Sound encoding system

A signal encoding system for generating at least two encoded signals in response to a plurality of information signals containing audio data representative of virtual images of sources of the data so that the encoded signals can be subsequently decoded by a decoding system including a decoding matrix means for decoding the encoded signals so as to regenerate the information signals. The signal encoding system comprises signal encoding means for generating the encoded signals in response to the information signals; and a duplicate of the decoding matrix means coupled to the encoding means for generating a feedback signal corresponding to each of the information signals; wherein the encoding signals are modified in response to the feedback signals so as to reduce the noise artifacts generated by the decoding matrix when the encoded signals are decoded by the decoding system. A system is also disclosed for modifying the signal gain impressed on an input signal so as to modify the dynamic range of at least a portion of the input signal wherein the system is of the type including operational amplification means for generating an output signal in response to the input signal and feedback means disposed in a feedback path of the operational amplification means for generating a feedback signal for the operational amplification means, wherein the feedback means includes means responsive to the output signal for impressing a gain on the feedback signal in response to and as a function of a control signal, and means for generating the control signal in response to and as a function of the output signal. The improvement comprises: the operational amplification means including means responsive to the input signal and the feedback signal for modifying the signal gain impressed on the input and feedback signals in response to a second control signal; and means for generating the second control signal in response to at least a portion of the feedback signal.

The present invention relates generally to audio signal processing systems, 
and an improved noise reduction system for use in such systems and, more 
particularly, to systems for encoding a plurality of input signals 
(containing audio data relating to virtual sound images) so as to produce 
two encoded information signals adapted to be decoded and substantially 
reproduce the input signals, and an improved noise reduction system 
particularly adapted for use in such signal encoding systems. 
As the term is used herein, "virtual image" means the apparent source of 
sound as psycho-acoustically perceived by the listener that is created by 
the combination of sonic signals generated by the loudspeakers disposed in 
such predisposed patterns in response to the information signals. 
Systems for recording and/or broadcasting two encoded signals 
representative of at least four information signals representative of a 
corresponding number of sources of audio information are well-known. 
Decoder systems can be used on reception or playback to decode the two 
encoded information signals to substantially reproduce the original 
information signals. Such prior art systems include quadraphonic systems. 
Quadraphonic sound systems have been developed based on the fact that 
sufficient audio data can be encoded in the two encoded signals so that 
the latter can be decoded into at least four unique information signals. 
These information signals can in turn be used to drive at least four 
loudspeakers disposed in a predisposed pattern around a listener so as to 
create virtual images. See, for example, U.S. Pat. Nos. 3632886; 3944735; 
and the references cited therein, including, respectively, U.S. Pat. Nos. 
2714633; 2845491; 3067287; 3067292; 3401237; and 3786193; 3794781; 
3798373; 3812295; 3821471; 3856684; 3829615 and 3836715. 
One common system, which has been used extensively in the movie industry, 
is used to encode four information signals respectively representative of 
audio information to be reproduced in the front, right, left and rear 
positions of the proposed listener. Such systems include an encoding 
matrix for encoding the four information signals into two encoded signals 
which are then recorded for the soundtrack of the film. When the audio 
signals are reproduced in the movie theatre, the encoded signals are 
applied to a decode matrix. The latter is the complement of the encode 
matrix so as to substantially reproduce the four information signals. The 
four information signals can then be applied to the appropriate 
loudspeakers positioned at the front, sides and rear of the movie theatre. 
Since the sound track offers a limited dynamic range for the recorded 
signals, it has been suggested that the two encoded signals each can be 
compressed in accordance with known techniques before recording the two 
signals onto a recording track. The recorded signals are then expanded in 
a manner complementary to the compression used in recording before they 
are decoded by the decoder matrix. 
