Head related transfer function pseudo-stereophony

An apparatus for producing pseudo-stereophonic sound from a monaural signal including a monaural source having a first speaker disposed in an anechoic room and having a sound output generated by the monaural signal. Second, third, fourth and fifth speaker disposed in the anechoic room (substantially) symmetrically about a listener having two ears. The monaural signal from the source is processed to output processed signals to each of the second, third, fourth and fifth speakers, each speaker producing a sound output corresponding to the received processed signal. A pair of microphones are disposed in the ears of the listener for receiving the sound outputs of the first, second, third, fourth and fifth speakers and producing two differentiated audio channels.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates generally to the field of stereophony and, 
more specifically, to a head related transfer function pseudo-stereophony 
in which two differentiated audio channels are derived from a single 
channel. 
2. Description of the Prior Art 
"Pseudo-stereophony" is a generic description of a family of techniques 
that allow the derivation of two channels of sound from a 
monaurally-recorded, one channel source. Signal processing techniques for 
achieving pseudo-stereophony have existed for at least thirty-six years, a 
time period roughly paralleling the commercial introduction of home stereo 
systems. In general, these techniques all involve a process where the two 
channels of output are derived according to a decorrelation process, i.e., 
the monaural input is processed to each channel in a differential manner 
so as to create two non-coherent signals. One technique, known as 
differential filtering, uses a high-pass filter for output channel one and 
a low-pass filter for output channel two. Other differential filtering 
techniques have used two comb filters with complimentary frequency 
responses, or two all-pass filters with differential phase response. 
Another type of pseudo-stereophony can be referred to as reverberation and 
time delay techniques where, for example, the technique involves adding 
differentially time delayed, scaled versions of the signal to each output 
channel, with the possibility of adding recirculated sound. 
Yet another type of pseudo-stereophony involves differential sub-audio 
frequency modulation of the output channels. This is sometimes called 
"pitch shifting". 
The aforementioned techniques suffer from one or more drawbacks, including 
the overt "coloration" of the sounds timbre in each separate channel of 
the output. Also, with loudspeaker reproduction the listener is required 
to sit near the center of the speakers. Further, there is a general 
inability to mix the pseudo-stereo output to monaural without disturbing 
the timbral distortions. Also, with stereo reverberation techniques, the 
problem of additional reverberation time is appreciable. The sound becomes 
"muddy" (overly reverberant) as a result of convolving an originally 
reverberant signal with the reverberation of the signal processor (and 
eventually that of the listening space). 
The pinnae of the human ears are shaped to provide a transfer function for 
received audio signals and thus have a characteristic frequency and phase 
response for a given angle of incidence of a source to a listener. This 
characteristic response is convolved with sound that enters the ear and 
contributes substantially to our ability to listen spatially. This is 
known as the "head related transfer function" (HRTF). The HRTF was 
described by Jens Blauret in "The Psychophysics of Human Sound 
Localization", MIT Press, Cambridge, 1983. 
HRTFs have been mentioned in some U.S. patents. For example, U.S. Pat. No. 
4,388,494, issued to Schone et al describes a pseudo-stereophonic 
reproduction circuit utilizing HRTFs. Similarly, U.S. Pat. No. 4,359,605, 
issued to Haramoto et al describes pseudo-stereophonic signal generation 
utilizing HRTFs. 
U.S. Pat. No. 4,219,696, issued to Kooure et al, discloses stereo signal 
generation having localized sound images dependent on HRTFs. U.S. Pat. No. 
4,192,969 discloses HRTF-related stereophonic signal generation with 
gauged adjustable attenuators for variable frequency correlation and 
response. 
A continuing need exists for improved pseudo-stereophonic signal generation 
capable of producing two channel sound derived from a monaural source. 
SUMMARY OF THE INVENTION 
An object of the present invention is to derive two differentiated audio 
channels from a single channel based on the head related transfer 
function. 
