9L D901.
USOO5333201A
United States Patent [191
Waller, Jr.
[54]
[75]
' [11]
Patent Number:
5,333,201
[45]
Date of Patent:
Jul. 26, 1994
‘
MULTI DIMENSIONAL SOUND CIRCUIT
Inventor: James K. Waller, Jr., Lake Orion,
Mich.
[57]
ABSTRACT
An audio sound system decodes from non-encoded
two'channel stereo into at least-four channel sound. The
rear channel information is derived by taking a differ
ence of left minus right and dividing that difference into
a plurality of bands. In a simplistic implementation, at
least one band is dynamically steered while the other
[73] Assignee: Rocktron Corporation, Rochester
Hills, Mich.
[21] Appl. N0.: 4,591
Jan. 14, 1993
[22] Filed:
band is unaltered so as to avoid any perceived pumping
effects while providing transient information to left/
Related US. Application Data
[63]
right, as well as directional enhancement. In a preferred
Continuation-in-part of Ser. No. 975,612, Nov. 12,
1992.
[5 1l
[52]
[58]
Int. Cl..5 .............................................. .. H04S 3/00
US.
Cl.
.. . . .. .. . . . .
. . . . . . . . . . . . . . . . . . . ..
381/22
Field of Search ..................... .. 381/17, 18, 22, 23,
381/21
embodiment, multiple bands are dynamically steered
left or right, so as to enhance directional information to
the rear of the listener. In both schemes, the low pass
?ltered output of the sum of the left and right inputs is
also combined with the directionally enhanced informa
tion, so as to provide a composite left rear and right rear
References Cited
output. Furthermore, the center channel information
does not necessarily require a discrete loudspeaker, and
U.S. PATENT DOCUMENTS
can be divided so that low frequency information can be
[56]
applied to the rear channels while mid and high fre
4,680,796
7/1987
Blackmer et a1. ................... .. 381/23
5,216,718
6/1993
Fukuda
. .. ..... .. ......
. . , ..
381/22
Primary Examiner-Forester W. Isen
Attorney, Agent, or Firm—Catalano, Zingerman &
quency information from the center channel can be
applied to the front left and right channels to compen
sate for any perceived loss of center information.
McKay
20 Claims, 11 Drawing Sheets
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July 26, 1994
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July 26, 1994
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5,333,201
1
5,333,201
2
MULTI DIMENSIONAL SOUND CIRCUIT
acoustic illusion of four channel sound due to the fact
that the human car has a different frequency response to
CROSS-REFERENCE TO RELATED
APPLICATIONS
directed from the rear. For this reason, the current
signals directed from the front than it has to signals
four-speaker stereo system used in automotive applica
tions sounds much more desirable than attempting to
adapt a current surround system, such as Dolby’s Pro
Logic TM , to automotive applications. Furthermore,
there are some major drawbacks to adapting a system
Jr. and Derek F. Bowers.
10 such as Dolby’s. Since only difference information
BACKGROUND OF THE INVENTION
would be fed to the rear speakers, the rear channel
The present invention relates generally to audio
would have a bandwidth of only 7 KHz, and it would be
sound systems and more speci?cally concerns audio
mono in that there would be no directional information
sound systems which decode from two-channel stereo
perceived to the rear of the listener. As a result, in
into at least four channel sound, commonly referred to
comparing adapted Dolby Pro Logic TM with conven
This application is a continuation-in-part of applica
tion Ser. No. 07/975,612, ?led Nov. 12, 1992, for Multi
Dimensional Sound Circuit, inventors James K. Waller,
as “surround” sound.
Surround systems generally encode four discrete
tional four-speaker stereo, many listeners would prefer
the sound imaging of the conventional four-speaker
channel signals into a stereo signal which can be de
stereo system.
coded through a matrix scheme into the discrete four
The majority of the steering schemes devised to en
channel signals. These four decoded signals are then 20 hance directional information have been designed to
played back through loudspeakers con?gured around
enhance the normal left, right, center and surround
the listener as front, left, right and rear. This principle
information in a similar fashion to the Dolby Pro Lo
was adopted originally by Peter Scheiber in US Pat.
gic TM system. For example, using a scheme such as
No. 3,632,886 speci?cally for audio applications, and
that disclosed by Peter Scheiber, to further enhance
25
the method of encoding four discrete signals into two
and then decoding back into four at playback has be
come commonly known as “quadraphonic” sound.
Scheiber’s original surround system produces only lim
ited separation between adjacent channels and therefore
requires additional dynamic steering to enhance direc
tional information. The basic principle has been applied
directional imaging from a signal previously encoded,
David E. Blackaner, in U.S. Pat. No. 4,589,129, pro
vides a discrete rear left, right and center surround
channel system. This system is further enhanced for
encoding aspects in US. ‘Pat. No. 4,680,796 which was
also devised speci?cally for video applications. In US.
Pat. No. 4,589,129, a very elaborate compression/ex
pansion scheme for encode and decode is disclosed for
the purpose of providing noise reduction. However, a
commonly known as Dolby Stereo TM. The front-cen
major
drawback is encountered in this scheme in that
ter speaker is designed to be positioned behind the 35
the directional steering process is performed broadband
movie screen for the purpose of localizing dialogue
and, in the event that predominant steering information
speci?cally from the movie screen. The front-left and
is present, objectionable pumping effects are perceived
front-right channels provide effects, while the rear or
by the listener. This system also has little serious impact
surround channel provides both ambient information as
Well as sound effects. The Dolby Pro Logic TM system, 40 in high quality audio applications, due to the fact that
the left and right surround information is processed
a Dolby Stereo TM system adapted for home use, uses a
through comb ?lters. Should a signal be processed by
tremendous amount of dynamic steering to further en
the left or right surround channels, where the funda
hance channel separation, and is very effective in local»
mental frequency of that signal falls into the notch of
izing signals at any of the four channels as an indepen
dent signal. The Dolby system, however, provides lim 45 one of these comb ?lters, it would reduce any impact of
that signal appearing at the left or right output. More
ited channel separation with composite simultaneous
over, the comb ?lters will destroy any possibility for
signals.