As described in our prior U.S. patent application Ser. No. 581,660, filed 
Feb. 21, 1984, now U.S. Pat. No. 4,589,129, issued May 13, 1986, an 
improved sound reproduction system responsive to these four information 
signals produced in accordance with this standard recording technique can 
be utilized to achieve better sound reproduction. As described in this 
patent, when decoding the two encoded signals with the decoding matrix so 
as to generate the four information signals, better control can be 
achieved by controlling the gain in each output channel with a signal 
transmitted in that channel. Further, by using a signal expander for 
controlling the signal gain of the information signal generated in the 
output channel, greater dynamic range is achieved while improving the 
signal-to-noise ratio of the two input signals. Even better results can be 
achieved where multiband expanders are used to control the signal gain of 
each information signal. 
In accordance with another aspect of the patented invention described in 
our prior U.S. Pat. No. 4,589,129 the system includes more than one 
speaker for creating a surround signal to create greater sound effects 
behind a listener. Specifically, the difference between the left and right 
signals and the difference between the front and rear signals are used to 
generate steering signals to control the amount of signal to the left and 
right rear speakers so as to control the directionality of the virtual 
image. 
While the system described in our prior patent provides excellent sound 
reproduction from signals originally encoded and decoded in accordance 
with the standards noted above, it has been found that while the standard 
technique of encoding the four signals and subsequently decoding the audio 
signals works extremely well for such film formats as 35 mm film, it has 
been found to be inadequate for such film formats as 16 mm film. 
Nevertheless, there is a current trend in the motion picture industry to 
produce film in a 16 mm format since the prints are smaller, i.e., about 
40% the length of an equivalent 35 mm print, and thus less bulky and 
lighter in weight. As a consequence, a feature length film can be 
contained on a single cassette making a single loop film strip for an 
entire feature length motion picture feasible, and remote control of the 
film print more easily obtainable. Moving the 16 mm print at 40% the speed 
of 35 mm, however, poses problems in reproducing sound from the 
soundtrack. The slower speeds makes noise much more noticeable. Impact 
noise, such as noise generated by dirt or scratches on the soundtrack of 
the print, will have a more devastating effect when reproducing sound from 
the slower moving 16 mm film than the corresponding situation when 
reproducing sound from the faster moving 35 mm film. Standard 16 mm 
typically has a signal-to-noise ratio (S/N) of about 35 to 40 dB. It is 
preferable to provide 70 to 80 dB S/N for good signal reproduction. 
Theoretically, a good S/N can be achieve if the encoded signals are 
compressed prior to recording at a 2:1 compression ratio. However, the 
standard process of encoding the original signals in the 35 mm film format 
employs the well-known Dolby A Noise Reduction System, manufactured and 
sold by Dolby Laboratories, Inc. of San Franciscio, Calif. This encoding 
system employs a multi-band compressor constructed by using a standard 
Dolby A decoder in the feedback loop of an operational-type amplifier. 
Such a construction, however, restricts the gain change in each frequency 
band to less than 20 dB. 
If the expander employed in the feedback loop of an operational amplifier 
is constructed to provide a wide dynamic range output with a reasonably 
high expansion ratio so as to provide the gain change required to cover 
the full signal range of 70 to 80 dB, the gain bandwidth problem with both 
the expander and the operational amplifier conspire to make the feedback 
loop impossible to close without either oscillation due to excess phase 
shift at high frequencies occurring when the expander gain is high, or 
loss of high frequency response occurring when the expander gain is low. 
More specifically, wide range linear decibel expanders are constructed with 
voltage or current controlled amplifiers which commonly have a 45.degree. 
excess phase shift at about 1 MHz. Other components used in the feedback 
circuit formed by the expander and operational amplifier cause a total 
excess phase shift of 90.degree. to occur near this same frequency. At 
high levels the expander quite often has a gain well in excess of unity. 
For safe operation within the constraints of the Nyquist stability 
criterion, the gain around the entire loop must be less than unity at any 
frequency where the excess phase shift due to the expander and the 
operational amplifier reaches 180.degree.. The gain-bandwidth product of 
the operational amplifier viewed around the entire feedback loop is, 
therefore, constrained to be not much more than 1 MHz at maximum gain. It 
is common to use a single dominant pole stabilization in operational 
amplifiers. It then follows that the system will have a 1 KHz cutoff 
frequency when the relative gain of the high frequency channel of the 
expander is around -60 dB. This is clearly unacceptable, particularly for 
audio applications. 
