Stereo image separation and perimeter enhancement

A stereo enhancement system utilizes a difference signal derived from the left and right stereo channels in which the difference signal is delayed, amplified, and then added into the appropriate channels to cancel left/right speaker mixing at the listener's ears, thereby to improve stereo separation without center region distortion. Depending on the amplification level of the difference signal, an increase in the perimeter sound over that produced at the central region gives a "wrap around" effect with only two speakers by increasing the volume of only the left/right directional sound components relative to centrally located sounds which have no left/right directionality. In order to compensate for the angle between the listener and the speakers, a continuously variable delay is provided in the 100 microsecond -1 millisecond range by the utilization of a serial-analog delay bucket brigade device. Ultrashort delays are created, in one embodiment, by utilizing equal delays in the left and right stereo channels. The difference signal is then delayed by an amount equal to either of the two initial delays plus the amount of desired delay. When serial analog delay devices are utilized, the clock rate of the delay line used for the difference signal is made slightly lower than the clock rate of the delay lines utilized in initially delaying the left/right signals, thereby to provide for ultrashort delayed difference signals. Off-center listener positions are accommodated by a further continuously variable delay line in one or the other of the left and right channels.

FIELD OF INVENTION 
This invention relates to stereo reproduction techniques and more 
particularly to a method and apparatus for either increasing stereo 
separation or enhancing perimeter sound images, or both. 
BACKGROUND OF THE INVENTION 
In the past, complete separation of stereo signals has been accomplished 
through the utilization of ear phones in which a left channel is applied 
to the left ear phone and the right channel is applied to the right ear 
phone. While in 1960 Benjamin Bauer of CBS Laboratories described an 
effort to make ear phones sound more like loud speakers by simulating the 
delayed leakage of the left speaker output to the right ear of a listener 
and vice versa, up until recently, there has been very little effort made 
to increase stereo separation when utilizing loud speakers. 
It will be appreciated that there is a certain amount of mixing of sounds 
from the left and right speakers at the listener's ears due to the fact 
that sounds from the left speaker, for instance, will arrive first at the 
left ear, then at the right ear of a listener. The sounds which arrive at 
the right ear are mixed with sounds arriving at the right ear from the 
right channel speaker. Thus the separation which was initially established 
by virtue of recording techniques is in some sense destroyed since one ear 
can hear sounds produced by both the stereo speakers. 
While Benjamin Bauer did suggest the aforementioned method to make ear 
phones sound more like loud speakers, he also suggested the reverse; that 
it would be possible to cancel stereo mixing when using loud speakers and 
suggested that this would give loud speakers the separation of head 
phones. 
Subsequent work on stereo loud speaker separation was done in Germany by 
Damaska et al. In the Damaska et al system, the right signal is delayed by 
the transit time for a sound to cross the listener's face to the left ear. 
This delayed signal is then frequency contoured, inverted and added to the 
left speaker signal. The correction signal in the left speaker is then 
used to acoustically cancel the right speaker leakage at the left ear. The 
same is done between the left and right channels such that acoustic 
cross-talk between the ears is presumably cancelled. 
One of the basis difficulties with delaying the left channel or right 
channel speaker signals is that there is a broad central region in which 
there is a "muddying" effect. This effect is due to the utilization of the 
full left and right channel signals. Note, in the central region no 
left/right information exists and this signal is essentially a monaural 
signal. As a result, not only is there frequency distortion, but also 
volume attenuation. The frequency distortion is due to a comb filtering 
effect in which the levels of various frequency components are increased 
or decreased in the central region. Thus the clarity and fidelity of the 
original recording is lost for central region produced sound. 
Moreover, in most recordings the most important sound producing instruments 
or performers create their sound at center stage. Thus the muddying or 
frequency distortion of sounds at center stage creates significant 
problems in a stereo reproduction system when utilizing the aforementioned 
enhancement technique. 