very successfully in cinematic applications, con?gured
in front-left, front-center, front-right and rear surround,
Although highly effective for audio/video applica
tions, the Dolby Pro Logic TM system is not the most
desirable for exclusive audio applications. The rear
side imaging from a system in which a common signal
appears at the front and rear of either side, as the rear
signal will no longer have the same phase characteris
tics as the front signal. In addition, if the comb ?lter is
generated with time delays, it would not have the same
provide an acceptable amount of low frequency infor
time domain aspects.
mation. The mono center channel, while perfectly
An additional drawback to this system is that it does
suited for dialogue in theater applications, is not desir
able for exclusive audio. The center channel has the 55 not lend itself to automotive applications because the
1 surround information is generated strictly by the differ
effect of producing a very mono front image.
ence from left and right and there is typically no low
It is desirable to provide a multi-channel scheme
frequency energy present in the difference information
which can produce four directional channels of infor
signal. In automotive sound systems, the majority of the
mation designed speci?cally for high quality audio ap
plications. It is also desirable that the system have the 60 bass is derived from the rear channels because the rear
surround channel is limited to 7 KHz, and it does not
capability to generate its four directional signals di
speakers are typically larger and the acoustic cavity in
rectly from a standard two-channel stereo recording,
which the speakers are enclosed can typically be much
therefore eliminating any requirement for encoding.
larger and thus provide better bass response.
One of the most desirable applications for a system
With the success of Dolby Pro Logic TM , which has
such as this would be automotive sound, con?gured as 65 become a standard feature on commercial audio/video
left/right front, and left/right rear. Current automotive
receivers, many manufacturers have attempted to pro
audio systems send the same left/right information to
the rear as is fed to the front. This produces a psycho
cally applied to audio. In particular, these schemes have
vide additional surround schemes that can be speci?
3
5,333,201
added arti?cial delays and/or ambient information to
4
bands. In a simplistic implementation, at least one band
is dynamically steered while the other band is unaltered
so as to avoid any perceived pumping effects while
providing transient information to left/right, as well as
directional enhancement. In a preferred embodiment,
multiple bands are dynamically steered left or right, so
the rear of the listener. More sophisticated and elabo
rate systems have been devised and implemented in
which the signal is processed through DSP or Digital
Signal Processing. Virtually all the attempts made in
DSP have also included the addition of arti?cial rever
beration and/or discrete delays to the rear speakers.
The addition of information not present in the source
signal is not desirable, as the music that is then per
as to enhance directional information to the rear of the
listener. In both schemes, the low pass ?ltered output of
the sum of the left and right inputs is also combined
with the directionally enhanced information, so as to
provide a composite left rear and right rear output.
In virtually all of the prior art surround systems,
ceived no longer accurately re?ects its original in
tended sound.
While DSP holds much promise for the future, it is a
very expensive system by today’s standard and it is
center channel information, which is derived as a left
desirable to provide a system that could be integrated,
plus right signal from the decoding matrix, is applied as
incorporating the advantages disclosed, for perhaps
a separate and discrete channel. This results in a per
one-tenth of the cost of such a system implemented in
DSP.
ceived loss of center information because center infor
mation is distributed equally to all four channels in a
In light of the prior art, and the drawbacks of at
conventional four-speaker system. In a preferred em
tempting to adapt any of the prior art systems speci?
cally to automotive applications, it is a primary object 20 bodiment of the present invention, this center channel
information does not necessarily require a discrete loud
of the present invention to provide four-channel sound
speaker, and can be divided so that low frequency infor
which greatly enhances the conventional four-speaker
mation
can be applied to the rear channels while mid
stereo system commonly used in auto sound systems. It
and
high
frequency information from the center chan
is also an object of the present invention to achieve a
nel
can
be
applied to the front left and right channels to
system that requires decode-only for use in high quality 25
compensate
for a perceived loss of center information.
audio sound systems which receives an input from a
conventional stereo signal, thus allowing for compati
BRIEF DESCRIPTION OF THE DRAWINGS
bility with all stereo recorded material, and decodes
Other
objects and advantages of the invention will
from this two-channel stereo signal an audio sound
become apparent upon reading the following detailed
description and upon reference to the drawings in
system incorporating at least four speakers located
left/right front and left/right rear. In particular, it is
desirable to be able to improve the ambient perceived to
the rear of the listener. It is also an object to provide
rear directional information without the necessity of
which:
without encountering the objectionable pumping per
the multi-band level sensor of FIG. 3;
FIG. 1 is a partial block/partial schematic diagram of
a simplistic implementation of the invention;
adding any arti?cial information such as delays, reverb, 35 FIG. 2 is a partial block/partial schematic diagram of
the steering signal generator of FIG. 1;
phase correction or harmonics generation that is not
FIG. 3 is a partial block/partial schematic diagram of
already present in the original source material. It is also
a three-band implementation of the present invention;
desirable to provide steering aspects to further enhance
FIG. 4 is a partial block/partial schematic diagram of
left/right directional imaging to the rear of the listener
FIG. Sis a partial block/partial schematic diagram of
another embodiment of the invention incorporating
further enhancements for improving decoded localiza
tion of audio signals;
provide discrete left/right imaging to the rear without 45 FIG. 6 is a partial block/partial schematic diagram of
a phase coherent implementation of the invention;
the necessity of providing comb ?lters disposed at the
FIG. 7 is a partial block/partial schematic diagram of
audio path, due to the fact that comb ?lters do not
an alternative phase coherent implementation of the
provide results considered to be musically pleasing in
ceived with a single-band system. Furthermore, it is an
object to provide emphasis to one side for directional
enhancement while providing an increased amount of
de-emphasis to the other side. It is also an object to
high quality audio applications. It is another object of
the invention to provide the possibility of localizing
simultaneous images to the rear speakers, i.e. a given
signal can be perceived as coming from the left while
invention; and
50
FIG. 8 is a partial block/partial schematic diagram of
yet another phase coherent implementation of the in
vention;
another signal is simultaneously coming from the right.
FIG. 9 is a graph illustrating the frequency response
Another object of the present invention is to provide
curve of an embodiment of the invention more sensitive
suf?cient bass information to the rear speakers of the 55 to high than mid frequency information;
auto sound system since the majority of the bass deliv
FIG. 10 is a partial block/partial schematic diagram
ered in automotive sound is generated from the rear. A
of an embodiment of the invention utilizing the fre
further object of the invention is to de?ne a system that
quency response of FIG. 9; and
can also lend itself to future DSP applications that can
FIG. 11 is a partial block/partial schematic diagram
further enhance the basic concept of the present inven 60 of a split band embodiment of the invention utilizing the
tion.
frequency response of FIG. 9.
