LEFT OUT
USO05420929A
UllltGd States Patent [19]
[11] Patent Number:
Geddes et al.
[45]
[54] SIGNAL PROCESSOR FOR SOUND IMAGE
ENHANCEMENT
Date of Patent:
Inventors:
[73] Assignee:
Earl R. Geddes, Livonia; J. William
May 30, 1995
FOREIGN PATENT DOCUMENTS
8401257
[75]
5,420,929
_
3/1984
WIPO ................................... .. 381/1
_
_
Whikehart, Novi, both of Mich.
jm?f'ytlgmmléekg‘émséu?tz
sszs an
xammer- mg
.
e
Ford Motor Company, Dearborn,
Attorney’ Agent, 0’ Firm—R°g¢1' L- May; Mark L
Mi¢h_
Mollon
[21] Appl. NO.: 888,087
[57]
ABSTRACT
A signal processor for sound image enhancement of
[22] Filed?
May 26, 1992
stereophonic signals provides ?uctuating coherence
'
[51] Int. 01.6 ............................................. .. H04R 5/00
between the left channel and right channel outputs by
crossfeeding a high pass Portion of thE left channel to
[52] UES. Cl. ............................... .. 381/1; 3381/22
[58] Fleld of Search ................. .. 381/1, 25, 22, 27, 26,
381/86
the right output and a like portion of the right channel
to the left output. Preferably, the crossfeed path in_
cludes a high pass ?lter to eliminate crossfeeding of
.
[56]
lower frequency range com onents which are often
References cued
reproduced monaurally in prlerecorded materials. The
US. PATENT DOCUMENTS
filtering is compensated for by a shelving ?lter intro
4,060,696 11/1977 Iwahara etal. ..................... .. 381/26
4,192,969 3/1980 Iwahara ......... ..
381/1
duced in the respective channel inPut ‘0 b°°$t the
Power of the lower frequency components to be added
4,219,596 g/1930 Kogure et a1,
179/1
with the crossfed signal to produce the channel output.
4,309,570 1/ 1982 Carver ....... ..
4,329,544 5/1982 Yamada ..... ..
179/1
179/1
-- 331/1
In the preferred embodiment, an automatic gain control
varies the gain of the crossfeed in accordance with the
stereo content in the input channels. In addition, the
4,388,494 6/1983 Schtine et al
4,394,536 7/1983 Sh‘ma 6‘ a1‘
179/1
gain control includes a control for user variation of the
4’495’637
V1985 Bmney """" "
‘ 381”
amount of coherence to be generated at the output.
4’603’429
7/1986 carver """" "
' 381”
Furthermore
4,622,689 11/1986 Horbrough .... ..
381/27
stereo detectgr circll’m
4,868,878
Kunu ' et al.
. 381/1
4,980,914 12/1990 Kunugi et a1.
38l/l
9/1989
4,980,915 12/1990 Ishikawa ............................. .. 381/27
the
resent invention also
rovides a
P
‘
29 Claims, 3 Drawing Sheets
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US. Patent
May 30, 1995
Sheet 2 of 3
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US. Patent
May 30, 1995
Sheet 3 0f 3
5,420,929
FIG 6
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FIG 7
1
5,420,929
2
SIGNAL PROCESSOR FOR SOUND IlVIAGE
ENHANCEMENT
Like U.S. Patents to Yamada and Kunugi et al., U.S.
Pat. No. 4,219,696 to Kogure et al. reproduces sound
from two loudspeakers located in front of the listener to
TECHNICAL FIELD
generate relocalized sound in a manner that simulates
sound reproduction sources to the rear of the listener.
The present invention relates generally to stereo
phonic reproduction systems, and more particularly to
such systems in which the stereo signals are processed
to enhance the sound image pattern in a sound area
serviced by speakers mounted at discrete locations.
The apparatus includes transfer functions canceling
sound in the direct path and imposing a time difference
between sound waves applied to the left and right ears
-of the listener. Similarly, U.S. Patent No. 4,192,969 to
10 Iwahara discloses a stereophonic sound reproduction
BACKGROUND ART
In the art of sound reproduction systems, it is well
known that the location of transducers, often referred to 15
as loudspeakers, has a substantial affect upon sound
reproduction of stereophonic signals. Accordingly,
speakers are preferably arranged in order to produce
psychoacoustically pleasurable sounds to the area occu
pied by the listeners. However, particularly in motor
vehicles, the number and position of the speakers is
often dictated by other packaging considerations and
cannot be arranged for the sole purpose of providing
maximum listening pleasure to the vehicle occupants.
Accordingly, there have been several developments to
process the signals to be emitted from the speakers in
system simulating an expanded stage by crossfeed paths
between the channels with a ?rst transfer function rep
resentative of ratio of the crossfeed transfer function to
the direct transfer function corresponding to a hypo
thetical sound location with respect to the listener’s
ears, and a second transfer function corresponding to
the ratio of crossfeed transfer function to direct transfer
function corresponding to the actual sound direction.