SUMMARY OF THE INVENTION 
Rather than utilizing the full left or right channel signal, delaying it, 
inverting it and then inserting it in the opposite channel, the subject 
system achieves separation by utilizing a left minus right difference 
signal, delaying it, frequency contouring it, and inserting it into the 
left channel and its inverse into the right channel. In one embodiment, 
this is accomplished by feeding the left and right channels to a 
difference amplifier, the output of which is delayed by a bucket brigade 
device such as a serial analog delay, which is clocked at the appropriate 
rate for the required delay. The output of the delay unit is then 
amplified, and then coupled to a phase splitter which inserts right minus 
left channel signals into the right channel and left minus right channel 
signals into the left channel. In so doing, sounds which have no 
left/right directionality, such as those emanating from center stage, are 
not delayed or inverted and in fact are not utilized. This leaves the 
center stage sounds completely undistorted, with no comb effect being 
superimposed. 
In one embodiment, the types of delays to be produced are ultrashort and on 
the order of between 100 microseconds and 1 millisecond. These delays are 
not easily produced. They are produced in the subject system by the 
utilization of a serial-analog delay (SAD) such as a bucket brigade 
device. These delay devices are clocked, in one embodiment at close to 3 
MegaHertz, which is ten times the normal rate, to achieve delays as short 
as 100 microseconds. In an alternative embodiment, in order to achieve a 
further shortness in the acoustic delay or allow use of slower SADs, SADs 
of equal delay are inserted into the left and right channels with a third 
delay used for the difference signal. The third delay may be provided by 
an SAD device which is clocked at a rate slightly lower than the rate at 
which the delay lines for the undifferenced signals are clocked, with the 
difference in clocking rates determining delays as short as ten 
microseconds or less. 
By the utilization of variable clock rates, continuously variable delays 
may be utilized which can compensate for listener position between the two 
speakers. This is an important aspect of the subject invention, since 
fixed delays will produce the required separation only at a given angular 
location relative to the two speakers. Rather than having to drag around 
the speakers, a variable delay provides that a listener may provide for 
maximum separation regardless of his angular relationship to the speakers. 
In one embodiment, the delay is set by merely determining the ratio 
between the distance that separates the two speakers versus the distance 
between the left speaker and the listener's left ear. 
For off-center listening in one embodiment, a broad band delay may be 
utilized in one channel to compensate for a listener being closer to one 
of the two speakers. 
It has been found, that in addition to achieving excellent separation, a 
"wrap around" sound can be achieved which is akin to the positioning of as 
many as one hundred speakers around the periphery of the room. This effect 
is achieved with only two loud speakers. In the subject system this is 
accomplished by amplifying the output of the delay circuit beyond the 
level required for image separation. What this achieves is as much as a 
ten dB volume increase in the perimeter or peripheral sound level over the 
central sound level, since it is only the left/right components which are 
amplified and inserted in this manner. This peripheral enhancement is 
achieved without center image distortion and may be varied by the listener 
to his own listening requirements. 
Further, it has been found that peripheral enhancement is effective to an 
adequate extent at positions removed from the position at which separation 
enhancement is maximal. Thus, peripheral sound enhancement may be achieved 
for listeners in the room at different positions, although the separation 
effect will be diminished for positions at the room's perimeter. 
The image separation and peripheral sound enhancement corresponds to 
speakers capable of projecting sounds at 180 degrees as opposed to the 
40-60 degrees of typically placed speakers with respect to the listening 
position. Thus, in one aspect of the subject invention, the utilization of 
the subject system results in a spreading of the apparent audio source 
beyond the angle of the loud speakers toward that of the original sounds 
with respect to the microphones originally used in the recording. Not only 
is there a left/right spreading effect, but also there is better spreading 
of the forward and rear portions of the performance, such that a total 
immersion in sound is achieved by this process. 
By so doing, the subject system does not affect the frequency response of 
the central image and therefore monaural or centrally located equal volume 
and phase signals in each channel are not adversely affected. 
It will be appreciated that most rumble is produced by the vertical 
movement of the stylus which corresponds to the left/right signal, while 
very low frequencies contain no left/right components. By utilizing a high 
pass filter having a low frequency cut off of approximately 70 Hertz, for 
the difference signal, rumble which is produced by vertical motion of the 
stylus in the recording or reproduction of a record is not accentuated in 
the subject system while bass response is not affected.