While the invention will be described in connection
' SUMMARY OF THE INVENTION
with a preferred embodiment, it will be understood that
In accordance with the invention, an audio sound
it is not intended to limit the invention to that embodi
system decodes from non-encoded two-channel stereo 65 ment. On the contrary, it is intended to cover all alter
into at least four channel sound. The rear channel infor
natives, modi?cations and equivalents as may be in
mation is derived by taking a difference of left minus
cluded within the spirit and scope of the invention as
right and dividing that difference into a plurality of
de?ned by the appended claims.
5
5,333,201
6
as to provide additional perceived localization to the
rear and side of the listener. The ?xed localization EQ
23 provides a frequency response to simulate the fre
quency response of the human ear responding to sound
DETAILED DESCRIPTION
Referring ?rst to FIG. 1, normal left/right stereo
information is applied to the left/right inputs 9L and
9R. The left and right input signals are buffered by
buffer ampli?ers 10L and 10R, providing a buffered
signal to drive the rest of the circuitry. These buffered
outputs are applied directly to summing ampli?ers 11L
and 11R which feed the majority of the composite sig
nal to the front left and right outputs 12L and 12R. The
outputs from the buffer ampli?ers 10L and 10R are also
fed to a summing ampli?er 20 which sums the left-and
right signals to provide an output which is further pro
cessed by a high pass ?lter 21 and fed to the summing
ampli?ers 11L and 11R which provide the additional
information for the front left and right channels. The
addition of the sum ?ltered signal is helpful in automo
tive applications to compensate for the decrease in cen
ter channel information due to the fact that primarily
from either side of the listener. Many studies have been
done in the area of interaural differences, and these
studies have been documented in publications such as
“The Audio Engineering Handbook” (Chapter 1:
“Principles of Sound and Hearing”) and “Audio” Mag
azine (“Frequency Contouring for Image Enhance
ment”, February, 1985). While in operation the left and
right rear speakers of the invention should be located
behind the listener, additional separation between the
front and rear channels can be achieved by the inclusion
of the ?xed localization EQ 23. The circuit of the EQ 23
would provide a frequency response approximating that
of the frequency response from either 90° or 135°. The
design of active ?lters is commonly known, and anyone
possessing normal skill in the art could design a ?lter
difference information is fed to the rear channels, al 20 with the frequency response characteristics described.
though adding the sum ?ltered signal may not be neces
The ?xed localization EQ 23 can additionally be used to
sary in some applications. It may even be desirable to
correct frequency response characteristics of a particu
feed unaltered left/right signal information to the front
lar vehicle or listening environment. While the addition
channels.
of a ?xed equalization circuit such as this can provide
The outputs from input buffers 10L and 10R~ are also 25 bene?ts for many applications, it is not necessary that it
applied to a differential ampli?er 30, which provides the
be included to achieve the desired objects of the inven
difference between the left and right signals at its out
tion.
put. The left and right buffered outputs of ampli?ers
The output of the ?xed localization EQ 23 is then fed
10L and 10R are also applied to high pass ?lters 13L
to a high pass ?lter 31 and a low pass ?lter 32 for divid
and 13R, respectively, for removing the bass content 30 ing the audio spectrum into two bands. The low band
from the buffered left and right input signals. This is
portion at the output of the low pass ?lter 32 is applied
preferred so that any steering information is derived
directly to summing ampli?ers 40L and 40R; The out
strictly from mid band and high band information pres
ent in the left and right signals.
The outputs of the high pass ?lters 13L and 13R are 35
then fed to level sensors 14L and 14R, respectively,
which, preferably, provide the log of the absolute value
of the ?ltered outputs from the sensors 13L and 13R,
and provide substantially a DC signal at the outputs of
put of the high pass ?lter 31, which contains substan
tially upper mid band and high band information, is
applied to the VCAs 34R and 35L, which control the
gain of the high band signal for the right and left out
puts, respectively. The outputs of the VCAs 34R and
35L are then applied to summing ampli?ers 40R and
40L, respectively. The VCAs 34R and 35L are func
the sensors 14L and 14R. The DC outputs from the 40 tional blocks of Rocktron’s integrated circuit
sensors 14L and 14R are applied to a difference ampli
HUSH TM 2050. Voltage-controlled ampli?ers are
?er 50. The output of the difference ampli?er 50 will be
commonly known and used, and many alternatives may
substantially proportional to the logarithm of the ratio
be used for the VCAs 34L and 35R.
of the amplitudes of the mid and high band information
The output of the summing ampli?er 20, after being
of the left and right signals. Other level sensing meth 45 processed by a low pass ?lter 22, is applied to the sum
ods, such as peak or averaging, are known and can be
ming ampli?er 40L and an ampli?er 41R for providing
used in place of that which is disclosed, although per
bass response of the summed channels to the rear left
haps with less than optimal results. With a dominant
and right outputs 43L and 46R, respectively.
energy level in the left band, the output of the differen
A level sensor 42 receives the output from the high
tial ampli?er 50 will be positive. With a dominant en 50 pass ?lter 31 and is con?gured so as to provide an in
ergy level in the right band, the output of differential
crease in DC voltage at the output of the level sensor 42
ampli?er 50 will be negative. The level sensors 14R and
14L have been set up with a relatively fast time con
stant, so as to provide very accurate instantaneous left/
when the signal energy at the output of the high pass
?lter 31 drops below —40 dBu, where OdBu=0.775
VRMS. The level sensor 42 provides noise reduction
right steering information at the output of the difference
ampli?er 50. A more moderate time constant is applied
in the steering generator 60 and will be discussed in
greater detail in relating to FIG. 2. The output signal
from the differential ampli?er 50 is applied to the steer
ing signal generator 60, which then decodes from this
55 aspects for the invention which are desirable due to the
difference signal the DC steering signal required to
control the voltage-controlled ampli?ers 34R and 35L
disposed in the signal path for the left and right rear
gain reduction or low-level downward expansion for
the VCAs 34R and 35L and noise reduction aspects are
channels as will be hereinafter explained.
fact that, in operation, the boosted difference informa
tion fed to the rear channels typically contains much of
the high frequency information present in the audio
signal. This would, therefore, increase the noise per
ceived by the listener. Thus the level sensor 42 provides
provided.