TECHNICAL PROBLEM RESOLVED
The present invention is distinguishable from the
above-identi?ed disclosures by processing each channel
input signal in a crossfeed path having a transfer func
tion circuit for frequency weighting the coherence of
25 the sound signals emitted from the left and right channel
output speakers. A processed crossfeed signal is added
order to adjust the audio reproduction image of a ste
to the opposite channel signal to produce each channel
reophonic signals.
output. The result is that the psycho-acoustic image is
Several attempts have been made to generate signals
narrower than the speaker separation although signals
that simulate a relocation of the speakers as if they had 30 at selected frequencies continue to maintain their origi
been spread further apart or located in a different direc
nal stereo separation. Accordingly, the present inven
tion from the listening area. U.S. Pat. No. 4,329,544 to
tion avoids the hole-in-the-center effect perceived when
Yamada discloses a sound reproduction system attempt
speakers are spaced far apart. As a result, the present
ing to audibly simulate a wider distance between the
invention provides a psycho-acoustic impression that
speakers. A transfer function equalizes sound pressures 35 the speakers are actually located closer to the speaker
from a signal representative of a third speaker location
and the conventional output emitted from stereo speak
positions of a more ideal listening environment where
ers. The system also includes a delay circuit in one
mined angular alignment with the listening position.
channel to compensate for the difference in distances
between the listener and each of the speakers, and also
sound sources are forward of and within a predeter
For example, an ideal environment might be considered
one in which speakers are aligned toward a listening
position and positioned about 40° off the central axis
between the speakers.
includes a reverberation circuit. U.S. Pat. No. 4,394,536
also discloses an apparatus for acoustic spreading and
reverberation effects for reproduced sound and the
Preferably, the transfer function circuit includes a
effects can be adjusted by the user.
signal processor for imposing repeated phase reversal
U.S. Pat. No. 4,868,878 to Kunugi et a1. discloses a 45 continuously throughout a predetermined band of sig
sound ?eld correcting system in which the transfer
nal frequencies, preferably implemented by delay. The
function adjusts a level in delay of the signal to compen
transfer function H is a function of the frequency and
sate for the distance between the travel of direct and
preferably, also a function of the crossfeed gain G. The
re?ective sound waves to a listening point. U.S. Pat.
processor controls the crossfeed of mono signals to
No. 4,980,914 issued‘ from a continuation-in-part appli
avoid annoying frequency coloration should mono sig
cation of U.S. Pat. No. 4,868,878 and discloses the addi
nals be present. The low-frequency content of input
tional feature that high pass or low pass ?lters may be
stereo signals are typically mono (left and right chan
used as desired at appropriate points of the system.
nels are coherent). Furthermore, broadcast speech and
U.S. Pat. No. 4,980,915 to Ishikawa discloses an inte
music pieces or passages can be mono, and this mono
grated circuit switch for use with a system including a 55 content can be over all frequencies. Mono signals
center input signal as well as left and right input signals.
should not be crossfed, since the resulting output signals
U.S. Pat. No. 4,495,637 discloses a method and appa
will consist of signals added to a delayed version of
ratus for’ enhancing psycho-acoustic imagery by asy
themselves. Such adding causes substantial frequency
metrically crossfeeding left and right signal inputs. The
coloration. In particular, a frequency component of an
asymmetry is designed to complement the listener’s 60 input signal having a period of 2T, where T is the cross
brain processing of perceived acoustic signals due to
feed delay time, would disappear completely from the
output since it is added to itself 180° out of phase. In a
naturally occurring left or right half brain dominance of
similar manner, a component with period T would add
the listener. The system employs out of phase crossfeed
to itself in phase, producing twice as much output for
without ?ltering or delays.
U.S. Pat. No. 4,388,494 to Schone et al. discloses a 65 that component. Therefore, the processor must remove
stereophonic reproduction system using the dummy
low-frequency signal content in the crossfed signals.
head recording process and a headphone reproduction
process with ?ltering and crossfeeding of the channels.
frequencies, removing on the low-frequency content is
For signals with mono content over substantially all
3
5,420,929
not suf?cient. Therefore, the system of the present in
vention includes a gain control circuit that turns off the
imaging effect when the signal is mono. The gain con
trol of the preferred embodiment includes user operable
control over the amount of imaging effect and auto
matic control depending upon the amount of mono
content in the input signal, preferably after low fre
quency content has been removed. Accordingly, a gain
control circuit according to the present invention in
cludes a stereo signal detection circuit for control of the
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more clearly under
amount of gain in the crossfeed path.