As a result, the overall signal compression ratio (over the spectral range 
of the signals) of the Dolby compressor is limited to about 1.3:1, which 
in turn limits the expansion of the decoder to the complementary expansion 
ratio of about 1:1.3. Consequently, employing a multi-band audio 
compressor in the process of encoding the audio information signal which 
allows exact decoding with a simple form of expander is difficult to 
realize when the amount of gain change is large. 
Accordingly, it is a general object of the present invention to provide an 
improved system for processing audio signals which substantially reduces 
or overcomes the above-noted problems. 
Another object of the present invention is to provide an improved system 
for encoding audio information signals so as to provide greater separation 
among the information signals upon reproduction. 
And another object of the present invention is to provide an improved 
system for encoding information signals so as to substantially reduce 
signal artifacts which otherwise tend to occur when decoding the encoded 
signals on reception or playback. 
Still another object of the present invention is to provide an improved 
signal encoding system of the type described adapted to be used in 
recording audio signals for 16 mm film. 
Yet another object of the present invention is to provide an improved 
signal encoding system of the type described providing encoded signals 
having greater S/Ns. 
And still another object of the present invention is to provide an improved 
signal encoding system of the type described providing greater compression 
ratios when encoding information signals. 
These and other objects are achieved by an improved signal encoding system 
for generating at least two encoded signals in response to a plurality of 
information signals containing audio data representative of virtual images 
of sources of the data so that the encoded signals can be subsequently 
decoded by a decoding system including decoding matrix means for decoding 
the encoded signals so as to regenerate the information signals. The 
signal encoding system comprises: 
signal encoding means for generating the encoded signals in response to the 
information signals; and 
a substantial duplicate of the decoding matrix means coupled to the 
encoding means for generating a feedback signal corresponding to each of 
the information signals; 
wherein the encoding signals are modified in response to the feedback 
signals so as to reduce the noise artifacts generated by the decoding 
matrix when the encoded signals are decoded by the decoding system. 
Another aspect of the present invention relates to an improved device for 
providing signal gain change in the form of signal compression at much 
greater overall compression ratios over the entire spectral range of the 
signals during the encoding process which, as will be evident hereinafter, 
has other applications besides its use in the signal encoding system 
described herein. 
Accordingly, another object of the present invention is to provide an 
improved device for providing signal gain changes which substantially 
reduces or overcomes the above-noted problems. 
And another object of the present invention is to provide an improved 
device for providing signal compression at relatively high compression 
ratios which can easily be reproduced using complementary expansion. 
Yet another object of the present invention is to provide an improved 
device of the type including an expander connected in a feedback loop of 
an operational amplifier for providing signal compression at relatively 
large gain changes, without oscillation due to excess phase shift at high 
frequencies when the expander gain is high or loss of high frequency 
response when the expander gain is low. 
Still another object of the present invention is to provide an improved 
device of the type including an expander connected in a feedback loop of 
an operational amplifier for providing signal compression which will not 
cut off the high frequency spectral portions of the information signal 
compressed at relatively large gain settings. 
These latter and other objects are achieved by a system for modifying the 
signal gain impressed on an input signal so as to modify the dynamic range 
of at least a portion of the input signal, wherein the system is of the 
type including operational amplification means for generating an output 
signal in response to the input signal and feedback means disposed in a 
feedback path of the operational amplification means for generating a 
feedback signal for the operational amplification means, wherein the 
feedback means includes means responsive to the output signal for 
impressing a gain on the feedback signal in response to and as a function 
of a control signal, and means for generating the control signal in 
response to and as a function of the output signal. The improvement 
comprises: 
the operational amplification means including means responsive to the input 
signal and the feedback signal for modifying the signal gain impressed on 
the input and feedback signals in response to a second control signal; and 
means for generating the second control signal in response to at least a 
portion of the feedback signal. 