DETAILED DESCRIPTION 
Referring now to FIG. 1, in a typical prior art system, a stereo unit 10 
which may be a receiver, an amplifier, or any type of equipment which 
produces left and right channel signals, is coupled via lines 12 and 14 
respectively to left and right speakers 16 and 18. With respect to left 
channel to right channel cancellation, a line 20 is coupled from the left 
channel line 12 to a delay unit 22 which delays signals on this line by an 
amount .DELTA.. The output of delay unit 22 is applied to an inverter 24, 
the output of which is applied to a mixer or summing point 26. 
Assuming that the full left channel signal is indicated by the letter L, 
then the output of the inverter 24 is -L. 
Assuming a listener 30 located midway between speaker 16 and 18 and 
assuming that the individual has a left ear 32 and a right ear 34, then, 
as can be seen from the diagram, sound may reach left ear 32 from speaker 
16 along a line 36. Sound from this speaker will also reach right ear 34 
along a line 38 as it crosses the face of the listener. It will be 
appreciated from inspection of this diagram that the sound transmitted to 
the right ear travels a longer distance than the sound transmitted to the 
left ear and will be delayed at the right ear by an amount .DELTA. 
corresponding to the difference in path length between the sound paths to 
both of the ears. 
As can also be seen, sound impinges on right ear 34 from speaker 18 along a 
path 40 such that there exists at the right ear a certain amount of mixing 
of left and right channels. This destroys stereo separation to a certain 
extent. 
In order to eliminate such mixing, delay unit 22 and inverter 24 couple an 
inverted left signal into the right channel so as to exactly cancel the 
left channel signal at the right ear. Thus at the right ear, a delayed and 
inverted left channel signal arrives at the same time as the right channel 
signal, with the delayed inverted left signal cancelling the signal which 
arrives from the left speaker. 
In so doing, a distortion region 42 is created which occupies generally the 
center region between the two speakers in which sound in this region is 
"muddied". What this means is that there is frequency distortion due to a 
comb filtering effect because the entire left channel signal is inverted, 
delayed, and mixed with the right channel signal. 
It will be appreciated that for cancellation at the left ear, a reverse 
type situation is envisaged in which the right channel signal is delayed 
by .DELTA., inverted and then coupled into the left channel. Thus the 
distortion region is symetric about a center line between the two 
speakers. 
While stereo separation is in fact enhanced by the system illustrated in 
FIG. 1, signals lacking left/right directionality will be significantly 
attenuated and distorted in the central region illustrated. 
Referring now to FIG. 2, a system is illustrated in which not only is there 
distortionless stereo separation enhancement, but also perimeter 
enhancement. 
In the embodiment illustrated, stereo unit 50 is coupled to left and right 
speakers 52 and 54 respectively, via respective lines 56 and 58. A 
difference amplifier 60 is coupled across lines 56 and 58 such that a left 
minus right difference signal is coupled to a delay unit 62 which delays 
the signal by an amount described hereinabove. Thereafter, the signal from 
the delay unit 62 is amplified at 64. The signal from the amplifier 64 is 
applied to a phase splitting circuit 66 which inverts the amplified signal 
and couples right minus left directional signals to a summing node or 
mixer 68 interposed in the right channel, and left minus right channel 
signals into a summing node or mixer 70 interposed in the left channel. A 
listener 72 having a left ear 74 and a right ear 76 is located along a 
center line 78 which defines the midpoints between the left and right 
speakers. 
In operation, the system responds to the difference signal between the left 
and right channels, delays it by an amount .DELTA., and in some cases 
amplifies it at which point it is applied to the phase splitter. By this 
operation, that which is subtracted from the right channel is the purely 
directional left channel signal, whereas that which is subtracted from the 
left channel is the purely directional right channel signal. It will be 
appreciated that monaural signals having no left/right information, or 
signals coming from center stage which have no left/right information, 
will be ignored by this system. Thus the central region 80 is one in which 
the subject system has no effect and in which no distortion occurs. 
With proper adjustment of the amplifier section of unit 64 sounds coming 
from the periphery can be made to have a level equal to that of sounds 
which originated from center stage. This results in extremely effective 
stereo image separation and eliminates close to if not all of the 
cross-mixing of signals from the two speakers at the listener's ears. 
