also explained below.
In both embodiments, the L, C, R, and S outputs
from the decoding and equalization means 2 feed a
respective loudspeaker or respective loudspeakers 10,
12, 14, and 16. In home listening environments the
center channel loudspeaker 12 is frequently omitted
(some matrix decoders intended for home use omit
entirely a center channel output). Suitable
amplification is provided as necessary, but is not
shown for simplicity.
The arrangements of both Figures 1 and 2 thus
provide for the coupling of at least the left, right,
and surround (and, optionally, the center) sound
channels encoded in the LT and RT signals to a
respective loudspeaker or loudspeakers. The
loudspeakers are intended to be located in operating
positions with respect to a listening room in order to
generate sound fields responsive to at least the left,
right, and surround (and, optionally, the center)
channels within the listening room.
Because of the requirement to accurately preserve
relative signal phase of the LT and RT input signals
for proper operation of the matrix decoder 4, which
responds to amplitude and phase relationships in the LT

1310919
14
and RT input signals, the placement of the
re-equalizing means 6 ~a type of filter, as explained
below) before the decoder 4, as in the embodiment of
Figure 1, is less desirable than the alternative
location after the decoder 4 shown in the embodiment of
Figure 2. In addition, the re-equalizing means 6, if
placed before decoder 4, may affect proper operation of
the noise reduction expander, if one is employed, in
the matrix decoder 4. The arrangement of Figure 2 is
lU thus preferred over that of Figure 1. The preferred
embodiment of re-equalizer means 6 described below
assumes that they are located after the matrix decoder
4 in the manner of the embodiment of Figure 2. If the
re~equalizer means 6 are located before the matrix
decoder 4 in the manner of Figure 1 it may be necessary
to modify their response characteristics in order to
minimize effects on noise reduction decoding that may
be included in the matrix decoder 4 and, also, it may
be necessary to carefully match the characteristics of
the two re-equalizer means 6 (of the Figure 1
embodiment) in order to minimize any relative shift in
phase and amplitude in the LT and RT signals as they
are processed by the re-equalizer means 6.
Figure 3 shows curve X of the International
Standard IS0 2969-1977(E) with the response
extrapolated to 20 kHz, beyond the official 12.5 kHz
upper frequency limit of the standard. It is common
practice in many theaters, particularly dubbing
theaters and other theaters equipped with high quality
3~ surround sound systems, to align their response to an
extended X characteristic. The extended X curve is a
de facto industry standard. The X characteristic
begins to roll off at 2 kHz and is down 7 dB at 10

131~J919
kHz. The extended curve is down about 9 dB at 16 kHz,
the highest frequency employed in current alignment
procedures for dubbing theaters. In public motion
picture theaters, which are larger than dubbing
theaters, the X curve is extended only to 12.5 kHz
because the high frequency attenuation of sound in the
air becomes a factor above about 12.5 kHz in such large
auditoriums. The X curve, and particularly its
extension, are believed by some in the industry to be
too rolled off at very high frequencies. In contrast
to the X curve and the extended X curve, a good quality
modern home consumer sound system, although not aligned
to a specific standard, tends not to exhibit such a
high-frequency room-loudspeaker response roll off.
Relative to the X curve and extended X curve, modern
home consumer systems may be characterized as
relatively flat at high frequencies.
As explained above, in the creation of a motion
picture soundtrack, the soundtrack is usually monitored
in a theater that has been aligned to the extended X
response curve, with the expectation that such motion
picture films will be played in theaters that have been
aligned to that standardized response curve. Thus,
motion picture soundtracks inherently carry a built-in
equalization that takes into account or compensates for
playback in theater-sized auditoriums whose
loudspeaker-room response is aligned to the
standardized curve. However, for the reasons discussed
above, this built-in equalization is not appropriate
for playback in home listening environments: the
soundtracks of motion pictures transferred to home
video software media have too much high frequency sound
energy when reproduced by a home system. Correct