Another object of the present invention is to provide a method and 
apparatus for producing pseudo-stereophony which avoids coloration of the 
sound timbre in each separate channel of a two channel output derived from 
a single channel source. 
Another object of the present invention is to provide a method and 
apparatus for producing pseudo-stereophony in which the listener is not 
required to be positioned in the center of two speakers. 
Yet another object of the present invention is to provide a method and 
apparatus for producing pseudo-stereophonic output channels which can be 
mixed to monaural (one channel) without disturbing coloration effects that 
result from phase cancellation. 
Still another object of the present invention is to provide a method and 
apparatus for producing pseudo-stereophony in which the sound image has an 
increased dimension of spaciousness. 
These and other objects of the present invention are met by providing an 
apparatus for producing pseudo-stereophonic sound from a monaural signal. 
The apparatus is based on the concept of a monaural source having a first 
speaker disposed in an anechoic room, the first speaker having a sound 
output based on the monaural signal, a second, third, fourth and fifth 
speaker disposed in the anechoic room substantially symmetrically about a 
listener having two ears, signal processing means for receiving the 
monaural signal from the source and outputting processed signals to each 
of the second, third, fourth and fifth speakers, each speaker producing a 
sound output corresponding to the received processed signals, and a pair 
of microphones, one disposed in each ear of a listener for receiving the 
sound outputs of the first through fifth speakers and producing two 
differentiated audio channels. Preferably, the signal processing means 
includes a gain unit which increases the monaural signal to the second, 
third, fourth and fifth speakers, and four variable delay units, one 
corresponding to each of the second, third, fourth and fifth speakers. 
In another aspect of the present invention, a method for producing 
pseudo-stereophonic sound from a monaural signal includes the steps of 
feeding a monaural input signal to an A/D converter to produce a digitized 
output signal, distributing the digitized output signal across six lines 
to produce six digital output signals, passing two of the six output 
signals respectively to left and right summation devices, passing each of 
the remaining four output signals through individual delay devices to 
establish four different delayed signals, multiplying each of the four 
different delayed signals by a common value, filtering each of the four 
multiplied signals through FIR filters having an impulse frequency 
response matching ipsilateral and contralateral magnitude responses of the 
head related transfer functions of an average listener and a linear phase 
response to produce left and right channel output signals, summing the 
left channel output signals with the unprocessed digital signal in the 
left summation device, summing the right channel output signals with the 
unprocessed digital signal in the right summation device, and converting 
the summed signals in a D/A converter to produce analog left and right 
channel signals. 
These and other features and advantages of the method and apparatus for 
producing pseudo-stereophonic sound according to the present invention 
will become more apparent with reference to the following detailed 
description and drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a pseudo-stereophony apparatus is generally 
referred to by the numeral 10 and includes a monaural sound source 12 
having an associated speaker 14. The speaker is driven by a sound signal 
produced by the source 12 to thereby generate audible sound waves. The 
speaker 14 is positioned in an anechoic chamber 16, such as a sound-proof 
room, directly in front of a listener 18 having a pair of ears. Four 
additional speakers 20, 22, 24 and 26 are placed at ear level around the 
listener 18 at 90.degree., 120.degree., 240.degree. and 270.degree. 
azimuth, respectively. These four additional speakers receive a processed 
version of the signal emitted from the monaural speaker 14. The loudness 
of the signal is first boosted by a gain unit 28 and then is fed in 
parallel to four individual variable delay units 30, 32, 34 and 36, each 
serving one of the four speakers 20, 22, 24 and 26. 
Two probe microphones 38 and 40 are placed inside the ears of a listener 
whose pinnae match the average shape and transfer function of an average 
listener. The two channel signal for the pseudo-stereophonic output is 
obtained from the output of the two microphones, which can be delivered to 
a left channel speaker 42 and a right channel speaker 44 which produce 
pseudo-stereophonic sound for a listener 46. 
