Note: Descriptions are shown in the official language in which they were submitted.
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~r~Tt F nF THE INVE TN ION
POST-AMPLIFICATION STEREOPHONIC TO SURROUND SOUND
DECODING CIRCUIT
~Jm D OF THE INVENTION
The present invention relates to stereophonic surround sound
decoding circuits. More specifically, the present invention relates to a post-
amplification stereophonic to surround sound decoding circuit to be installed
between the outputs of a conventional stereophonic amplifier and conventional
loudspeakers.
~3AGKGROUND OF THE INVENTION
While monophonic sound is still widely used in amplitude
modulation (AM) radio and in telephone networks, stereophonic sounds are now
considered the standard in high fidelity (HI-FI) applications.
Stereophonic sound systems aim at reproducing two different
sound channels, via loudspeakers, in such a way that the sounds reaching each
ear of the listener give the impression to the listener that he is brought at
the
location of the sound recording. Of course, the realism of this impression
depends on many factors such as, for example, the relative position of the
listener with respect to the loudspeakers and the quality of the recording.
Surround sound systems have been introduced in consumer
electronics to create what is now called home theatres where the listener is
submitted to different sounds coming from remote locations in the room.
Standard home theater surround sound systems are equipped with five
loudspeakers each reproducing a different sound channel.
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The surround sound decoding circuits used in surround sound
systems may generally be divided into two major types, i.e., pre-amplification
decoding circuits and post-amplification decoding circuits.
Surround sound systems using pre-amplification decoders
receive a conventional low level two-channel stereophonic sound signal and
expend it to a five-channel surround sound. Each of these five channels is
amplified separately and then supplied to a predetermined loudspeaker. The
five channels are generally determined as follows:
a main left channel corresponds to the left channel
of the stereophonic sound signal and is
reproduced by a front left loudspeaker;
a main right channel corresponds to the right
channel of the stereophonic sound signal and is
reproduced by a front right loudspeaker,
a secondary left channel (also called the left
surround channel) is decoded by subtracting the
main right channel from the main left channel and
is reproduced by a rear left loudspeaker;
2p a secondary right channel (also called the right
surround channel) is decoded by subtracting the
main left channel from the main right channel and
is reproduced by a rear right loudspeaker; and
a central channel is decoded by adding the main
25 left channel and the main right channel, and is
reproduced by a central loudspeaker usually
mounted on top of the television screen.
A major drawback of the surround sound systems using pre-
30 amplification d~oders is that a five-channel amplifier is necessary to
reproduce
the surround sound since the power amplification is done after the decoding of
the surround sound from the stereophonic sound. Users of this technology
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must therefore acquire a dedicated power amplifier, which increases the total
cost of the system.
Post-amplification surround sound decoders usually solve the
above-mentioned drawback of the pre-amplification decoders by providing an
apparatus that may be installed between the left and right power amplified
stereophonic signal outputs of a conventional stereophonic amplifier and the
five loudspeakers mentioned hereinabove. The stereophonic to surround
sound decoding is therefore done after the power amplification which allows
the
use of a conventional stereophonic amplifier.
United States Patent N° 5,265,166, issued on November 23,
1993 to Madnick et al. and entitled: "MULTI-CHANNEL SOUND SIMULATION
SYSTEM°, describes such a post-amplification stereophonic to
surround sound
decoder. The system disclosed by Madnick et al. suffers major drawbacks
generally leading to a decrease in the quality of sound reproduction and to an
increased complexity of the load imposed on the stereophonic amplifier. As
wilt
be easily understood by one skilled in the art, the return to the ground of
the
negative terminal of the rear loudspeakers causes the power output to
°see" an
impedance other than the conventional 8 ohms speaker impedance.
Furthermore, the return to the ground of the negative terminal of the central
loudspeaker via an inductance will also increase the complexity of the load of
the power amplifier wich will inevitably Isad to an increase in sound
distortion
and an overall general decrease in the sound reproduction. It is also to be
noted that the combination, via resistors, of the right and left channels to
generate the central channel will decrease the clarity of the sound
reproduction
of the main right and left channels since there are no provisions to prevent
some "bleeding" of the right channel in the left channel and vice-versa.
United States Patent N° 5,497,425, issued on March 5, 1996 to
Robert J. Rapoport and entitled: °MULTI-CHANNEL SURROUND SOUND
SIMULATION DEVICE°, describes a hybrid stereophonic to surround
sound
decoder provided with features from both the pre-ampification and post-
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amplification decoding schemes described hereinabove. The system proposed by
;. Rapoport has many drawbacks. for example, a supplementary power amplifier
must be provided to amplify the central channel before the reproduction by the
central loudspeaker. Furthermore, the drawbacks discussed hereinabove with
respect to the system of Madnick ef al. generally apply to the system of
Rapoport
since the design philosophy is similar.