' 16 1310919
timbre is not preserved and details in the soundtrack
can be heard that are not intended to be heard.
According to one aspect of this invention, a
correction curve is provided to compensate for the
large room equalization inherent in motion picture
soundtracks when played back in small listening rooms.
The correction curve was empirically derived using a
specialized commercially-available acoustic testing
manikin. The correction curve is a difference curve
l~ derived from measurements of steady-state one-third
octave sound level spectra taken in representive
extended X curve aligned large auditoriums in
comparison to a good quality modern home consumer
loudspeaker-room sound system. The correction curve is
lS shown in Figure 4 as a cross-hatched band centered
about a solid line central response characteristic.
The correction band takes into account an allowable
tolerance in the correction of about +l dB up to about
10 kHz and about ~2 dB from about lO kHz to 20 kHz,
2~ where the ear is less sensitive to variation in
response. In practice, the tolerance for the initial
flat portion of the characteristic, below about 2 kHz,
may be tighter. The form of the correction curve band
is generally that of a low-pass filter with a shelving
response: the correction is relatively flat up to
about 4 to 5 kHz, exhibits a roll off, and again
begins to flatten out above about 10 kHz. About 3 to 5
dB roll off is provided at 10 kHz. The extended X
curve response is also shown in Figure 4 for
3~ reference. As mentioned above, the X curve, and
particularly its extension are believed by some in the

17 t~?~
industry to be too rolled off at very high frequencies.
It will be appreciated that the optimum correction
curve would change in the event that a modified X curve
standard is adopted and put into practice.
A filter/equalizer circuit can be implemented by
means of an active filter, such as shown in Figure 5,
to provide a transfer characteristic closely
approximating the solid central line of the correction
curve band of Figure 4. The correct frequency response
for the filter/equalizer is obtained by the combination
of a simple real pole and a ~dip" equalizer section.
The real pole is realized by a single RC filter section
with a -3dB frequency of 15 k~z. The dip equalizer is
a second order filter with a nearly flat response. The
transfer function of the section is:
S2+Y7r+~2
The complex pole pair and the complex zero pair have
the same radian frequency but their angles are slightly
different giving the desired dip in the frequency
response with minimum phase shift. The same dip could
be achieved with the zeros in the right half plane, but
the phase shift would be closer to that of an allpass
filter--180 degrees at the resonant frequency. The
parameters of the dip section in the filter/equalizer
are:
fo=1231~1z
Q=081
y=0733

18 1 3 1 0 91 9
where fo=2~o . Another way of interpreting these
parameters is that the Q of the poles is 0.81 and the Q
of the zeros is 081 . The dip section can be
realized by a single operational amplifier filter stage
and six components as shown in Figure 5. The filter
stage in effect subtracts a bandpass filtered signal
from unity giving the required transfer function and
frequency response shape. The circuit topology, one of
a class of single operational amplifier biquadratic
circuits, is known for use as an allpass filter
(Passive and Active Network Analysis and Synthesis by
Aram Budak, ~oughton Mifflin Company, Boston, 1974,
page 451).
The rectangular coordinates of the poles and zeros
of the overall filter equalizer are as follows (units
are radians/sec in those locations on the s-plane):
Real Pole:
a,p = -9.4248xl 04
2 0 Complex Poles:
~p + j~p =-4.70~6xlO4+ jS.9962xl04
Complex 7,eros:
a, + j~, =-3.4485xl04+ j6.7967xl04
Figure 6 shows the location of the poles and zeros on
the s-plane.
When implemented with the preferred component
values listed below, the resulting characteristic
response of the filter/equalizer circuit of Figure 5
is:

1~109~q
19
Frequency, Hz Response, dB
0
100
500 0
1,000
2,000 -0.2
-3,150 -0-4
4,000 -0.7
5,000 -1.1
6,300 -1.8
8,000 -2.8
10,000 -4.2
12,500 -5.2
16,000 5-4
20,000 ~5-7
As mentioned above, there is an allowable tolerance of
about +l dB up to about 10 kHz and about +2 dB from
about 10 kHz to 20 kHz. The preferred component values
of the circuit shown in Figure 5 are as follows:
Component 5% tolerance 1% tolerance
Rl 6K8 6K81
(6.81 kilohms)
R2 18K 17K4
Cl=C2 1.2N 1.2N (1.2
nanofarads)
RA 2K2 2K00
RB lOK lOK0
RP 4K7 4K87
CP 2.2N 2.2N