Other objects of the invention will in part be obvious and will in part 
appear hereinafter. The invention, accordingly, comprises the apparatus 
possessing the construction, combination of elements, and arrangement of 
parts which are exemplified in the following detailed disclosure, and the 
scope of the application of which will be indicated in the claims.

In the drawings, the same numerals are used to designate like or similar 
parts. 
The signal decoding system shown is FIG. 1 is of the type shown in our U.S. 
Pat. No. 4,589,129, which is particularly adapted to be used with the 
loudspeaker placement shown in FIG. 2. Generally, the encoded signals are 
applied to the two input terminals 20A and 20B of the decoding matrix 22 
shown in FIG. 1. As described in our patent, the decoding system is 
originally designed for use with signals encoded in accordance with the 
split variable area (SVA) standard used in the motion picture industry. As 
described in our prior patent, matrix 22 is preferably the standard matrix 
for generating the four information signals at the respective output 
terminals 23. The four decoded information signals contain the information 
of the location of the virtual image when reproduced on loudspeakers 
positioned at the four positions relative to the listener 12 depicted in 
FIG. 2, i.e., the front, left, right and rear positions. The decoding 
system shown in FIG. 1 is adapted to utilize the information in these four 
signals to produce additional signals for rear speakers so that different 
sounds can be produced from the left rear and right rear positions of the 
listener in addition to the center rear, when the signal information 
indicates such emphasis. 
Accordingly, as shown in FIG. 2, in addition to the front speaker 10A, left 
speaker 10B, right speaker 10C, and center rear (surround) speaker 10D, 
the left and right surround speakers 10E and 10F are employed. The outputs 
of matrix 22 are respectively connected to the center, left, right, and 
surround channel signal control units 24A, 24B, 24C and 24D. Generally, 
each control unit 24 preferably includes a voltage control amplifier 
(VCA), set for expansion, and at least one level sensor adapted to sense 
the signal level input to that VCA and for generating a control signal to 
the voltage control amplifier for controlling the signal gain impressed on 
the respective channel information signal. The signal gain provided on 
each channel information signal is, thus, a function of the signal level 
of the channel signal and will be expanded as a function of the signal 
level sensed. As is well-known, expansion is a technique wherein very low 
level input signals are attenuated while very large signals are amplified 
so as to increase the dynamic range of the channnel information signal and 
increase the S/N of the signal. 
Preferably, as shown in our U.S. Pat. No. 4,589,129, each control unit 24 
is preferably a three band expander wherein each channel signal has its 
spectral content divided into three bands, a low band, mid-band, and high 
band, separately expanded and subsequently combined. The preferred 
expansion process employed is taught in U.S. Pat. No. 3,789,143 issued to 
David E. Blackmer on Jan. 29, 1974. 
The outputs of units 24A, 24B, 24C are provided to the respective speakers 
10A, 10B, and 10C. The output of the mid-band level sensor of each unit 24 
is provided at the respective level output terminals 26. The mid-band 
outputs of units 24B and 24C are provided to the L-R steering signal 
generator 28A for comparing the mid-band levels of the left and right 
channel signals and for generating a signal as a function of and in 
response to the difference. Similarly, the mid-band outputs of units 24A 
and 24D are provided to the C-S steering signal generator 28B for 
comparing the mid-band levels of the center and surround channel signals 
and for generating a signal as a function of and in response to the 
difference. The outputs of the steering signal generators provide the 
inputs to the surround extender steering generator 30, which in turn 
provides a control signal to the surround extender control unit 32. The 
latter modifies the surround channel signal output from control unit 24D 
so as to provide directionality to and enhance virtual sound images 
produced by the surround loudspeakers 10D, 10E and 10F. The control unit 
32 is, thus, designed to weight the signals applied to the surround 
speakers 10D, 10E and 10F based on the differences between the right and 
left information signals and between the center and surround information 
signals. 