In the preferred embodiment, the crossfeed paths
include high pass ?lters to avoid crossfeeding the low
frequency signal content. Since the output of each chan
stood by reference to the following detailed description
of a preferred embodiment when read in conjunction
with the accompanying drawing in which like reference
characters refer to like parts throughout the views and
nel is the sum of a delayed ?rst channel input added to
the opposite input signal, the image control circuit
could produce an output power spectrum with in
creased magnitude at high frequencies. Accordingly, a
shelving ?lter is included for each channel input line to
be added to the crossfeed signal from the other channel,
4
of speakers are often ?xed by considerations unrelated
to the acoustic environment within the vehicle.
The present invention also provides automatic con
trol of the imaging effect by controlling the amount of
crossfeed gain according to the amount of stereo con
tent in the left and right signals. The power spectrum
response of the system is preferably maintained at a
substantially constant level regardless of the amount of
crossfeed.
in which:
FIG. 1 is a diagrammatic view of the overall circuit
20
con?guration for sound image enhancement according
to the present invention;
so that a predetermined amount of boost at the low
FIGS. 2A-2B is a graphical representation of the
frequencies compensates for the added output at the
input channel signals delivered to and the output chan
higher frequencies. In the preferred embodiment, a
nel signals produced by the circuit shown in FIG. 1;
branch line with a low pass ?ltered version of the input
FIGS. 3A-3B is a graphical representation of the
signal is added to the channel input line and the cross 25
transfer
function employed in the crossfeed path 10 of
feed line to obtain the ?at net output response.
the circuit shown in FIG. 1;
While the gain of the crossfed signal controls the
FIG. 4 is a diagrammatic view of a more detailed
amount of the imaging effect, the gain adjustment cir
modi?cation of the general circuit con?guration shown
cuit should also adjust the gains of the direct input and
'
branch paths to keep the output power spectrum ?at 30 in FIG. 1;
FIG. 5 is a diagrammatic view of a stereo detector
given a ?at input spectrum.
circuit for use with the circuit shown in FIG. 4;
When using a lowpass ?lter in a branch line to obtain
FIG. 6 is an enlarged graphical representation of a
a shelving ?lter response, and with the direct, branch,
portion of the output signal curves shown in FIG. 2 and
and crossfeed gains adjusted properly, the output power
spectrum is ?at except for possibly near the lowpass and 35 generated by the circuit shown in FIG. 4; and
FIG. 7 is a graphical representation of the output of
highpass ?lters’ cutoff frequencies, where ripple can
the
shelving ?lter employed in the circuit of FIG. 4.
occur. This ripple can be signi?cant for some applica
tions. As will be described later, it is computationally
BEST MODE OF THE INVENTION
desirable to make the lowpass and highpass ?lter cutoff
Referring
first to FIG. 1, the stereo imaging process
frequencies the same. In this situation, a 0.5 dB dip 40
ing circuit 10 is shown comprising a left channel input
occurs at the cutoff frequency due to the phase relation
line 12 as well as a right channel input line 14 receiving
ship of the ?lters in this region.
signals from a left channel source 16 and a right channel
To compensate for this unadjusted effect, one ap
source 18 as diagrammatically represented in FIG. 1. Of
proach is to add an all-pass ?lter in the direct path that
has the same delay response as the low pass ?lter in the 45 course, the left channel source 16 and the right channel
source 18 may be parts of a single stereophonic repro
branch line, but with a ?at magnitude response. A sec
duction component such as a tuner, preamp or the like.
ond approach is to add a phase-equalizer (an all-pass
In addition, the circuit 10 generates a left channel out
?lter) after the low pass ?lter in the branch path to make
put line 20 and a right channel output line 22 coupled to
the net response in the branch path phase linear so that
the same amount of delay is imposed at all frequencies. 50 respective transducers such as speakers 24 and 26. '
Still referring to FIG. 1, input line 12 is branched to
The net delay of the branch path would also be added to
a crossfeed path 28 including a transfer function 30
the direct path. A still further approach which is an
which is added to the right channel direct path 32 by
approximate solution and the simplest is to add a ?xed
appropriate adding circuitry 34. Similarly, the right
delay to the direct path since the delay in the low fre
channel input line is branched through a crossfeed path
quency content in the branch path signal can be approx
imated by a constant delay. The amount of constant
36 including a transfer function 38 which is added to a
direct path 40 from the left channel input line 12 at an
delay added to the direct path should also be added to
appropriate adding circuit 42.
the delay in the crossfeed path to keep the net delay
The crossfeed transfer functions 30 and 38 contain a
between these paths the same.