Also known is GB 2,014,404 which describes a stereophonic speaker
~y system for use in an automobile, where five speakers are connected to the
two
'A' 10 standard outputs of an amplifier. This system does not provide a
surround sound
decoding environment.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide an improved
post-amplification stereophonic to surround sound decoding apparatus free of
the
above-mentioned drawbacks of the prior art.
More specifically, in accordance with the present invention, there is
provided a post-amplification stereophonic to surround sound decoding circuit
comprising:
an input for receiving a power amplified stereophonic signal including a left
channel signal and a right channel signal;
a first output configured to be connected to a secondary left loudspeaker;
a second output configured to be connected to a secondary right
loudspeaker;
a third output configured to be connected to a central loudspeaker;
secondary channels decoding means for decoding a secondary left channel
~r 30 signal and a secondary right channel signal from the left and right
channel signals
IY
of the power amplified stereophonic signal; the secondary left channel signal
being
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supplied to the fifst output and the secondary right channel signal being
supplied
r to the second output; and
central channel decoding means for decoding a central channel signal from
the left and right channel signals of the power amplified stereophonic signal;
the
central channel decoding means including first and second decoupling means
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signals; the central channel decoding means including means for combining the
left and right decoupled channel signals into the central channel signal; the
central channel signal being supplied to the third output;
whereby the decoupling means of the central channel decoding means allow
the left and right channel signals of the power amplified stereophonic signal
to
be combined without modifying the original left and right channel signals.
According to another aspect of the present invention, there is
provided a post-amplification stereophonic to surround sound decoding circuit
comprising:
an input for receiving a power amplified stereophonic signal
including a left channel signal and a right channel signal;
a first output configured to be connected to a secondary left
loudspeaker,
a second output configured to be connected to a secondary right
loudspeaker;
a third output configured to be connected to a central
loudspeaker;
a fourth output configured to be connected to a main left
loudspeaker,
a fifth output configured to be connected to a main right
loudspeaker;
secondary channels decoding means for decoding a secondary
left channel signal and a secondary right channel signal from the left and
right
channel signals of the power amplified stereophonic signal; the secondary left
channel signal being supplied to the first output and the secondary right
channel
signal being supplied to the second output;
central channel decoding means for decoding a central channel
signal from the left and right channel signals of the p~ver amplified
stereophonic signal; the central channel decoding means including first and
second decoupling means respectively decoupling the left and right channel
signals; the central channel decoding means including means for combining the
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felt and right decoupled channel signals into the central channel signal; the
central channel signal being supplied to the third output; and
main channels volume control means for controlling the amplitude
of (a) a left channel signal supplied to the fourth output ftom the left
channel
signal and (b) a right channel signal supplied to the fifth output from the
right
channel signal;
whereby the decoupling means of the central channel decoding means allow
the left and right channel signals of the power amplified stereophonic signal
to
be combined without modifying the original left and right channel signals.
According to yet another aspect of the present invention, there
is provided a central channel decoding circuit for a post-amplification
stereophonic to surround sound decoding apparatus comprising:
an input for receiving a power amplified stereophonic signal
13 including a left channel signal and a right channel signal;
an output configured to be connected to a central loudspeaker;
and
central channel decoding means for decoding a central channel
signal from the left and right channel signals of the power amplified
stereophonic signal; the central channel decoding means including first and
second decoupiing means respectively decoupling the left and right channel
signals; the ~ntra! channel d~oding means including means for combining the
left and right decoupled channel signals into the central channel signal; the
central channel signal being supplied to the output;
whereby the decoupling means of the central channel decoding means allow
the left and right channel signals of the power amplified stereophonic signal
to
be combined without modifying the original left and right channel signals.
Other objects and advantages of the present invention will
become more apparent to one skilled in the art upon reading of the following
non restrictive description of a preferred embodiment thereof, given by way of
example only with reference to the appended drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure 1 illustrates, in a block diagram, a post-amplification
stereophonic to surround sound decoding circuit according to an embodiment
of the present invention; the decoding circuit being connected to a
stereophonic
power amplifier and to five loudspeakers;
1 p Figure 2 illustrates, in a schematic view, the post-amplification
stereophonic to surround sound decoding circuit of figure 1; and
Figure 3 illustrates, in a schematic view, an alternate embodiment
of the central channel decoding circuit illustrated in figure 2.