Finally, the center information signal, representative of the L+R 
information, is applied to a detector 34 preferably of the type described 
in U.S. Pat. No. 4,404,427 issued to David E. Blackmer, wherein a level 
sensor is adapted to distinguish information signals which quickly change 
from those that slowly change so as to distinguish between such signals as 
those associated with closed-miked speech, impulse noise, or staccato 
music which are adapted to be reproduced in a localized manner, from those 
associated with background music which are adapted to be reproduced by 
speakers connected to more than one channel. The detector 34 is connected 
to the control buffer 36 for reversing the polarity output of the detector 
34 and applying the resulting signal to a control input terminal of the 
VCA of the center channel signal control signal unit 24C. As described in 
our prior U.S. Pat. No. 4,589,129, the detector 34 is being used for 
stereophonic presentation and utilizes the L+R stereo information in the 
center information channel to turn up (i.e., amplify) the mid-range VCA of 
the control signal unit 24C so as to amplify those signals indicating 
localization in the center channel, such as closed-mike speech. Since 
background information produced on multiple channels is often more 
reverberant than localized information, amplifying these localized signals 
tends to improve the intelligibility of the localized information, such as 
speech, and improve the balance between localized information and the 
background information produced on multiple channels under adverse 
conditions. 
The system described in our prior patent and generally described with 
respect to FIG. 2, provides excellent sound reproduction from signals 
originally encoded on 35 mm film and decoded in accordance with the 
standards noted above. As shown generally in FIG. 3, the encoding process 
includes encoding the four left, center, right and surround information 
signals with an encoding matrix 50 which encodes the signals applied to 
the respective encoding input terminals 48 in a manner well known in the 
art. The encoded signals provided at the output terminals 52 of the matrix 
50 can then be recorded in the recording channel 54. On subsequent 
playback the decoding matrix 22 provides the four information signals at 
the respective output terminals 23, which in turn apply the four signals 
to the FIG. 1 embodiment as described in FIG. 1. 
In accordance with techniques well-known in the art, each encoded signal 
can be compressed with a multi-band compressor 56 prior to recording the 
encoded signal in the recording channel 54 (see FIG. 3). Each compressed 
encoded signal is expanded in a complementary manner with expander 58, 
prior to decoding the encoded signals. The compression-expansion process 
will improve the S/N of the decoded information signals provided at the 
output terminals 23 and enhance the dynamic range of the information 
signals provided to the FIG. 1 playback system. 
It has been found that while the standard technique of encoding the four 
signals and subsequently decoding the audio signals (whether with the FIG. 
1 decoding technique or other techniques known in the art) works 
reasonably well for such film formats as 35 mm film, it has been found to 
be inadequate for such film formats as 16 mm film. 
As previously described, moving the 16 mm print at 40% the speed of 35 mm 
poses problems in reproducing sound from the soundtrack. The slower speed 
results in noise from the print becoming more noticeable. Impact noise, 
for example, generated by dirt or scratches, will have a more devastating 
effect when reproducing sound from the slower moving 16 mm film. Standard 
16 mm typically has a signal-to-noise ratio (S/N) of about 35 to 40 dB. It 
is preferable to provide 70 to 80 dB S/N for good signal reproduction. 
Theoretically, the latter S/N can be achieved if the encoded signals are 
compressed with the compressors 56 prior to recording at a 2:1 compression 
ratio. However, the standard compression techniques currently used in the 
signal encoding systems developed by Dolby Laboratories provides a limited 
overall compression ratio of about 1.3:1. Consequently, it was necessary 
prior to the present invention to limit the expansion ratio of the 
expanders 58 so that the expanders expand the decoded signal by a 
complementary expansion ratio of between about 1:1.3. 
In accordance with the present invention, an improved encoding system is 
provided for achieving the compression ratios desired without concern 
about the limitations imposed by the Dolby Laboratories compressor. 
More specifically, FIG. 4 shows the preferred encoder system. The latter 
includes means for substantially reducing the effects of artifacts 
generated by the decoder matrix 22. In particular, the encoder of FIG. 3 
is modified to include the portions of the decoder of FIG. 1 which 
generate the artifacts, i.e., the decoding matrix 22. 