As a result, the present invention provides stereo 60 frequency weighting circuit. The transfer function em
ployed in the preferred embodiment is shown in FIG.
phonic reproduction of stereo channel signals with a
narrower psycho-acoustic image than the spacing be
3a and FIG. 3b. The magnitude of the function, as
tween the speakers. The system acoustically simulates
shown in FIG. 3a, is at a maximum above a predeter
mined frequency so that the lower frequency signal
substantially closer presence of the program material to
the listener by ?uctuating the coherence of the channel 65 oomponents of the channel inputs are substantially at
signal outputs without adjusting the physical location of
the speakers. As a result, this is especially useful in
motor vehicle passenger compartments where positions
tenuated by the transfer function since such frequencies
often have mono content. The rapidly changing phase
response in FIG. 3b is due to a frequency independent
5
5,420,929
6
cantly mono at frequencies that pass through the high
delay, preferably between 2 and 10 milliseconds, which
pass ?lters will be desirable. The switching is best ac—
is part of the crossfeed transfer function.
complished by controlling the crossfeed gain in re
The result of this signal processing is graphically
sponse to the amount of mono content in the signal that
demonstrated in FIGS. 2a and 2b. In FIG. 2a, the plot
ted line 50 represents left channel and right input chan 5 is crossfed. Thus, a gain control circuit with a stereo
detector is illustrated in the circuit con?guration shown
nel signal spectrums. Plotted line 52 indicates the sum of
in FIGS. 4 and 5.
the signal strengths of the left and right channel. Line 54
As shown in FIG. 4, the imaging circuit 100 includes
demonstrates the output signal spectrum of each chan
nel output line 20 and 22, while line 56 demonstrates the
sum of the signal strengths transmitted at output chan
nels 20 and 22. It is desired that for a ?at and equal input
spectrums, the output spectrums should be ?at and
equal. However, this is not the case as can be seen in 54.
To make the output spectrums ?at, the low-frequency
a shelving ?lter 53 in each transfer function 44 and 46
implemented by coupling a branch line to the channel
input line. The left branch path 102 includes a transfer
function 104 in the form of a low pass filter whose out
put is added to the sum of the direct input line 40 and
the crossfeed path 136. The transfer function 104 in
part of the outputs must be boosted. A shelving ?lter
could be used to boost the low frequency signal power
output to the same level as the higher frequency compo
nents. The effect of the boost is illustrated in phantom
line at 55 in FIG. 2a. Preferably, the gains are con
branch line 102 may be an exact complement of the high
trolled for a net 0 db output as described in greater
cient signal processing manner. Similarly, the right
detailbelow.
In FIG. 2b, the coherence of the left channel and
right channel input lines 12 and 14 is demonstrated at
channel input line 114 includes a branch line 106 with
the transfer function 108 in the form of a low pass ?lter
for adding to the sum of the direct line 132 and cross
curve 58. The curve 58 demonstrates that the lowest
feed path’ 128 from the left channel.
frequency signal components are substantially mono as 25
In addition, each of the three signals added at each of
the channel output lines 120 and 122 must be multiplied
by related gain constants in order to control the output
response to obtain a ?at power spectrum output. As
they are reproduced substantially equally in both chan
nels in prerecorded material. Conversely, the higher
frequency signal components maintain their separated
stereo imaging. In other words, the coherence is valued
closer to 0 for the signal components with higher fre
quencies. Graphic trace 60 demonstrates the ?uctuating
coherence of the output signal generated at the channel
output lines 20 and 22. Thus, as a result of processing in
the imaging circuit 10, the stereo separation at certain
frequencies varies between 0 and 1 throughout the en 35
tire upper range of frequencies in the signals processed.
Accordingly, the output from the speakers 24 and 26
is demonstrated to be coherent at numerous frequencies
pass ?lter used for crossfeeding. These ?lters may be
provided by a state-variable ?lter 103, as indicated dia
grammatically in FIG. 4, so that the low pass and high
pass functions are obtained simultaneously in an ef?
discussed above, the gain of the crossfeed signal con
trols the amount of the imaging effect. The gains in each
of the direct paths 140, 132 and in the branch paths 102
and 106 are correspondingly controlled to compensate
for or offset the crossfeed gain and keep the spectrum
flat. The gain control will be discussed in greater detail
with respect to FIG. 5.
The use of the state-variable ?lter to obtain lowpass
and highpass ?lters simultaneously results in the ?lters
having the same cutoff frequencies. The addition of the
lowpass ?lter output to the direct path, with the correct
through a wide band while other frequency compo
nents remain entirely right or left channel outputs. As a 40 gain settings, results in the desired shelving ?lter spec
tral response, except near the cutoff frequency. Near
result, the acoustic image of the sound reproduction is
the cutoff frequency, due to the phase relationships of
perceived to be narrower than the physical distance
the low pass ?lter and the direct path, an error of 0.5 dB
between the left channel speaker 24 and the right chan
occurs relative to the desired shelving ?lter response 66
nel speaker 26. Such a feature is particularly useful
when the speakers are located at the outermost borders 45 in the shelving ?lter response as shown in 62 in FIG. 7.
This in turn results in a 0.5 db error in the ?nal output
of the passenger compartment of a motor vehicle.
spectral response as shown at 112 in FIG. 6, and in
While the circuit described above provides the de
sired stereo imaging effect, the presence of mono signals
many applications, this may be undesirable.