1 31 Oq 1 9
The filterJequalizer circuit of Figure 5 is one
practical embodiment of the re-equalizer means 6 of
Figure 2. Many other filter/equalizer circuit
configurations are possible within the teachings of the
invention.
Referring again to the embodiments of Figures 1
and 2, the loudspeaker or loudspeakers 10, 12 (if
used), and 14 are preferable directional loudspeakers
that generate, when in their operating positions in the
lU listening room, left, center (if used), and right
channel sound fields in which the free (direct~ sound
field component is predominant over the diffuse sound
field component of each sound field at listening
positions within the listening room. The loudspeaker
or loudspeakers 16 is (or are) preferably
non-directional so as to generate, when in its or their
operating positions in the listening room, a surround
channel sound field in which the diffuse sound field
component is predominant over the free (direct) sound
2~ field component at listening positions within the
listening room. A non-directional sound field for
reproducing the surround channel can be achieved in
various ways. Preferably, one or more dipole type
loudspeakers each having a generally figure-eight
radiation pattern are oriented with one of their
respective nulls generally toward the listeners. Other
types of loudspeakers having a null in their radiation
patterns can also be used. Another possibility is
to use a multiplicity of speakers having low
3U directivity arranged around the listeners so as to
create an overall sound field that is diffuse. Thus,
depending on their placement in the listening room and
their orientation with respect to the listening

1~lo9lrj
21
positions, even directional loudspeakers are capable of
producing a predominantly diffuse sound field.
In order to obtain the full sonic benefits of
directional and non-directional speakers as just set
forth, it is preferred that the arrangements of the
Figure 1 and Figure 2 embodiments use the optional
direct/diffuse equalizer 8. Such an equalizer
compensates for the differences in listener perceived
timbre between direct and diffuse sound fields. The
lo use of a direct/diffuse equalizer with the directional
and non-directional speakers as just set forth is
applicable to both large (theater-sized) auditoriums
and to small (home) listening rooms. As applied to
large (theater-sized) auditoriums, the arrangements of
Figures 1 and 2 would, of course, not require the
re-equalizer means 6.
The preferred embodiment of the direct/diffuse
equalizer 8 is an active filter/equalizer circuit that
substantially implements (within 0.3 dB) the inverse of
2~ the curve defined by the difference data set forth in
IS0 454-1975(E). In practice, such a close tolerance
is not required. The difference data in that standard
is a table of the amount by which the sound pressure
level in a free field exceeds that in a diffuse field
for equal loudness. The data is as follows:
Frequency. Hz Difference, dB
0
63 0
o
100 0

22 1 3 1 Q q 1 9
125 o
160 0
: 200 0.3
250 0.6
315 0.9
: 400 1.2
500 1.6
630 2.3
800 2.8
1,000 3.0
1,250 2.0
1,600
2,000 - 1.4
2,500 - 2.0
3,150 - 1.9
4,000 - 1.0
5,000 0.5
6,300 3.0
8,000 4.0
; 20 10,000 4.3
There is a suggestion in the above-cited article by
Theile that ISO 454 does not properly take into account
the SLD effect, discussed above. Accordingly, the
compensation provided by the standard may be somewhat
in error. It is intended that the either ISO 454 or a
corrected version thereof should provide the basis for
the practical implementation of the equalizer 8.
Figure 7 shows a schematic diagram of a practical
embodiment of the direct/diffuse equalizer 8 that
implements the inverse of the curve defined by ISO 454-
1975(E). It will be noted that the standard provides
data up to 10 kHz. This is more than adequate because