Turning now to figures 1 and 2 of the appended drawings, a post-
amplification stereophonic to surround sound decoding circuit 10 will be
described.
The decoding circuit 10 illustrated in figure 1 is shown connected
to a stereophonic power amplifier 12. The decoding c~rcun ~ v mc~uaes a yen
input 14, a right input 16 and a ground connection 18 respectively connected
to a left output 20, a right output 22 and a ground connection 24 of the
sten~ophonic pov~r amplifier 12. Of course, the connections 20, 22 and 24 of
the power amplifier 12 are usually connected to two conventional main left and
main right loudspeakers.
The decoding circuit 10 also includes a two terminal main left
loudspeaker output 26, 27 connected to a main left loudspeaker 28, a two
terminal main right loudspeaker output 30, 31 connected to a main right
loudspeaker 32, a two terminal secondary left loudspeaker output 34, 35
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connected to a secondary left loudspeaker 36, a two terminal secondary right
loudspeaker output 38, 39 connected to a secondary right loudspeaker 40 and
a two terminal central loudspeaker output 42, 43 connected to a central
loudspeaker 44.
The decoding circuit 10 includes a main channels volume control
circuit 46, a secondary channels decoding circuit 48 and a central channel
decoding circuit 50.
As can be seen from figure 1, the main channels volume control
circuit 46 interconnects the inputs 14, 16 and 18 and the main loudspeakers
outputs 26, 27, 30 and 31; the secondary channels decoding circuit 48
interconnects the inputs 14 and 16 and the secondary loudspeakers outputs 34,
35, 38 and 39; and the central channel decoding circuit 50 interconnects the
inputs 14, 16 and 18 and the central loudspeaker outputs 42 and 43.
Turning now to figure 2 of the appended drawings, the arcuits 46,
48 and 50 will be described in greater details.
The main channels volume control circuit 46 includes a variable
resistor 52 connected to the left input 14 and to the output 26 to the main
left
loudspeaker and a variable resistor 54 connected to the right input 16 and to
the output 30 to the main right loudspeaker. By varying the value of the
resistors 52 and 54, one modifies the amplitude of the signal supplied to the
main loudspeakers, thus varying the volume of the sound reproduced by the
main loudspeakers, since the portion of the power amplified signal dissipated
as heat in the resistors is modified. As can be seen from figure 2, the ground
connections 27 and 31 are interconnected and connected to the ground input
connection 18.
It is to be noted that the variable resistors 52 and 54 could be
advantageously embodied together in a stereophonic L-pad (with no ground
connection) and having an appropriate power rating. tf this is the case, the
user
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will be faced with only one volume control for the two main loudspeakers.
Alternatively, the variable resistors could also be embodied by a plurality of
discrete high quality resistors (not shown) and switches (not shown) or by a
high power levels variable resistance integrated circuit (not shown).
10
It is therefore to be noted that the term "variable resistor is to be
construed, herein and in the appended claims, as any electronic element or
arrangement of resistive andlor other electronic elements allowing the
modification of a resistance between two points of an electrical circuit.
The secondary channels decoding circuit 48 includes a first
capacitor 56 connected to the left input 14 and to the output 34 to the
secondary left loudspeaker, a second capacitor 58 connected to the right input
16 and to the output 38 to the secondary right loudspeaker and a variable
resistor 60 connected to the ground output connection 35 to the secondary left
loudspeaker and to the output ground connection 39 to the secondary right
loudspeaker. It is to be noted that the ground output connections 35 and 39
are
not connected to the input ground connection 18.
2p As it will be apparent to one skilled in the art, the connection of
the variable resistor 60 between the output ground connections 35 and 39
causes the secondary loudspeakers to reproduce respective secondary channel
signals that are obtained by a substraction of the signal supplied to the
inputs
14 and 16. More specifically, the secondary left channel signal reproduced by
the secondary left loudspeaker is the signal supplied to the left input 14
minus
the signal supplied to the right input 16. Similarly, the secondary right
channel
signal reproduced by the secondary right loudspeaker is the signal supplied to
the right input 16 minus the signal supplied to the right input 14. Since the
secondary channels decoding circuit 48 is not connected to the ground, the
secondary left and right channel signals are equal since the subtraction of
the
input 14 ftom the input 16 is equal to the subtraction of the input 16 from
the
input 14. Furthermore, since capacitors 56 and 58 are, in a sense, decoupling
capacitors and since the secondary channels decoding circuit 48 is not
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connected to the ground, the interconnection of the ground connections 35 and
39 will have no ill effect on the signals supplied to the main channels volume
control circuit 46 and to the central channel decoding circuit 50 since the
capacitors 56 and 58 prevent the signal supplied to the input 14 to "bleed" in
the
signal supplied to the input 16 and vice-versa.