Any of several preferred approaches may be em
in the crossfeed branches causes identical signals to be
added at the adding circuits 34 and 42. This substan 50 ployed to compensate for the error. A transfer function
110 in the direct line 140 can comprise an all-pass ?lter
tially changes the frequency spectrum of the resulting
that has the same delay response as the low pass ?lter of
signal for the reason that mono signal components are
transfer function 104. A further approach would be to
added to delayed versions of themselves due to the
include an all-pass ?lter in the branch line 102 after the
crossfeed signal added. As previously discussed, com
low pass ?lter to make the net response in the low pass
ponents with various periods are attenuated or boosted
path phase linear. The phase linear response means that
depending on their periods relative to the time delay. As
all frequencies have the same amount of delay. A corre
a result, control may be provided to avoid undesirable
sponding constant delay 110 would also be added to the
frequency coloration occurs that substantially effects
direct path 140. Furthermore, similar ?lters would be
the audio output of the program material.
Mono input cannot be avoided since the low-fre 60 employed in the branch and direct lines of the opposite
channel.
quency content of most signals is mono as previously
The most preferred approach, which is chosen for its
shown in FIG. 2b. Also, the voice content heard as
simplicity, is to approximate the delay of the low pass
normal speech on a stereo broadcast, and particular
?lter in the branch path by imposing a constant delay
music pieces or passages are transmitted monaurally.
The highpass response of the crossfeed paths prevents 65 110 in path 140. The use of a constant delay is justi?ed
by the fact that frequencies from 0 to about the corner
the substantially mono low-frequency content from
frequency of the low pass ?lter have a generally con
being crossfed. A circuit improvement which would
turn off the imaging effect when the signal is signi?
stant delay. Appropriate selection of a predetermined
7
5,420,929
delay in the direct path can reduce the ripple in the
output power spectrum to as low as plus or minus 0.08
db as is illustrated in FIG. 6 at curve 113. In contrast,
8
to reduce the number of samples which need to be used
in order to calculate the difference to sum ratio. Deci
output power spectrum has a 0.5db dip at the corner 5
frequency as demonstrated by curve 112 in FIG. 6. Of
course, the amount of constant delay added to the direct
mation is appropriate since the integrators reduce the
signal bandwidth, thus allowing a lower sample rate.
Decimation reduces the computational load for the
subsequent processing shown in FIG. 5. The decimated
result is predivided at 168 to avoid complications under
path 140 must also be added to the crossfeed path 136 to
keep the net delay between these paths the same as they
are added at the adder circuit 120. Similarly, the right
special conditions such as when the detected sum level
is 0 and when the detected difference is larger than the
detected sum due to the operation of the envelope de
channel processor paths can be modi?ed as discussed
tectors 165.
without any delay offered by a transfer function 110, the
above with respect to the left channel paths, and the
discussion need not be repeated in order to provide a
' complete disclosure. Nevertheless the shelving ?lter
output is adjusted as shown at 64 in FIG. 7 and closely
conforms with the ideal shelving ?lter output curve 66.
Referring now to FIG. 5, a preferred gain control
mechanism with a stereo detector for automatically
An additional processing section 156 for the stereo
dependent gain of the imaging circuit to assure that the
amount of stereo, originally represented by the value N
output from ratio 170, is multiplied by a sensitivity fac
tor which is adjustable by a user control 162. The sensi
tivity control controls how much the crossfeed gain G
is affected by the stereo detector. A factor of 2 at con
trol 174 allows a multiplied net sensitivity of 0 to 2. In
controlling the crossfeed gains provides two useful
functions. In particular, the gain G of the crossfeed path 20 addition, sensitivity can also be adjusted by an arbitrary
can be automatically varied in response to the stereo
curve function 176.
content of the signals running through the left and right
In the preferred embodiment, the function 176 pro
channel inputs. Secondly, the imaging effect can be
vides a piece wise linear curve that varies the rate of
varied as desired by the listener in order to produce the
desired acoustical effect. FIG. 5 diagrammatically rep 25 change of the signal level with respect to the amount of
stereo content in the signal. A modi?ed stereo content
resents the circuit features of signal processing accord
ing to the present invention to generate the crossfeed
gain G with control signal 160. In addition, the circuit
generates the compensating gain GA with control sig
nal 150 for the direct paths 140 and 132 and the compen
sating gain GB with control signal 152 in the branch
paths 102 and 106 that maintain a flat power output
spectrum by offsetting the varying crossfeed gain gen
signal output from the curve circuit 176 is subjected to
a dead zone function in order to prevent small changes
in the signal level N due to noise or other inconsisten¢
cies, from modulating the crossfeed gain. An adjustable
dead zone circuit 178 provides a dead zone around the
current value of the modified signal representing cross
feed gain, so that the gain output of the circuit 156
erated as a function of the stereo separation detected.