In FIG. 4, the four information signals L, C, R and S are applied to the 
four respective input terminals 48 of the encoder circuit. Each input 
terminal 48 is applied to the input of a compressor 62, preferably of the 
multi-band type, with three bands being adequate. The preferred embodiment 
of each compressor 62 is described in greater detail with regard to FIG. 
6. Each compressor 62 compresses the corresponding signal received at its 
input at the desired compression ratio which can be chosen to be the same 
for all the compressors 62, or they can vary to provide weighting among 
the different channels so as to enhance the channel separation between the 
various channels and define a more pronounced virtual image. In the 
preferred embodiment, the left and right channels are compressed by the 
same compression ratio of 1.3:1, while the center signal is compressed 
less, at a ratio of 1.2:1, and the surround signal is compressed more, at 
a ratio of 1.5:1. The output of the encode matrix 50A is the encoded 
signals representative of the total left and total right stereophonic 
information L.sub.T and R.sub.T. The matrix 50A preferably is modified 
from the prior art matrix shown in FIG. 3 in a manner described in greater 
detail in FIG. 7. The output of the encode matrix 50A is applied to the 
input of the decode matrix 22, identical to the decode matrix used in the 
FIG. 1 embodiment. The four outputs of the decode matrix are preferably 
respectively connected to the four inputs of control units 24, also 
identical to those used in FIG. 1. 
The outputs of the lowpass, mid-band and highpass filters of each of the 
expanders 24A, 24B, 24C and 24D are connected to the inputs of the level 
sensors of the lowpass, mid-band and highpass bands of the corresponding 
compressors 62A, 62B, 62C and 62D, as described in greater detail in FIG. 
6. The output of the expanders can be connected to a monitor 64 for 
monitoring the recording process. In this regard, the expanders 24 provide 
the complement to compressors 62 by providing the various weighting 
factors to the expansion of each of the signals which are opposite to 
those provided by the compressors. Thus, preferably the left and right 
information signals are expanded by an expansion ratio of 1:1.3, the 
center information signal is expanded by a ratio of 1:1.2, and the 
surround information signals is expanded by a ratio of 1:1.5. This 
expansion is the same as that provided in the decoding units of the type 
shown in FIG. 1, preferably used to decode the signals encoded by the 
encoder system so that the monitor 64 will detect the signals as they will 
appear at the output of the respective expanders of the preferred decoding 
system. The output signals of the encoding system are respectively 
provided at the output terminals 52A and 52B of the encoding matrix 50A, 
where they can be fed directly to the recording channel 54 as shown in 
FIG. 4, or through the two respective compressors 56A and 56B (shown in 
FIG. 5). The latter, in turn, are applied to the recording channel 54, as 
shown in FIG. 5. 
As constructed, the matrix 22 and the lowpass, bandpass and highpass 
filters of the expanders 24 provide a feedback path for the encoding 
signals so as to eliminate or substantially reduce the artifacts which 
would otherwise be produced by the decoding matrix 22 upon playback. In 
this way the recording medium is provided with the two encoded signals 
which have been processed so that artifacts produced during the decoding 
process will be minimized. 
In FIG. 5, the compressors 56 provide additional compression to the encoded 
signals. The encoded signals are expanded in a complementary manner by the 
expanders 58 when decoding. The use of compressors 56, in addition to the 
compressors 62, has a multiplicative effect on the total compression of 
the signals prior to recording. Specifically, although the compressors 56 
are used to compress the encoded signals it has the effect, when combined 
with the effect of the compressors 62, of compressing the left and right 
information signals by a ratio of 1.95:1, the center information signal by 
a ratio of 1.8:1 and the surround information signal by a ratio of 2.25:1. 
This provides the needed compression for recording the audio signals on 
such medium as 16 mm film. The combined expansion provided by the 
expanders 24 and 58 will be the complement of these compression ratios 
upon playback. 
In FIG. 6, the compressor 62 is preferably of the type in which an expander 
90 is placed in the feedback path of an operational amplifier 100. In 
accordance with the present invention, the gain-bandwidth product of the 
operational amplifier 100 is modified to be the inverse of the gain 
control unit used in the high-band portion of the expander 90. The overall 
closed loop frequency response of the circuit in FIG. 6 is, therefore, 
nearly constant in the high frequency band. When this is so, the loop 
stability problem at the lower frequency bands is minimized. 