Referring to FIG. 2, the steering signal generator 60
The output of the differential ampli?er 30, which 65 receives the substantially-DC output level from the
contains the audio difference information of left-minus
differential ampli?er 50. The output from the differen
right, is fed through a ?xed localization EQ 23. This
tial ampli?er 50 is applied to an inverting ampli?er 61
?xed localization EQ 23 further enhances the system so
and a diode 62L. The output of the inverting ampli?er
7
5,333,201
8
61 will provide a signal of opposite polarity to that of
the difference ampli?er 50, so that when the left channel
74R to control the right limiter 73R which controls the
has a dominant signal energy, the output of the invert
nel through the right summing ampli?er 40R. The op
posite is true if signal energy is dominant in the right
channel, as the voltage at the output of the right level
sensor 14R goes positive, causing the output of the
differential ampli?er 50 to go negative and invert
through the inverting amplifier 61. The right diode 65R
then becomes conductive and the output of the right
buffer ampli?er 70R becomes positive. The maximum
amount of gain is determined by the right limiter 73R,
and this DC voltage is applied to the control port of the
right VCA 34R, which then turns up the right rear
ing ampli?er 61 will go negative. When the right chan
nel has a dominant signal energy, the output of the
inverting ampli?er 61 will go positive. The output of
the inverting ampli?er 61 is applied to another diode
65R. Thus diodes 62L and 65R provide peak detection
from the output of the differential ampli?er 50 and the
inverting ampli?er 61, so as to provide a positive-going
voltage at the cathode of the ?rst diode 62L when there
is a predominant signal energy in the left channel, and a
positive-going voltage at the cathode of the other diode
65R when there is a predominant right channel signal.
Capacitors 63 and 66 provide ?ltering, and resistors 64
and 67 provide release characteristics for the positive
peak detectors. The time constant of the steering de
coder is typically at least two times that of the time
constants in the level sensors 14R and 14L so as to avoid
right VCA 34R so as to turn down the right rear chan
channel through the right summing ampli?er 40R. The
output of the right summing ampli?er 40R is then in
verted via the inverting ampli?er 41R so as to maintain
phase coherency between the left front and left rear
channels, as well as between the right front and right
rear channels. This coherency allows the system to
any jittering or pumping effects in the decoded-direc 20 preserve the possibility for side-imaging.
tional signal. Buffer ampli?ers 69L and 70R provide
isolation for the peak detectors and output drive to
drive the additional steering circuitry. The output of
Conversely, the positive output of the right buffer
ampli?er 70R is inverted through the right inverting
amplifier 71R. This negative-going voltage is applied to
one buffer ampli?er 69L will provide a positive-going
the left limiter 72L to control the left VCA 35L through
DC voltage with a predominant left channel signal, and 25 a resistor 77, and turns down the left channel. Because
the output of the other buffer ampli?er 70R will provide
the output of the differential ampli?er 50 is negative in
a positive-going DC voltage with a predominant right
this case, the left diode 62L is not conductive. While the
channel signal. The outputs of the buffer ampli?ers 69L
gain of the VCAs 34R and 35L is limited to between 3
and 70R are applied to limiters 72L and 73R, respec
and 18 dB, the de-emphasis provided to the opposite
tively, for limiting the maximum voltage possible to 30 channel is typically 15 to 30 dB.
drive the voltage-controlled ampli?ers 34R and 35L.
Due to the fact that the difference signal contains the
The limiters 72L and 73R are contained internally to the
majority of spacial information, rear ambience is greatly
HUSH 2050 IC as expander control ampli?ers which
enhanced for a more natural perception by the listener.
provide an output voltage in one quadrant. These am
Also, due to the fact that the difference information that
pli?ers are designed to only swing positive and to satu 35 is dynamically steered through the VCAs 34R and 35L
rate at zero volts DC. The circuitry is con?gured such
is only upper mid and high frequency information pro
that the limiters 72L and 73R will hit maximum nega
cessed by the high pass ?lter 31, and the lower mid band
tive swing or zero volts DC at the desired point, provid
information that is passed through low pass ?lter 32 is
ing the maximum gain desired for the VCAs 34R and
unaltered, there will be perceived directional informa
35L. In practice, the limiters 72L and 73R will limit,
tion from the rear of the listener. The system provides
between 3 and 18 dB, the maximum output gain from
an extremely fast attack time so as to allow enhance
the VCAs 34R and 35L. The outputs of the limiters 72L
ment of transient information. However, there will not
and 73R are connected to the control ports of the VCAs
be a perceived pumping effect, due to the fact that the
35L and 34R, respectively, and through resistors 74R
steering is not achieved by broadband means. The
and 75L. The output of the ?rst buffer ampli?er 69L is 45 lower midband signal contains less directional informa
also inverted by an inverting ampli?er 68L and cross
coupled through the resistor 74R to the right channel’s
limiter/control ampli?er 73R so as to provide gain re
tion and, therefore, does not require steering for subjec
tively excellent results.