By varying the value of the resistor 60, one modifies the
amplitude of the signal supplied to the secondary loudspeakers, thus varying
the volume of the sound reproduced by the secondary loudspeakers, since the
portion of the power amplified signal dissipated as heat in the resistors is
modified.
It has been found advantageous to select the value of the
capaators 56 and 58 so that a high-pass filter of about 100 Hz is created with
respect to the impedance of the secondary loudspeakers.
The central channel decoding circuit 50 includes a first fixed
value resistor 62 connected to the left input 14, a first capacitor 64
connected
to the first resistor 62, a second fixed value resistor 66 connected to the
right
input 16, a second capacitor 68 connected to the second resistor 66 and a
variable resistor 70 interconnecting both capacitors 64 and 68 to the output
42
to the central loudspeaker. The ground connection 43 is connected to the
ground input connection 18.
The signal supplied to the central loudspeaker output 42 is
therefore the addition of the signal supplied to the left and right inputs 14
and
16 by the power amplifier 12.
It is to be noted that since the capacitors 64 and 68 are, in a
sense, decoupling capacitors, the interconnection of the output of these
capaators to cause the addition of the left and right inputs 14 and 16 will
have
no ill effect on the signals supplied to the main channels volume control
circuit
46 and to the secondary channels decoding circuit 48.
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It has been found advantageous to select values of the fixed
resistors 62 and 66 so that each is equivalent to half the nominal impedance
of
the main loudspeakers. It has also been found advantageous to select the
values of the capacitors 64 and 68 so that a high-pass filter of about 100 Hz
is
created with respect to the impedance of the central loudspeaker.
Again, by varying the value of the resistor 70, one modifies the
amplitude of the signal supplied to the central loudspeaker, thus varying the
volume of the sound reproduced by the central loudspeakers, since the portion
of the power amplified signal dissipated as heat in the resistor is modified.
It is to be noted that the variable resistors 60 and 70 could be
advantageously embodied in separate monophonic L-pads (with no ground
connections) and having an appropriate power ratings. Alternatively, the
variable resistors 60 and 70 could also be embodied by a plurality of discrete
high quality resistors (not shown) and switches (not shovm) or by high power
levels variable resistance integrated circuits (not shoHrn).
Turning now to figure 3 of the appended drawings, an alternate
central channel decoding circuit 150 will be briefly described. The major
difference between the decoding circuit 150 and the decoding circuit 50 of
figure 2 consists in the replacement of the variable resistor 70 provided
downstream from the interconnection of the left and right inputs by a pair of
variable resistors 170, 170' respectively provided between the fixed value
resistors 62, 66 and the capacitors 64, 68, thus upstream from the
interconnection of the left and right inputs. The decoding circuit 150
improves
the impedance and frequency stability as well as allowing more usable power
to be supplied to the central loudspeaker.
Again, it is to be noted that the variable resistors 170 and 170'
could be advantageously embodied together in a stereophonic L-pad (with no
ground connection) and having an appropriate power rating. Alternatively, the
variable resistors could also be embodied by a plurality of discrete high
quality
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resistors (not shown) and switches (not shown) or by a high power levels
variable resistance integrated circuit (not shown).
Returning to figure 2, it is to be noted that the impedance of the
post-amplification stereophonic to surround sound decoding circuit 10 "seenp
by the outputs 20, 22 and 24 of the power amplifier 12 is essentially similar
to
the impedance of the main loudspeakers since the decoding circuits 48 and 50
use decoupling capacitors and are therefore virtually "invisible" as far as
impedance is concerned. Indeed, the impedances of the decoding circuits 48
and 50 are so much higher than the impedance of the main loudspeakers 28
and 32 that the connection of the circuits 48 and 50, generally in parallel,
therewith will not modify significantly the impedance seen by the outputs 20,
22
and 24 of the power amplifier.
As can be seen from figure 2, fuses 72, 74 and 76 are provided
between the inputs 14, 16 and 18 and the circuits 46, 48 and 50 as protection.
It is also to be noted that an oNoff switch (not shown) could advantageausiy
be
provided to disconnect the circuits 48 and 50 from the inputs 14, 16 and 18
when the user wants to use only the main loudspeakers and does not want to
hear a surround sound.
It has been found that non-polarized capacitors are required for
the decoding circuits of the present invention. Of course, as will be easily
understood by one skilled in the art, matched pairs of polarized capacitors
could
be substituted.