changes only when large changes occur. When the
a similar signal 149 from the crossfeed path 136 are
matically increased or lowered. As a result, noise or
In the block diagram of FIG. 5, a high pass ?ltered 35 input to the function circuit 178 increases or decreases
more than the width of the dead zone, the gain is auto
signal 147 from the left-to-right crossfeed path 128 and
distortion does not modulate the crossfeed gain affect
ing the right and left output signals. The dead zone
channel signal and the right channel signal are gener 40 circuit 178 includes a manual adjustor 180 for varying
the width of the dead zone.
ated and then envelope-detected to determine their
In addition a limit function 182 may be used to limit
respective levels. When the signal pair is mono (coher
introduced to the adder 151 and subtractor 153 as
shown. The sum of and the difference between the left
ent), the left signal level equals the right signal level and
so the detected difference level is 0. When the signals
the value of the crossfeed gain or to turn off the imaging
effect if desired. A limit adjustor 184 controls the limit
are not mono (non-coherent), the detected difference 45 imposed upon the crossfeed gain control signal before
level is non-zero. Thus, the detected difference level
varies according to the amount of stereo content. How
the signal is delivered to the gain controllers in the
crossfeed paths 128 and 136. In addition, the compensa
tor 186 varies the compensatory gain control signals
applied to the gain controllers in the direct paths 132
ever, the detected difference level also changes accord
ing to the absolute levels of the left and right signals.
To compensate for normal stereophonic reproduc 50 and 140 as well as in the branch paths 102 and 106 to
maintain a ?at power output at the channel outputs 120
tion in which the left and right signals will vary in level
and 122.
independent of stereo content, the detected difference is
As a result of the above description, it will be under
normalized. Accordingly, the detected difference is
stood that the present invention provides a signal pro
divided by the detected sum of the left and right signals
as at 170 to provide a quantity representing the amount 55 cessor for reducing the width of the stereo image pro
duced during stereophonic reproduction. As a result,
of stereo content N in the signal. The result (called N)
the present invention eliminates the hole-in-the-middle
varies from 0 for fully coherent left and right signals to
response typically associated with sound reproduction
1 for fully non-coherent left and right signals.
systems having widely spaced speaker locations with
The basic stereo detector circuit 154 of the preferred
embodiment includes an additive gain reducing function 60 respect to the listener position. Moreover, the automatic
gain control automatically varies the amount of imaging
164. In addition, an absolute value detector 165 pro
vides an output signal that is integrated at 166 with a
effect in response to the amount of stereo content being
predetermined integrator attack time constant and a
delivered to the processor. Furthermore, the circuit is
predetermined integrator decay time constant, prefera
arranged so as to provide a ?at power output response
bly in the range of one millisecond and one hundred 65 given a ?at input response and it avoids frequency col
milliseconds, respectively. The signals are then simulta
oration of the sound output produced. The stereo detec
neously decimated as diagrammatically shown at 167,
tor circuit may also be employed for other imaging or
preferably reducing the sampling rate by an 8 to 1 ratio,
signal functions.
9
5,420,929
10
frequency components to said left adder circuit and said
right channel input line communicates with a right
branch line having a low pass ?lter for adding low
Having thus described the present invention, many
modi?cations thereto will become apparent to those
skilled in the art to which it pertains without departing
from the scope and spirit of the present invention as
de?ned in the appended claims. For example, the pre
ferred embodiment has been described in terms of the
frequency signal power to said right adder circuit.
9. The invention as de?ned in claim 1 and further
comprising at least one gain control for limiting each of
the said signals added at said left channel and right
digital signal processing (DSP) preferably employed in
the environment of a motor vehicle, where such pro
channel outputs to obtain a ?at response over the audio
cessing capability for its implementation is readily avail
able. However, it is readily apparent that other tech
niques and apparatus, for example, hardwired analog
circuits, could be used to generate the circuits of the
present invention.
signal frequency range.
10. The invention as de?ned in claim 9 and further
We claim:
1. An apparatus for narrowing stereo imaging of 15
stereophonic signals to be delivered to at least one pair
of loudspeakers comprising:
a left channel output line coupled to a ?rst speaker of
said at least one pair;
a right channel output line coupled to a second
speaker of said at least one pair;
a left direct line coupled to said left channel input line
and added to the output from said left low pass
a left channel input line;
a right channel input line;
a left-to-right crossfeed path initiating at said left
channel input line,
?lter;
25
a right adder for adding said left-to-right crossfeed
path to said right input line at said right channel
a right direct line coupled to said right channel input
line and added to the output of said right low pass
?lter;
a left-to-right crossfeed path coupled to said left
output line;
a right-to-left crossfeed path initiating at said right
channel input line;
channel input line and added to said right direct
line including a left high pass ?lter and ?rst means
for delaying the signal in said left-to-right crossfeed
path to provide ?uctuated frequency weighted
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan
coherence from the loudspeakers;
nel output line;
a right-to-left crossfeed path coupled to said right
each of said crossfeed paths having a transfer function
circuit for repeatedly ?uctuating the phase of the
respective input signal passing through the cross
comprising a ?rst gain controller in each said crossfeed
line and a second gain controller in each said input line.