A control line SA provides a DC voltage simulta
neously to parallel resistors 78L and 79R, which in turn
50
feed the negative inputs to the limiters 72L and 73R,
of the buffer ampli?er 70R so as to provide a negative
respectively, and provide DC control for the VCAs
going voltage and reduce the gain at the right VCA 34R
34R and 35L through right and left control lines SR and
and de-emphasize the signal energy that is being empha
SL. This is a means of providing high band noise reduc
sized by the left VCA 35L. In operation, should there
tion when the signal level at the output of the high pass
be a predominant high frequency energy in the left 55 ?lter 31 drops below approximately —40 dBu. The
channel, the DC voltage at the output of the left level
values for the components shown in FIG. 2 are dis
sensor 14L will be larger than the DC voltage at the
closed in Table 1.
output of the right level sensor 13R. Therefore, the
TABLE 1
output of the differential ampli?er 50 will be positive
duction to the signal applied to the right channel. Con
versely, the inverting ampli?er 71R inverts the output
going and the output of the left buffer ampli?er 69L will 60
be positive-going, which will provide gain based on the
amplitude difference between left and right. The left
limiter 72L will determine the maximum amount of gain
provided by the left VCA 35L, so as to turn up the left
rear channel through the left summing ampli?er 40L. 65
However, when the left buffer ampli?er 69L is positive,
the left inverting ampli?er 68L goes negative and ap
plies a negative-going DC signal through the resistor
61
62 L
LF 353
1N4l48
63
.47 pf
64
65 R
470 K9.
1N4l48
66
.47 pf
67
68 L
69 L
70 R
71 R
470 K!)
LF 353
LF 353
LF 353
LF 353
5,333,201
9
-
TABLE l-continued
72 L
73 R
HUSH 2050 TM
HUSH 2050 TM
74
75
76
77
78
79
81
82
83
84
85
86
87
88
39 K!)
43 K9
43 K0
39 Kit
43 KS2
43 K0.
20 K0.
20 K0.
20 K0.
20 KO.
20 K!)
20 Kit
20 K!)
20 K0
L
R
L
L
R
R
10
low pass ?lter 22 is fed to the sun, ming ampli?ers 40R
and 40L to provide bass information to the rear chan
nels. This low frequency information also assists in
preventing any perceived image-wandering in the rear
channels, as well as pumping affects that can occur
when steering broadband signals.
FIG. 8 discloses yet another embodiment of the in
vention having another mearis of providing low fre
quency information to the rear channels. Common
block numbers are used where common functions are
performed. In this embodiment, the buffered outputs of
the buffer ampli?ers 10L and 10R are individually fed
to low pass ?lters 22L and 22R, respectively, and fed
directly to the summing ampli?ers 40L and 40R. Low
15
pass ?ltering the individual buffered inputs maintains
stereo separation of the rear channel bass content. A
Now referring to FIG. 6, another embodiment of the
invention is illustrated which offers improvements for
further improvement is gained by raising the comer
frequency of the low pass ?lters 22L and 22R to include
lower mid band information. This will increase the
listener perception of this stereo separation, as well as
assist in preventing any perceived image-wandering or
pumping effects in the rear channels.
Referring now to FIG. 3, a more elaborate implemen
tation of the invention than that shown in FIG. 1 is
rear center imaging in that the rear channels are phase
coherent, i.e. not out of phase. To compensate for the
phase error that would take place between the right rear
and the right front, all-pass phase circuits are inserted.
One all-pass phase circuit 27 shifts the phase of the
difference information at the output of the ?xed local
ization EQ 23, and provides a phase-shifted signal that is 25 disclosed. Block numbers common to FIG. 1 are used
then applied to both the left and right rear outputs 43L
where common functions are performed.
and 43R. All-pass ?lters 26L and 26R shift the phase of
Left and right inputs 9L and 9R, respectively, are
the front left and right channels such that the difference
buffered by the buffer ampli?ers 10L and 10R. Sum
between the left front 12L and left rear 43L outputs will
ming ampli?ers 11L and 11R receive the buffered out
be 90° and the difference between the right front 12R 30 puts from the buffer ampli?ers 10L and 10R. The left/
and right rear 43R outputs will also be 90° . This com
right summing ampli?er 20 also receives the outputs
pensates for the 180° phase shift that would be present at
from the buffer ampli?ers 10L and 10R and provides
the right rear output 43R without the phase inversion
the sum of left-plus-right. The summed signal from this
derived by the ampli?er 41R shown in FIG. 1. In this
summing ampli?er 20 is ?ltered through the high pass
embodiment of the invention, due to the fact that the 35 ?lter 21 and summed with the buffered left/right chan
rear right and left channels are 100% phase coherent,
nel information by summing ampli?ers 11L and 11R to
rear center stability is greatly improved. All pass phase
provide composite left-front 12L and right-front 12R
circuits such as those disclosed in FIG. 6 are commonly
outputs. The outputs from the buffer ampli?ers 10L and
known in the art, and anyone skilled in the art could
design all-pass phase shift circuits capable of providing
a difference of 90° phase shift between the front and
rear channels, as provided by the all pass phase shift
circuits 26L, 26R and 27.
10R are also fed to the differential ampli?er 30 to pro
40
vide a signal equal to left-minus-right. This difference
signal is then fed to the ?xed localization EQ23, which
is identical to that disclosed and discussed in FIG. 1.
The output of the ?xed localization EQ 23 is then split
Comparing FIGS. 1 and 6, the all-pass ?lters 26L,
into three discrete bands via a high pass filter 31, a band
26R and 27 have been inserted and the right inverting 45 pass ?lter 33 and a low pass ?lter 32. The outputs from
ampli?er 41R has been omitted. The right inverting
the buffer ampli?ers 10L and 10R are also each split
ampli?er 41R, which corrects the phase error between
into three discrete bands. The buffered left channel
the right rear 43R and right front 12R in FIG. 1, is
signal is fed to a high pass ?lter 101L, a band pass ?lter
omitted in FIG. 6 to regain a stable rear center image
102L and a low pass ?lter 103L. Likewise, the buffered
due to the fact that the left 43L and right 43R rear 50 right channel signal is fed to a high pass ?lter 101R, a
channels regain phase coherency. The alternate method
band pass ?lter 102R and a low pass ?lter 103R. The
shown in FIG. 6 compensates for the 180° phase error
outputs from the left ?lters 101-103L and the right
that would take place between the right rear 43R and
?lters 101-103R are then fed to left and right level
right front 12R by inserting the all-pass circuits 26L,
sensors 104-106L and 104-106R, respectively, which
26R and 27. The bass signal that is fed to the rear chan 55 provide a substantially DC output equal to the absolute
nels from the low-pass ?lter 22 is simply fed to the
inputs of both summing ampli?ers 40L and 40R.
value of the logarithm of the energy present in each
discrete band.