It is to be noted that even though the circuits 46, 48 and 50 have
been described as separate circuits, these circuits may advantageously be
embodied together onto a printed circuit board (not shown).
As will be understood by one skilled in the art, the circuits 46, 48
and 50 illustrated in figure 2 and described hereinabove are given by way of
example only and could be modified without departing from the s~pe of the
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present invention. It is also to be noted that each circuit includes volume
controls accessible to the user to modify the volume of each of the three
groups
of loudspeakers, i.e., the main loudspeakers, the secondary loudspeakers and
the central loudspeaker. Indeed, it has been found that some users prefer to
hear the surround channels (reproduced by the secondary loudspeakers) and
the central channel (reproduced by the central loudspeaker) at a greater or
lesser volume than is considered optimal. Furthermore, since the decoding
circuit 10 may be connected to a number of different stereophonic power
amplifiers and to a number of different loudspeakers, the separate volume
controls help compensate for these differences. However, a simpler and less
expensive post-amplification stereophonic to surround sound decoding circuit
could be done by removing the variable resistors 52, 54, 60 and 70 (or,
alternatively 170, 170'). Similarly, the fuses 72, 74 and 76 could be removed
from the circuit since they are provided only for overload protection. It will
be
noted that such a simplfied circuit (not shown) would be less versatile since
the
user would have no independent control on the three groups of loudspeakers.
It is to be noted that even tough the post-amplification
stereophonic to surround sound decoding circuit 10 described hereinabove
includes outputs to the main left and main right loudspeakers, it would be
within
the skills of one knowledgeable in the art to design a decoding apparatus (not
shown) not including these outputs if the decoder was intended to be used with
a power amplifier provided with two pairs of left-right outputs. Indeed, one
pair
of outputs could be directly connected to the main left and main right
loudspeakers, while the other pair of outputs could be supplied to the
decoding
apparatus to yield the se~ndary left, secondary right and central outputs as
desc~bed hereinabove. Of course, such a decoding apparatus would be less
versatile since no independent control on the signal supplied to the main
loudspeakers would be provided and since the circuit would be only usable on
the above-mentioned type of power amplifiers or with multiple integrated
amplifters provided with a common floating ground..
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It is also to be noted that the post-amplification stereophonic to
surround sound decoding circuit 10 may advantageously be packaged in an
enclosure provided with adequate connectors for the input and outputs of the
circuit 10 to yield a post-amplification sten~phonic to surround sound
decoding
apparatus. The actuators of the variable resistors 52, 54, 60 and 70 are
advantageously positioned so as to be accessible to the user. Alternatively,
the
decoding circuit 10 could be installed in a conventional power amplifier to
yield
a surround power amplifier without requiring five amplification channels.
It is finally to be noted that the output pairs 34, 35; 38, 39; and 42,
43 could be provided with adequate switching elements (not shown) allowing
the polarity of these output pairs to be inverted. Indeed, selective inversion
of
these output pairs would allow the listener to tailor the surround sound
reproduction.
As will be apparent to one skilled in the art, the post-amplification
stenrophonic to surround sound decoding circuit of the present invention has
many advantage over the stereophonic to surround sound of the prior art, such
as:
the decoding is done downstream from the power amplification, allowing the
user to keep his conventional stereophonic power amplifier;
the decoding circuit is compatible with every conventional consumer and
professional loudspeaker;
the decoding circuit is compatible with every conventional consumer and
professional audio power amplifier;
the decoding circuit allows the user to control his listening experience by
independently varying the volume of the three groups of
loudspeakers;
the decoding is compatible with every known two channels mixed encoding
standards such as, for example, the home theater surround
sound developed by Dolby laboratories, the Pro-Logic' ""
technique and the THXT"' technique developed by Lucas Arts
Entertainment Co.;
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the decoding circuit does not require an additional power source;
the decoding circuit offers a wide bandwidth to the center and surround
channels;
the decoding circuit may easily be designed to allow unusual and complex
loudspeaker impedance load;
the decoding circuit may easily be designed to handle high power signals by
providing components having an adequate power rating;
the decoding circuit may easily be designed for different applications such
as,
for example, multimedia computing, automobile sound systems,
virtual reality applications; and
the decoding circuit may easily be designed with a computer-
controlled interface, to control the separate volumes, for
example, if the particular decoding circuit is intended to be used
by a computer controlled application.
Although the present invention has been described hereinabove
by way of a preferred embodiment thereof, this preferred embodiment can be
modified at will, without departing from the spirit and nature of the subject
invention as defined in the appended claims.
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