As shown in FIG. 6, the output of the decode matrix 22 is applied to the 
input of the control unit 24 in the form of a three band expander as 
described in our U.S. Pat. No. 4,589,129. Generally, the expander includes 
the lowpass, bandpass and highpass filters 80a, 80b and 80c, respectively, 
for providing the lowband, midband and highband portions of the 
corresponding signal received from the decoding matrix. The outputs of the 
filters are applied to the the inputs of level sensors 82a, 82b and 82c, 
which in turn have their respective outputs connected to the control 
inputs of gain control units 84a, 84b and 84c. The latter have their 
inputs connected to respectively receive the outputs of the filters 80, 
and their respective outputs connected through the summing amplifier 86 to 
the monitor 64. 
Expander 90, substantially identical to the expander 24, is also utilized, 
wherein the expander 90 is connected in a feedback loop with the 
operational amplifier 100 so as to form the compressor unit 62. The 
expander 90 is identical to the expander 24. The output of the lowpass, 
bandpass and highpass filters 80a, 80b and 80c of the expander 24 are also 
connected to the respective inputs of the level sensor 82d, 82e and 82f of 
expander 90, so that the inputs to sensors 82d, 82e and 82f and gain 
control units 84d, 84e and 84f include both the outputs of the respective 
filters 80a, 80b and 80c and filters 80d, 80e and 80f. The input 102 
terminal of the compressor unit 62 receives the signal from the 
corresponding terminal 48. Input terminal 102 is connected through the 
resistor 104 to the junction 106. Junction 106 is also connected through 
resistor 108 to the output of the expander 90 so as to receive a feedback 
signal therefrom. Junction 106 is also connected to the input of gain 
control unit 110, which in turn has an output connected to the inverting 
input of the operational amplifier 100, the non-inverting input of the 
latter being connected to system ground. The operational amplifier 100 has 
a resistor 112 and capacitor 114 connected in series with one another and 
in a feedback loop between the output and inverting input of the 
amplifier. 
The output of the amplifier 100 is connected to the output 116 of the 
compressor unit, which in turn, as shown in FIGS. 4 and 5, is connected to 
the input of the encoding matrix 50A. The output of the operational 
amplifier 100 is connected through resistor 118 to the input of an 
inverting amplifier 120. Inverting amplifier 120 includes a feedback 
resistor 122 and a feedback capacitor 124. The output of inverting 
amplifier 116 is applied to the input 126 of the expander 90 so as to 
complete that feedback loop. The output of the amplifier 100 is also 
connected through resistor 128 to the inverting amplifier 130 having a 
feedback capacitor 132. The output of amplifier 130 is connected through 
resistor 134 to the junction 106 so as to provide DC stability in the loop 
formed by the operational amplifier 100 and the expander 90. 
The gain control unit 110 is set for compression and has its control input 
connected to receive the output signal from the high frequency level 
sensor 82f so that the latter controls the gain of the unit, as well as 
the stability of the loop. In operation, the expander 90 in the feedback 
loop of the operational amplifier 100 will produce a variable 
gain-bandwidth product which exactly complements the expander 24, while 
maintaining stability under all gain settings possible in the operation of 
the expander 24. 
The feedback technique described in FIG. 6 in which the gain control unit 
110 is disposed at the input of the amplifier 100 and the expander 90 
disposed in the feedback path of the amplifier can also be used for signal 
expansion. In such a case, the expander 90 is replaced with a compressor, 
preferably of the multi-band type. The gain control unit 110 will, 
however, be set for signal expansion. Further, the technique of using the 
gain control unit 110 at the input of the amplifier 100 can be employed in 
any circuit in which there are gain changing devices disposed in the 
feedback loop whether the end circuit is used for compression or expansion 
of substantially the entire input signal, or compression or expansion of a 
portion of the input signal, as, for example, in adaptive filters, such as 
described in U.S. Pat. No. 4,101,849, issued to David E. Blackmer et al., 
on July 18, 1978, or any other type of device utilizing a compressor or 
expander in a feedback loop around an operational amplifier. 