FIG. 7 illustrates an embodiment of the invention
Referring now to FIG. 4, a partial block/partial sche
similar to that disclosed in FIG. 6. Common block num
matic diagram of the circuitry contained in block 100 of
bers are used where con, non functions are performed. 60 FIG. 3 illustrates both the ?ltering network 101-103
In this embodiment, the buffered output signals of the
and the level sensors 104-106 for either channel, i.e. left
buffer ampli?ers 10L and 10R are fed to the differential
or right. The ?lter networks 101, 102 and 103 are com
ampli?er 30. The differenced output of the ampli?er 30
monly known in the art and include a 2-pole high pass
is then fed to the fixed localization EQ 23, followed by
?lter at the output of the high pass network 101 and a
the all pass phase shift circuit 27. The output of the 65 2-pole low pass ?lter at the output of the low pass net
phase shift circuit 27 is then fed directly to both VCAs
work 103. The outputs of the high pass network 101 and
34R and 35L, which therefore provide broadband rear
channel steering. The summed low pass output of the
the low pass network 103 are summed at the negative
input of a differential ampli?er 102. The direct input is
5,333,201
11
fed to the positive input of the differential ampli?er 102.
The difference output will be equal to the midrange
information present in the input signal. The 2-pole high
pass ?lter 101 has an output passing frequencies above
approximately 4 KHz, the low pass ?lter 103 has an 5
output passing frequencies below approximately 500 Hz
and the bandpass ?lter 102 has an output passing the
frequencies between the high pass ?lter 101 and the low
pass ?lter 103. Other frequencies may be used as alter
natives to those disclosed. The outputs from each of the 1 0
?lter sections are processed by a level sensor. One level
sensor 104, disclosed in detail for the high pass ?lter
101, is virtually identical to the other level sensors 105
and 106. The function of the level sensor 104 is served
by the custom integrated circuit HUSH TM 2050. The 1 5
HUSH TM 2050 IC contains the circuitry 104A shown
in FIG. 4. The output of the high pass ?lter 101 is AC
coupled through a capacitor C1 to the input of a log
detector which provides the logarithm of the absolute
value of the input signal. The log detected output is 20
applied to the positive input of an ampli?er A1, which
sets the gain of the full wave recti?ed, log-detected
signal by a feedback resistor R3 and a gain-determining
resistor R1. Another resistor R2 provides a DC offset so
12
tion of the right channel. A differential ampli?er 51
provides a positive-going output with a predominant
signal energy in the mid-band portion of the left channel
and a negative-going output with a predominant signal
energy in the mid-band portion of the right channel.
Likewise, a differential ampli?er 52 provides a positive
going output with a predominant signal energy in the
low-band portion .of the left 'channel and a negative
going output with a predominant signal energy in the
low-band portion of the right channel. The outputs of
the differential ampli?ers 50, 51 and 52 feed the steering
generators 60H, 60B and 60L of a steering decoder 80,
respectively. The steering generators 60H, 60B and 60L
are each virtually identical to the steering generator 60
disclosed in FIG. 2. The high pass steering generator
60H determines the left/right steering characteristics
for the high-band portion of the audio spectrum, the
mid band steering generator 60B determines the left/
right steering characteristics for the mid-band and the
low pass steering generator 60L determines the left/
right steering characteristics for the low-band. The
outputs of each of these steering generators provide the
proper DC voltage to control the VCAs 34-39 disposed
in the audio signal path for the right and left rear out
puts. These VCAs control the high, mid and low-band
that the output of the ampli?er A1 operates within the 2 5
proper DC range. The output of the ampli?er A1 is then
portions of the audio spectrum so as to enhance direc
peak-detected by a diode D1 and ?ltered by a capacitor
tional information for the left 43L and right 43R rear
C2. The ?lter capacitor C2 and a resistor R4 determine
outputs. The audio inputs to the high band VCAs 34
the time constant for the release characteristics of the
level sensor 104. This ?ltered signal is then buffered by 3 O and 35 are fed from the highpass filter 31, the audio
a buffer ampli?er A2 and inverted by a unity gain in
inputs to the mid band VCAs 36 and 38 are fed from a
verting ampli?er A3. The output of the inverting ampli
band pass ?lter 33 and the audio inputs to the low band
?er A3 feeds an input resistor R8 and is then fed to the
VCAs 37 and 39 are fed from the low pass ?lter 32. The
negative input of an operational ampli?er A4. A feed
outputs of the right VCAs 34, 36 and 37 are summed
back resistor R9 provides negative feedback to the op- 3 5 through the ampli?er 40R, so as to provide a composite
erational ampli?er A4. The output of operational ampli
output of the entire spectrum of difference information
?er A4 is a positive-going DC signal, linear in volts-per
that has been divided into a plurality of bands by the
decibel, proportional to the input signal level applied to
?lters 31, 32 and 33. Likewise, the suturing ampli?er
the input of the level sensor 104. The circuitry disclosed
40L combines the audio outputs of the left VCAs 35, 38
in FIG. 4 is virtually identical to that of the level sensors
40 and 39 to provide a composite output of the entire spec
13L and 13R in FIG. 1. The time constants may vary.
trum of difference information processed by the ?lters
The values for the components shown in FIG. 4 are
31, 32 and 33.
listed in TABLE 2.
The signal summed at the summing ampli?er 20 is
also low pass ?ltered through the low pass ?lter 22 and
TABLE2
A1
A2
A3
c2
LF 353
LF 353
LF 353
LF 353
LF 353
.47 Mfd
.1 Mfd
ca
470 pf
D1
R1
R2
R3
R4
R5
R6
R7
R8
R9
1N 4148
1 K0
91 K0
10 K0
1 M0.
20 R0
20 no
150 no
20 K0
20 no
A4
102
c1
45
also fed to the positive input of the differential ampli?er
41R to provide bass content as a portion of the signal of
50 the right rear output 43R. The differential ampli?er 41R
differences the low pass ?ltered output of the low pass
55
?lter 22 and the output of the right summing ampli?er
40R to maintain proper phase coherency between the
right rear 43R and right front 12R channels.