23 ~ 3~
the frequency response of the surround channel in the
standard matrix surround sound system is limited to
about 7 kHz. Equalizer 8 employs four sections having
a total of five operational amplifiers. Except for the
second section, which is a simple RC single pole low
pass filter (25 kHz) and buffer (op amp 40), the
sections are basically the same circuit topology
identified above as known for use as an allpass filter.
The first section, including op amp 38, functions as a
dip equalizer with a -5.6 dB gain at 1 kHz. The third
section, including op amps 42 and 44, uses op amp 42 to
provide a phase inversion, causing the section to
function as a boost equalizer having a gain of 9 dB at
2.5 kHz. The last section is a further dip equalizer
having a -6 dB gain at 8 kHz. The preferred circuit
values are as follows:
component value
48 6K98
6K19
52 22N
54 22N
56 6K98
58 6K81
2.4K
62 2700 pF
64 6K81
66 30Kl
68 4K99
lON
72 lON
74 5K81
76 lOK
78 lOK2

-
1310919
24
7K5
82 2N7
8~ 2N7
86 7K5
The equalizer circuit of Figure 7 is one practical
embodiment of the equalizer means 8 of Figures 1 and
2. Many other filter/equalizer circuit configurations
are possible within the teachings of the invention.
In a modification of the embodiments of Figures 1
l~ and 2, the monophonic surround-sound channel
advantageously may be split, by appropriate
de-correlating means, into two channels which, when
applied to first and second surround loudspeakers or
groups of loudspeakers, provide two surround channel
sound fields having low-interaural cross-correlation
with respect to each other at listening positions
within the listening room. Preferably, each of the two
de-correlated surround channel sound fields is
generated by a single loudspeaker. The use of more
than a single loudspeaker to generate each field may
make it more difficult to match the timbre of the
diffuse surround channel sound field to that of the
direct left, center, and right channel sound fields.
This may be a result of a comb filter effect produced
when more than two loudspeakers are used to generate
each of the de-correlated surround channel sound
fields.
It has previously been established that human
perception favors dissimilar sound present at the two
3U ears insofar as the reverberant energy in a listening
room is concerned. In order to provide such a
dissimilarity when using matrix audio surround-sound
technology, added circuitry is needed beyond simple

1310919
Z4~
encoding and decodi11g, since o1)ly a monaural surround
track is encoded. In principle this circuitry may
employ various known techniques for synthesizing stereo
from a monaural source, such as comb filtering.
110wever, many of these techniyues produce undesirable
audible side efrects. For example, comb filters suffer
~rom audible "~11asi11ess," w11ic11 can readily be
distinguis11ed hy careful lis~eners.
Preferably, tl1e decorrel~tio11 circuitry used in
the practical embodiment of this aspect of the

1 3 1 09 1 9
invention employs small amounts of frequency or pitch
shifting, which is ~nown to be relatively unobtrusive
to critical listeners. Pitch shifting, for example, is
currently used, besides as an effect, to allow the
increase of gain before feedback in public address
systems, where it is not easily noticed, the amount of
such shifts being small, in the order of a few Hertz.
A 5 Hz shift is employed in a modulation-demodulation
circuit for this purpose described in "A Frequency
lU Shifter for Improving Acoustic Feedback Stability," by
A.J. Prestigiacomo and D.J. MacLean, reprinted in Sound
Reinforcement. An Antholoqy, Audio Engineering Society,
1978, pp. B-6 - B-9.
Frequency or pitch shifting may be accomplished by
any of the well-known techniques for doing so. In
addition to the method described in the Prestigiacomo
and MacLean article, as noted in the Handbook for Sound
Enqineers, the New Audio Cyclopedia, Howard W. Sams &
Co. First Edition, 1987, page 626, delay can form the
2~ basis for frequency shift: the signal is applied to
the memory of the delay at one rate (the original
frequency) and read out at a different rate (the
sllifted frequency).
The surround channel signal is applied to two
paths. At least one path is processed by a pitch
shifter. Preferably, the frequency or pitch shift is
fixed and is small, sufficient to psychoacoustically
de-correlate the sound fields without audibly degrading
the sound: in the order of a few Hertz. Although more
complex arrangements are possible, they may not be
necessary. For example, pitch shifting could be
provided in both paths and the pitch could be shifted
in a comp]ementary fashion, with one polarity of shift