Assuming listener 72 is located along the center line, then the amount of 
delay .DELTA. is established by measuring the distance between the two 
speakers and by measuring the distance from the left speaker to left ear 
74. This ratio establishes the optimum delay via the geometry and if the 
delay is easily variable as will be discussed, adjustment may be either in 
accordance with this ratio or when the listener hears maximum separation 
in the stereo signals. 
An alternative method of establishing delay .DELTA. is to measure the angle 
between lines 84 and 86 and to establish the above-mentioned ratio by the 
utilization of the indicated trigometric relationship. 
It is a finding of this invention, that not only is separation enhanced in 
this distortionless fashion, but also peripheral sound which exists about 
the perimeter may be artificially enhanced to give an exceedingly pleasing 
effect of "wrap around" sound. This is accomplished by amplifying the 
signal from the delay line by an amount in excess of that which 
corresponds to the level of sound which exists in the central region. This 
may be increased by as much as ten dB to give the effect of having been 
surrounded completely by the sound. The effect, as noted hereinbefore, is 
equivalent to having the whole perimeter of the room lined with speakers, 
each projecting inwardly such that sound is projected as illustrated by 
arrows 82 towards the center of the room and towards the listener. 
It will be appreciated that it is the perimeter sound which is amplified 
due to the difference signal processing. In short, since monaural or 
directionless signals are not amplified, it is the signals which have a 
directionality, either left or right, which are amplified and inserted 
into the left and right channel speakers. 
Referring now to FIG. 3, an expanded block diagram is utilized in 
describing the subject system. In this case, a differential amplifier 
performing as a left/right discriminator is illustrated at 90 to which is 
coupled a left input and a right input as illustrated. The output of the 
left/right discriminator 90 is a left minus right signal which is applied 
to a rumble reducing high pass filter 92, to an anti alias low pass filter 
94 and finally to a delay line which, in one embodiment, is a 
serial-analog delay such as manufactured by Reticon Corporation. This is 
basically a bucket brigade device which is clocked via clock 100 under 
control of a variable control unit 102. The output of delay line 96 is 
applied to a low pass filter 98 which removes clocking components and 
lowers the high frequency content to compensate for the different response 
of the intended and other ear. The resultant signal is then applied to a 
variable gain amplifier 104 under control of a control unit 106. The 
output of the variable gain unit is applied to a by-pass switch 110 and 
thence to a simple phase splitter 112 to generate the left minus right and 
right minus left correction signals. The output of phase splitter 112 is 
applied to mixers 114 and 116 respectively in the left and right channel 
lines. In this embodiment, the output of each mixer is applied 
respectively to an additional amplifier 120 and 122 which are variable 
gain amplifiers under control of a master control 124. The outputs of 
these amplifiers are then applied to external left and right power 
amplifiers and speakers. 
It will be appreciated that utilizing the serial analog delay unit for 
delay unit 96 produces a delay which is continuously variable. The 
delaying of acoustic signals for ultrashort delay times is not an easy 
matter and has in the past been done with lumped constant elements. These 
circuit elements provide delays which are in general fixed at the factory. 
Therefore there is no ready adjustability for listener angle with respect 
to the speakers. 
In the subject system, however, this delay may be continuously varied so 
that maximum separation at a wide range of listener positions may be 
achieved. Thus adjustment unit 102 is in effect a cross-talk cancellation 
null adjusting unit and may be adjusted via remote control by the listener 
as he listens to his stereo recordings. 
Low pass filter 94 is interposed to take out ultrasonic signals that might 
beat with the delay clock frequency and high pass filter 92 is interposed 
so as to remove rumble under seventy Hertz. Note, as a conventionally 
accepted principle there is very little left/right information below one 
hundred or so Hertz. Thus by providing the high pass filter with a 
roll-off at seventy Hertz, no right/left information or overall bass level 
is lost in the process. This corrects for rumble which is usually produced 
by vertical stylis action within the grooves of the record, produced 
either during recording or reproduction. 
Referring now to FIG. 4, this embodiment is utilized for the off-center 
listener. This off-center listener may be located for instance as in FIG. 