In operation, the left and right buffered outputs from
the buffer ampli?ers 10L and 10R are each divided into
a three band spectrum, processed by the high pass, low
pass and band pass ?lters. The level sensors 104-106L
60
Referring again to FIG. 3, the outputs of all the level
fed to the input of the left summing ampli?er 40L to
provide bass content as a portion of the signal of the left
rear output 43L. The output of the low pass ?lter 22 is
and 104-106R following the outputs of the ?lters pro
vide DC signal levels representative of the spectral
sensors 104-106L and 104-106R are positive-going DC
energy present in each band of each channel. These DC
signal levels are fed to the differential ampli?ers 50, 51
voltages proportional to the output signal energy at the
and 52 which provide positive or negative steering
outputs of the ?lters 101-103L and 101-103R. The dif
information based on the predominant signal energy
ferential ampli?er 50 provides a positive-going output 6 contained in each portion of the spectrum. The steering
with a predominant signal energy in the high-band por
decoder 80 then provides proper DC control steering
tion of the left channel and a negative-going output
signals for the VCAs disposed in the signal path for the
with a predominant signal energy in the high-band por
right and left rear outputs 43R and 43L.
13
5,333,201
The left and right input signals buffered by the buffer
ampli?ers 10L and 10R, respectively, are differenced by
the ampli?er 30 and divided into high, mid and low
14
energy present in the right channel input. Conversely,
the difference ampli?er 50R provides a DC output
which is positive only when the signal energy is pre
bands by the ?lters 31, 32 and‘33. The outputs of these
dominantly right by greater than 10 dB over the signal
?lters are then applied to the inputs of the VCAs 34-39.
energy present in the left channel input.
The VCAs 34-39 provide the proper emphasis or de
Steering generator 160 is similar to that disclosed in
emphasis for each band within each channel. The com
FIG. 2. However, it has been re-con?gured so that
posite system, as disclosed in FIG. 3, allows for a pre
limiter/control amps 172L and 173R will provide unity
dominant high frequency signal to be emphasized in the
gain to the rear channel VCAs 34R and 35L, i.e. it will
left channel via the left high band VCA 35 and de-em 10 not provide upward expansion or emphasis to the left or
phasized in the right channel via the left high band
right rear channel when the difference in signal energy
VCA 35, while simultaneously emphasizing a predomi
between the left and right inputs is less than 10 dB.
nant mid frequency signal in the right channel via the
However, a de-emphasis of the opposite channel will be
right mid band VCA 36 and de-emphasizing that mid
achieved through inverting ampli?ers 168 and 171
frequency signal in the left channel via the left mid band 15 when a predominant signal energy (less than 10 dB) is
VCA 38. Thus it can be seen that in this embodiment it
detected in one channel. For example, if a predominant
is possible to provide instantaneous emphasis into the
signal energy is detected in the left channel (less than 10
left 43L and right 43R rear channels, based on signal
dB more than that of the right), no control voltage will
energy present in various portions of the audio spec
be present on the output SL, but a control voltage will
trum.
20 be present on the output of SR so as to attenuate the
Now referring to FIG. 5, yet another embodiment of
the invention incorporating further enhancements for
improving localization of the decoded audio signals is
signal within the high band portion of the spectrum for
the right channel. Conversely, if a predominant signal
energy is detected in the right channel (less than 10 dB
illustrated. Common numbers are used to denote com
mon circuit functions to those of other ?gures.
more than that of the left), no control voltage will be
25 present on the output SR, but a control voltage will be
Left/right audio inputs 9L and 9R are buffered by
buffer ampli?ers 10L and 10R. The buffered output
signals are then high pass filtered to provide substan
tially upper mid and high frequency information at the
outputs of the high pass ?lters 13L and 13R. The decod 30
ing matrix contains matrixing circuits 15L, 16L, 16R
present on the output SL so as to attenuate the signal
within the high band portion of the spectrum for the left
channel.
In operation, the left limiter 172L will limit at a pre
de?ned maximum VCA gain between 0 dB and +3 dB
with difference information less than 10 dB. Only when
the signal energy is predominantly left by greater than
10 dB will the output of the difference ampli?er 50L,
and 15R, where 15L is strictly information contained in
the high pass ?ltered left signal at unity gain, 15R is
strictly information contained in the high pass ?ltered
processed through a diode D101, increase the limiting
right signal at unity gain, 16L provides (left X
35 point of the left limiter 72 to increase the emphasis into
0.89 l)+(right x 0.316) and 16R provides (right x
0.891)+(X 0.316). The outputs from the decoding ma
also con?gured so as to limit VCA gain between 0 dB
trix each feed a level sensor (17L, 17LR, 17RL and
17R) which provide substantially DC outputs propor
the left channel. Conversely, the right limiter 73R is
and +3 dB. Only when the signal energy is predomi
nantly right by greater than 10 dB will the output of the
difference ampli?er 50R, processed through a diode
D102, increase the limiting point of the right limiter 73R
to increase the emphasis into the right channel via the
right channel’s VCA 34R.
tional to the logarithm of the absolute value of the sig
nal energy contained in the outputs of the decoding
matrix. The level sensor 17L, which re?ects strictly left
signal information is fed to the positive input of a differ
ential ampli?er 50L, while the minus input of the differ
The embodiment disclosed in FIG. 5 allows for a
ential ampli?er 50L is fed by the level sensor 17LR, 45 given individual signal to be localized at any location
which contains predominantly left signal information
within 360° of the listener, dependent upon the amount
plus a small portion of right. The exclusive left and right
that the given signal is panned to the left or to the right
outputs from the level sensors 17L and 17R, respec
input. A composite input signal would require that the
tively, are‘fed to the positive and negative inputs, re
energy level in one channel be at least 10 dB greater
spectively, of a differential ampli?er 50 virtually identi 50 than that of the other channel before the rear channel
cal to that disclosed in FIG. 1. The output of the differ
information will begin to be emphasized.