As shown in FIG. 7, the encoding circuit includes the input terminals 140A, 
140B, 140C and 140D for receiving the respective left, center, surround 
and right information signals. The input terminals are connected through 
the respective resistors 142A, 142B, 142C, and 142D to the corresponding 
buffers 144A, 144B, 144C and 144D. Buffers 144A and 144D have their 
respective outputs connected through respective resistors 145A and 145F to 
the corresponding inputs of summing amplifiers 146A and 146B. The output 
of amplifiers 146A and 146B are respectively connected to terminals 148A 
and 148B for providing the L.sub.T and R.sub.T encoded signals, 
respectively. Buffer 144B has its output connected through the resistors 
150A and 150B to the signal inputs of VCAs 152A and 152B. Buffer 144C has 
its output connected through the resistor 150C to the signal input of VCA 
152C and through signal inverter 151, through resistor 150D to the signal 
input of VCA 152D. The outputs of VCAs 152A and 152C are connected through 
resistors 145B and 145D to an input of the summing amplifier 146A. The 
outputs of VCAs 152B and 152D are connected through resistors 145C and 
145E to the summing amplifier 146B. VCAs 152A, 152B, 152C and 152D are 
preferably for setting the gain of the signal and selectively controlled 
by controls on the monitor 64. 
The control monitors 64 are suitably connected to a source of DC voltage 
158 and provide a control of the amount of relative weighting of the 
center and surround signals provided to each encoded signal so as to 
improve channel separation. This is preferred, since the listener 12 will 
be more influenced by the directionality provided by the surround speakers 
10D, 10E and 10F that are provided by speakers 10A, 10B and 10C. Further, 
by using the controls the operator monitoring the signals can simulate 
panning wherein a signal which is present on the center or surround input, 
can be made to appear to emanate from one side, wherein one pair of VCAs 
152A and 152C or 152B and 152D are set to provide maximum transmission 
while the other two provide minimum transmission. As the operator 
increases the DC control signals to one set of VCAs while decreasing the 
other set, the actual sound when reproduced will appear to pan from one 
side to the other. Obviously, one pair of VCAs 152A, 152B or 152C, 152D 
can be omitted where control is desired in only one of the channels. 
The foregoing encoding system provides greater separation among the 
information signals upon reproduction of the information signals because 
of the different compression and expansion factors used for each 
information signal in the FIG. 4 embodiment and for each information and 
encoded signal in the FIG. 5 embodiment. By using the decoding matrix 22 
in the feedback path of the compressor during the encoding process, the 
artifacts generated in the decoder will be substantially eliminated from 
the output signals from the expanders 24. The encoded signals provided to 
the recording channel in both FIGS. 4 and 5 will be provided with a 
greater S/N at greater compression ratios so that the encoding system can 
easily be used to process such noisy signals as audio signals generated 
with 16 mm film. Finally, by providing the VCAs 152 in the encoding matrix 
in FIG. 7, the front and/or rear surround signals can be selectively 
weighted so as to effect the left-right directionality of the virtual 
image on playback. 
In addition, several advantages are achieved by the gain control techniques 
embodied in the compressor shown in FIG. 6. In particular, relatively high 
compression ratios can easily be produced using complementary expansion. 
The compressor of the type including the expander 90 connected in a 
feedback loop of operational amplifier 100 provides signal compression at 
relatively large gain changes, without oscillation due to excess phase 
shift at high frequencies when the expander gain is high or loss of high 
frequency response when the expander gain is low. Such a compressor 
provides signal compression which will not cut off the high frequency 
spectral portions of the information signal compressed at relatively large 
gain settings. 
Since certain changes may be made in the above apparatus without departing 
from the scope of the invention herein involved, it is intended that all 
matter contained in the above description or shown in the accompanying 
drawings shall be interpreted in an illustrative and not in a limiting 
sense.