2 at position 130. The off-center position may be necessitated by 
furniture in the room or the inability to locate speakers in precisely a 
spot which would place the usual listening position at a point midway 
between the speakers. As can be seen, units 90, 92, 94, 96, 98 and 104 
have been duplicated for each of the channels as illustrated by units 90', 
92', 94', 96', 98' and 104'. In this embodiment, a phase splitter is not 
utilized to separate the signals to be fed back to the right and left 
channels. This function is accomplished by duplication of the various 
units. They function in the same manner and are adjusted as by 
potentiometers R1 and R2 for a listener located on a center line at some 
distance from the speakers as measured along a line perpendicular to the 
line joining the two speakers. 
Variable gain amplifiers 104 and 104' are controlled respectively by 
potentiometers R3A and R3B to achieve whatever gain is required either for 
enhancement or for absolute stereo fidelity. 
The outputs of variable gain amplifiers 104 and 104' are coupled 
respectively to mixers 134 and 134' coupled into the left channel right 
channel lines respectively. The mixers may themselves be amplifiers and 
their amplitude is adjusted through potentiometers R4A and R4B as 
illustrated. 
Assuming that the listener is off-center to the left as illustrated, the 
output from mixer 134 is applied to a low pass filter 136, to an 
additional serial analog delay line 138 and thence to a further low pass 
filter 140 from which point it is applied as the left channel output via 
switch S1. 
Delay 138 is clocked by a clock 142 controlled by potentiometer R5 and has 
a delay greater than or equal to ten microseconds in one illustrative 
embodiment. 
The purpose of delaying the left channel signal over the right channel 
signal is that the off-center listener in the above case is closer to the 
left channel speaker. Thus he will be receiving signals from the left 
channel speaker before signals from the right channel speaker arrive. In 
this manner, the system may be adjusted for off-center listeners, with the 
right channel being provided with the same type of delay unit should the 
listener be off-center to the right. 
Referring to FIG. 5, as has been mentioned hereinbefore, it is with some 
difficulty that ultrashort acoustic delays are achievable. This is because 
in standard low noise SAD lines there are 512 different charge transfer 
elements such that the delay line is said to be 512 units long. 
In order to utilize these standard delay units, and with their ultimate 
limit in clocking frequency, it is possible, in the subject case and in 
others, to achieve shorter delays through the utilization of three of 
these devices. 
In this embodiment, analog delay lines 150 and 152 are interposed in the 
left and right channel lines as illustrated and are clocked identically, 
for instance, at a clock rate of 128 kiloHertz. This gives a two 
millisecond delay for each line. The left and right inputs are also 
coupled to a differential amplifier 154, the output of which is coupled a 
third analog delay line 156. This analog delay is clocked at a rate 
slightly less than delays 150 and 152, for instance at 122 kiloHertz. This 
provides for a 2.1 millisecond delay such that the overall delay of the 
left minus right signal with respect to the left and right signals is 0.1 
millisecond or 100 microseconds. It can thus be seen that relatively short 
delays can be achieved between any two signals by the use of this 
differential delay technique. 
It will be appreciated that in the embodiment shown in FIG. 5, the delayed 
left or right channel signals and the difference channel may be 
appropriately mixed to achieve the stereo enhancement. 
In general, this technique may be utilized when it is desirable to delay 
any one signal with respect to another and the desired delay is too short 
to be achieved through the clocking of an existing serial analog delay 
line. In this case, the delays in each of the lines are made different so 
the desired delay is obtained as the differential. 
Note, capacitors 158, 159 and 160 are used as low pass filters to remove 
all clock fundamentals, thereby to avoid clock difference frequencies from 
appearing in the outputs. 
This sysem is useful when bandwidth is not critical, i.e., when the maximum 
signal frequency is much much less than the clock frequency. Thus the 
system is useful for short delays at bandwidths less than 1/3rd the clock 
frequency. 
Having above indicated a preferred embodiment of the present invention, it 
will occur to those skilled in the art that modifications and alternatives 
can be practiced within the spirit of the invention. It is accordingly 
intended to define the scope of the invention only as indicated on the 
following claims.