ence ampli?er 50 will be positive with a predominant
FIG. 9 is a graphical representation of a typical alter
signal energy in the left band and negative with a pre
native frequency response plot for the high pass ?lters
dominant signal energy in the right band. The output of
13R and 3L of FIGS. 1 and 5-8 which provides further
the level sensor 17RL which provides a DC signal rep 55 improvements in steering both broadband and limited
resentative of predominantly right signal information
bandwidth signals in the rear channels. As shown, the
plus a small portion of left is fed to the negative input of
curve has a corner frequency Fc of approximately 18
a differential ampli?er 50R, while the output of the
KHz, but could range from approximately 6 KHz to 20
level sensor 17R, representing strictly right channel
KHz depending on the requirements of a particular
information is fed to the positive input of the ampli?er 60 application. The critical factor is that the frequency
50R. The decoding matrix, level sensors and difference
response weights the level sensors 14R and 14L so that
ampli?ers operate in unison to provide a DC output at
they become sensitized to primarily high band informa
the difference ampli?er 50 which is positive when pre
tion or more sensitive to high than mid frequency infor
dominant signal energy is in the left channel and nega
mation. Such a frequency response can be applied to an
tive when predominant signal energy is in the right 65 embodiment such as that shown in FIG. 1, for example,
channel. The difference ampli?er 50L provides a DC
in which only high band information is steered to the
output which is positive only when the signal energy is
predominantly left by greater than 10 dB over the signal
left and right rear channels. Applying this method to an
embodiment such as FIG. 1 eliminates undesirable side
15
5,333,201
16
effects such as jittering and image-wandering when
cuitry. The basic functions of the generalized blocks of
signals are steered to the left and right rear channels.
the 2050 IC are well known to those skilled in the art.
Many alternatives exist as standard product ICs from a
However, referring to FIG. 10, another embodiment
of the invention is disclosed in which high pass ?lters
13LH and 13RH having the frequency response plot
shown in FIG. 9 feed level sensors 14R and 14L. By
weighting the level sensors 14R and 14L for the steering
detector in this manner, left and right steering becomes
based primarily on high frequency information. For
example, if predominant midband information is present
requiring left or right steering and a subtle amount of
high frequency information suddenly appears in either
channel 9L or 9R, the subtle high frequency would
large number of IC manufacturers, as well as discrete
circuit design.
The invention is intended to encompass all such mod
i?cations and alternatives as would be apparent to those
skilled in the art. Since many changes may be made in
the above apparatus without departing from the scope
of the invention disclosed, it is intended that all matter
contained in the above description and accompanying
drawings shall be interpreted in an illustrative sense,
and not a limiting sense.
become the dominant factor to steer the signal in that
What is claimed
direction. Weighting the level sensors 14R and 14L in 15
1. A circuit for decoding two channel stereo signals
this manner dramatically improves the aforementioned
into multi-channel sound signals comprising:
undesirable side-effects which occur when steering
means for differencing the two channel stereo signals
broadband signals.
The application of the principle of weighting the
level sensors to the split band embodiment of the circuit 20
is illustrated in FIG. 11 in which the output of the dif
ferential ampli?er 30 is enhanced by the ?xed equaliza
to provide a primary signal;
means for dynamically varying the level of said pri
mary signal to produce a ?rst dynamically varied
signal; and
'
means having a frequency response more sensitive to
tion circuit 23 to produce a primary signal which is then
high than mid-frequency information for control
divided into high and low bands by the high pass filter
31 and the low pass ?lter 32. The output signal of the
high pass filter 31 is then dynamically varied by a right
ling the gain of said varying means to increase the
level of said ?rst dynamically varied signal when
the level of one of the two channel signals is high
high band VCA 34 and a left high band VCA 35 while
the output of the low pass ?lter 32 is dynamically varied
by the right low band VCA 37 and the left low band
said ?rst dynamically varied signal when the level
of the other of the two channel signals is high rela
VCA 39. To control the gains impressed by the VCA’s, 30
tive to the one.
relative to the other and to decrease the level of
one of the input stereo signals 9R is fed to a high pass
2. A circuit according to claim 1, said controlling
filter 101R and a low pass ?lter 103R while the other
means comprising:
stereo input signal 9L is fed to a high pass ?lter 101L
means having a frequency response more sensitive to
and a low pass ?lter 103L. As before, each of these ?lter
high than mid frequency information for deriving a
outputs is level sensed and the difference between the 35
?rst dc signal proportional to one of the two chan
sensed high pass outputs is used to provide a ?rst con
nel stereo signals;
trol signal while the difference between the sensed low
means having a frequency response more sensitive to
pass outputs is used to obtain a second control signal.
high than mid frequency information for deriving a
The difference of the sensed high pass outputs is used by
second dc signal proportional to the other of the
the steering decoder 80 to control the high band VCA’s
two channel stereo signals;
while the control signal derived from the sensed low
means for differencing said ?rst and second dc signals
pass signals is used to control the low band VCA’s. The
to provide a dc control signal which is positive
high pass ?lters 101R and 101L are selected to provide
when one of the two channel stereo signals is domi
a frequency response which is more responsive to high
nant and which is negative when the other of the
than mid frequency information such as the frequency 45
two channel stereo signals is dominant; and
response curve illustrated in FIG. 9. This special sensi
means for impressing positive and negative gains on
tivity to the high rather than the mid frequency content
said varying means in response to said positive and
of these signals provides unexpectedly pleasing im
negative conditions of said dc control signal.
provements in the audibly directional aspects of the
3. A circuit according to claim 1 further comprising:
system.
50
second means for dynamically varying the level of
While a number of embodiments have been disclosed
said primary signal to produce a second dynami
with various features for enhancing the basic concepts
cally varied signal; and
of the invention, the invention also lends itself to imple
means having a frequency response more sensitive to
mentation as a DSP software algorithm. In a DSP im
plementation, it would be conceivable to divide the 55
audio spectrum into a larger number of frequency bands
to get even better frequency resolution, thereby provid
ing better localization at speci?c frequency bands
within the audio spectrum. The further enhancements
that can be provided through a DSP implementation
will become apparent to those skilled in the art, and are
well within the scope of the invention.
The invention disclosed has been reduced to practice
where many of the circuit functions are performed by
the custom integrated circuit HUSH 2050 TM. The
2050 IC is a proprietary IC developed by Rocktron
Corporation, and contains log-based detection circuits,
voltage-controlled ampli?ers and VCA control cir
high than mid frequency information for control
ling the gain of said second varying means to in
crease the level of said second dynamically varied
signal when the level of the other of the two chan
nel signals is high relative to the one and to de
crease the level of said second dynamically varied
signal when the level of the one of the two channel
signals is high relative to the other.
4. A circuit according to claim 1 further comprising
means for enhancing said primary signal before said
primary signal is dynamically varied.
5. A circuit according to claim 4, said enhancing
means comprising means for providing ?xed localiza
tion equalization simulating the frequency response
characteristics of the human ear.
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