25A 1 3 1 09 1 9
driving the surround chA1lllel signal in one path up in
frequency, and the other driving the signal in the
other path downward in frequency. Other possibilities
include varying the pitch shift by varying the clocking
of a delay line. The shift could be varied in
accordance with the envelope of the surround channel
audio signal (e.g., unàer control of a circuit
fol'owing the surround channel audio signal having a
syllabic time constallt--sucll circuits are well known
for use with audio compressors and expanders).
Although either analog or digital delay processing
may be empIoyed, the lower cost of digital delay lines
suggests digital processing, particularly the use of
adaptive delta modulation (~DM) for which relatively
inexpensive decoders are available. Conventional pulse
code modulation (PCM) also may be used. ~lthough
waveform discontinuities ("splices") occur at the
signal block sample junctions as the output signal from
the delay line is reconstructed whether ADM or PCM is
2n used, such splices tend to be inaudible in the case of
ADM because the errors are single bit errors. In the
case of PCM, special signal processing is likely
required to reduce the audibility of the splices.
According to the above cited IJandbook for Sound
Enaineers, several signal-processing techniques have
successfully reduced the audibility o~ such "splices."
Referring to Figure 8, the surround output ~rom
matrix decoder 4 (optionally, via direct/diffuse

2G 1 31 Oql 9
equalizer 8) of Figures 1 or 2 provides the input to
the decorrelator wllich is applied to an anti-aliasing
low-pass filter 102 in the signal processing path and
to an envel~pe generator 122 in the control signal
path. The filtered input signal is then applied to an
analog-to-digital converter (preferably, ADM~ 104, the
digital output of which is applied to two paths that
generate, respectively, the left surround and right
surround outputs. The assignment of the "left" and
"rigl-t" paths is purely arbitrary and the designations
may be reversed. The paths are the same and include a
clocked delay line 106 (114), a digital-to-analog
converter 108 (116) and an anti-imaging low-pass filter
110 (118).
The control signal for controlling the pitch shift
by means of altering the clocking of the delay lines
106 and 114 is fixed or variable, according to the
position of switch 124, which selects the input to a
very low frequency voltage controlled oscillator (VC0)
128 either from the envelope generator 122, which
follows the syllabic rate of the surround channel audio
signal, or from a fixed source, shown as a variable
resistor 126. VC0 128 operates at a very low fre-
quency, less than 5 Hz. The output of the low fre-
quency VC0 128 is applied directly to a high frequency
VCO 130 which clocks delay line 106 in the left
surround path and is also inverted by inverter 132 for
application to a second high frequency VC0 134 which
clocks delay line 114 in the right surround path.
When there is no output from the low frequency VC0 128,
the two high frequency VCOs are set to the same
frequency (in the megahertz range, the exact frequency
depending on the clock rate required for the delay

27 1310919
lines, which in turn depends on the digital sampling
rate selected). The low frequency oscillator 128
modulates the high frequency oscillators, pr~ducing
complementary pitch shifts.
Alternatively, the decorrelator of Figure 8 may be
simplified so that the surround output from the matrix
decoder is applied without processing in a first path
to either the left surround loudspeaker(s) 112 or right
surround loudspeaker(s) 120. The other path is applied
to the other of the loudspeaker(s) via frequency or
pitch shift processing, preferably fixed, including
anti-aliasing low-pass filter 102, analog-to-digital
converter 104, delay 106, digital-to-analog converter
108, anti-imaging low-pass filter 110. Delay 106 is
controlled as shown in Figure 8, preferably with switch
124 selecting the fixed input from potentiometer 126.
The amount of frequency shifting required in this
variation in which the pitch is shifted only in one
channel is about twice that provided to each of the
paths in the embodiment of Figure 8.
The output of the paths is applied (through
suitable amplification), respectively, to one
(preferably) or a group of left surround loudspeakers
112 and to one (preferably) or a group of right
surround loudspeakers 120. The loudspeakers should be
arranged so that they generate first and second sound
fields generally to the left (side and/or rear) and
right (side and/or rear) of listening positions within
the listening room. The aforementioned techniques
regarding the generation of a predominantly diffuse
sound field are preferably applied to the decorrelated
surround channel.

Representative Drawing