11. An apparatus for transmitting stereophonic signal
to at least a pair of loudspeakers comprising:
a left channel input line;
a right channel input line;.
a left branch line coupled to said left channel input
line and including a left low pass ?lter;
a right branch line coupled to said right channel input
line and including a right low pass ?lter;
35
channel input line and added to said left direct line,
including a right high pass ?lter and second means
for delaying the signal in said right-to-left crossfeed
path to provide ?uctuated frequency weighted
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
coherence from the loudspeakers;
loudspeakers.
whereby said loudspeakers emit sound signals simu
2. The invention as de?ned in claim 1 wherein said
transfer function circuit includes a signal processor for
lating a narrower perceived audible distance be
tween said loudspeakers than the physical distance
between said speakers.
imposing repeated phase reversal continuously along a
predetermined band of frequencies.
12. The invention as de?ned in claim 11 wherein each
3. The invention as de?ned in claim 2 wherein said
45 said ?rst and second means for delaying the signal com
signal processor includes a high pass ?lter.
prises a means for delaying the signal independent of
4. The invention as de?ned in claim 1 wherein said
frequency.
transfer function circuit imposes a delay upon the signal
13. In a stereo audio reproduction system having at
added from each said crossfeed path and further com
least one pair of two speakers physically spaced apart
for separated left channel output signal and right chan
nel output signal, and having a left channel input signal
and a right channel input signal, the improvement com-_
prising each of said left channel input line and said right
channel input line including a signal delay circuit for
delaying the signal added to the respective crossfeed
path.
prising:
5. The invention as de?ned in claim 4 wherein said
signal delay circuit includes means for imposing a pre
determined, frequency independent delay on said signal
in said respective input line.
55
6. The invention as de?ned in claim 1 and further
comprising a gain control for limiting the maximum
output to a ?at response over the audio signal frequency
range at said left channel output line and said right
channel output line by limiting at least one input to each
of said right adder and said left adder.
7. The invention as de?ned in claim 6 wherein said
gain control includes means for manually adjusting the
gain of each said signal added at said channel outputs. 65
8. The invention as de?ned in claim' 1 wherein each
said left channel input line communicates with a left
branch line having a low pass ?lter for adding low
means for delivering one of the left channel output
signal and the right channel output signal to one of
said speakers of a pair and delivering the other of
the left channel output signal and the right channel
output signal to another speaker of said pair; and
means for narrowing the psycho-acoustically per
ceived distance between said pair' of speakers by
crossfeeding a frequency weighted, delayed, non
inverted portion of said right channel input signal
as part of said left channel output signal and cross
feeding a frequency weighted, delayed, non
inverted portion of said left channel input signal as
part of said right channel output signal to obtain a
?uctuated frequency weighted coherence from
said pair of speakers.
11
5,420,929
14. An apparatus for processing stereo imaging of
stereophonic signals to be delivered to at least one pair
of loudspeakers comprising:
a left channel output line coupled to a ?rst speaker of
5
said at least one pair;
a right channel output line coupled to a second
speaker of said at least one pair;
a left channel input line;
a right channel input line;
channel input line,
'
a right adder for adding said left-to-right crossfeed
path to said right input line at said right channel
a right-to-left crossfeed path initiating at said right
channel input line,
channel input line;
a right adder for adding said left-to-right crossfeed
path to said right input line at said right channel
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan
output line;
nel output line;
a right-to-left crossfeed path initiating at said right
each of said crossfeed paths having a transfer function
channel input line;
circuit for repeatedly ?uctuating the phase of the
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan
respective input signal passing through the cross
nel output line;
each of said crossfeed paths having a transfer function 20
circuit for repeatedly ?uctuating the phase of the
respective input signal passing through the cross
loudspeakers; and
speaker of said atleast one pair;
a left channel input line;
a right channel input line;
a left-to-right crossfeed path initiating at said left
output line;
a left-to-right crossfeed path initiating at said left
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
12
a right channel output line coupled to a second
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
loudspeakers;
a gain control for limiting the maximum output at said
left channel output line and said right channel out
put line; and
25
wherein each said channel input line includes a shelv
ing ?lter transfer function for boosting the power
output of low frequency signal components.
15. The invention as de?ned in claim 14 wherein said 30
shelving ?lter comprises a branch line communicating
with said channel input line, having a low pass filter and
adding to said channel input line and the respective
-
wherein said gain control comprises a stereo detector
for controlling the crossfeed gain applied to the
signals added at said channel output line.
21. An apparatus for processing stereo imaging of
stereophonic signals to be delivered to at least one pair
of loudspeakers comprising:
a left channel output line coupled to a ?rst speaker of
said at least one pair;
a right channel output line coupled to a second
speaker of said at least one pair;
a left channel input line;
crossfeed path.
16. The invention as de?ned in claim 15 wherein said
low pass ?lter introduces a signal delay in said branch
a right channel input line;
line wherein the duration of said signal delay varies
with frequency, and wherein said channel input line
a left-to-right crossfeed path initiating at said left
includes an all pass ?lter with a time delay response
a right adder for adding said left-to-right crossfeed
path to said right input line at said right channel
channel input line,
corresponding to said signal delay for correcting the
shelving ?lter response.
output line;
17. The invention as de?ned in claim 15 wherein said
a right-to-left crossfeed path initiating at said right
low pass ?lter introduces a signal delay in said branch
line wherein the duration of said signal delay varies
with frequency, and wherein said branch line also in 45
cludes an all-pass ?lter to equalize the phase of the
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan
branch signals to obtain a frequency independent delay
in the branch line, and further comprising a frequency
independent delay in said channel input line down
stream of branch line to delay the channel input signal
channel input line;
nel output line;
each of said crossfeed paths having a transfer function
circuit for repeatedly ?uctuating the phase of the
respective input signal passing through the cross
50
an amount corresponding to the delay in said branch
line.
18. The invention as de?ned in claim 15 wherein said
low pass ?lter introduces a signal delay in said branch
line wherein the duration of said signal delay varies 55
with frequency, and further comprising a signal delay
circuit in said channel input line for imposing a constant
delay on the direct input signal.
19. The invention as de?ned in claim 18 and further
comprising a signal delay circuit in each crossfeed path 60
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
loudspeakers; and
a gain control for limiting the maximum output at said
left channel output line and said right channel out
put line;
wherein said gain control includes means for auto
matically adjusting the gain in response to the level
of stereo separation between said left and right
channel input lines.
22. The invention as de?ned in claim 21 wherein said
that is added to said channel input lines at said channel
output line to coordinate the phases of the signals added
at said channel output lines.
means for automatically adjusting the gain includes
of loudspeakers comprising:
of loudspeakers comprising:
means for proportionally adjusting the gain over at least
one predetermined range of stereo separation.
20. An apparatus for processing stereo imaging of
23. An apparatus for processing stereo imaging of
stereophonic signals to be delivered to at least one pair 65 stereophonic signals to be delivered to at least one pair
a left channel output line coupled to a ?rst speaker of
said at least one pair;
a left channel output line coupled to a ?rst speaker of
said at least one pair;
13
5,420,929
14
a right channel output line coupled to a second
speaker of said at least one pair;
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan
a left channel input line;
a right channel input line;
5
a left-to-right crossfeed path initiating at said left
each of said crossfeed paths having a transfer function
channel input line,
10
a right-to-left crossfeed path initiating at said right
loudspeakers; and
wherein each said left channel input line communi
cates with a left branch line having a low pass ?lter
for adding low frequency components to said left
adder circuit and said right channel input line com
channel input line;
a left adder for adding said right-to-left crossfeed
path to said left channel input line at said left chan~
nel output line;
each of said crossfeed paths having a transfer function
circuit for repeatedly ?uctuating the phase of the
respective input signal passing through the cross
circuit for repeatedly ?uctuating the phase of the
respective input signal passing through the cross
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
a right adder for adding said left-to-right crossfeed
path to said right input line at said right channel
output line;
nel output line;
municates with a right branch line having a low
15
pass ?lter for adding low frequency signal power
to said right adder circuit.
7
25. The invention as de?ned in claim 24 and further
comprising at least one gain control for limiting each of
the said signals added at said left channel and right
channel outputs.
feed path to vary the channel coherence of the
sound signal emitted from said at least one pair of
26. The invention as de?ned in claim 25 wherein said
loudspeakers;
a gain control for limiting the maximum output at said
left channel output line and said right channel out
25
put line; and
wherein said gain control includes means for manu
gain control includes means for automatically adjusting
the gain in response to the level of stereo separation
between said left and right channel input lines.
27. The invention as de?ned in claim 26 wherein said
gain control includes means for adjusting crossfeed gain
in each crossfeed path.
ally adjusting the gain of said signals added at said
28. The invention as defined in claim 27 wherein each
channel outputs.
of said left and right input lines include means for ad
24. An apparatus for processing stereo imaging of 30 justing the gain of the input signal added at said respec
stereophonic signals to be delivered to at least one pair
tive adder in correspondence with
of loudspeakers comprising:
a left channel output line coupled to a ?rst speaker of
said at least one pair;
a right channel output line coupled to a second 35
where G is the crossfeed gain in the crossfeed path.
speaker of said at least one pair;
29. The invention as de?ned in claim 28 wherein each
a left channel input line;
said
left and right branch line includes means for adjust
a right channel input line;
ing
the
gain of the branch line signal added at said re
a left-to-n'ght crossfeed path initiating at said left
W
channel input line,
a right adder for adding said left-to-right crossfeed
40
spective adder in correspondence with
path to said right input line at said right channel
output line;
a right-to-left crossfeed path initiating at said right 45 where G is the crossfeed gain in the crossfeed path.
channel input line;
*
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