Microphones are placed inside of the ears of the listener to capture the 
characteristic frequency and phase response for a given angle of incidence 
from a source to a listener. This characteristic response is convolved 
with sound that enters the ear and contributes substantially to our 
ability to listen spatially. This is known as the "head related transfer 
function" (HRTF), as described in the aforementioned article by Jens 
Blauret in his article "Spatial Hearing: The Phytophysics of Human Sound 
Localization" MIT Press, Cambridge, 1983, this article being incorporated 
herein by reference. 
It is possible to simulate the characteristic frequency and phase response 
described above by means of digital filtering according to the present 
invention. An effective simulation of the spatial dimensions of sound can 
be achieved by using filters whose impulse response and group delay 
characteristics match the free-field or monaural HRTF of the two ears of 
the listener. With speakers, spatial effects can also be obtained with 
HRTF filtering that are impossible to accomplish with other methods. This 
includes the ability to move the virtual sound image beyond the arc 
defined by stereo loudspeakers (so long as each ear is receiving a 
suitable amount of signal from each speaker). 
Referring now to FIG. 2, a signal processing arrangement according to the 
present invention mimics the heuristic description of the invention shown 
in FIG. 1. The monaural input signal is fed to an analog to digital (A/D) 
converter 48, typically having a 44.1 kHz sampling rate and 16 bit word 
length. The digitized signal is then distributed across six lines, 
numbered 1-6 in FIG. 2. Lines 1 and 6 pass their signal directly to left 
and right digital summation devices 50 and 52, while lines 2, 3, 4 and 5 
each pass through an individual digital delay device 54, 56, 58 and 60. 
These are either set to fixed values or are interactively set by the user 
as further described below. The value of time delay used for lines 2, 3, 4 
and 5 are all different, as will also be described in more detail below. 
The output of each delay device 54, 56, 58 and 60 is passed through an 
individual digital multiply device or gain devices 62, 64, 66, and 68. The 
value added to each signal by the gain devices is identical and are 
nominally set to a value whereby the signal of lines 2, 3, 4 and 5 are 
each increased by for example, six dB above the amplitude level of lines 1 
and 6. The value of the multiply can be adjusted by the user. 
The output of each gain device 62, 64, 66 and 68 is fed through a separate 
HRTF filter network 70, 72, 74, and 76. As shown in FIG. 3, each HRTF 
filter network includes two FIR (Finite Impulse Response) filters 78 and 
80 whose impulse frequency response matches the ipsilateral HRTFs of an 
average listener at 90, 120, 240 and 270 degrees azimuth, 0 degrees 
elevation. Each HRTF filter network takes a single input and derives two 
differentiated outputs, one for each output channel, using the two 
separate FIR filters 78, 80 shown in FIG. 3. The FIR filter 78 for the 
left output summed at 50 in FIG. 3 uses a particular frequency and group 
delay response for a particular angle of incidence, as shown in FIGS. 4 
and 5, or alternatively in FIGS. 6 and 7. The FIR filter 80 for the right 
output summed at 52 in FIG. 3 uses the frequency and group delay responses 
shown in FIGS. 4 and 5, or alternatively in FIGS. 6 and 7, of the 
symmetrically opposed angle of incidence of the HRTF measurement used in 
FIR filter 78. Specifically, referring to FIG. 2. HRTF filter network 70 
uses an HRTF measurement for 90 degrees for FIR filter 78 and 270 degrees 
for FIR filter 80-HRTF filter network 72 uses an HRTF measurement for 120 
degrees for FIR filter 78 and 240 degrees for FIR filter 80-HRTF filter 
network 74 uses an HRTF measurement for 240 degrees for FIR filter 78 and 
120 degrees for FIR filter 80; and HRTF filter network 76 uses an HRTF 
measurement for 270 degrees for FIR filter 78 and 90 degrees for FIR 
filter 80. 
The left output channel of each of the four HRTF filtering networks are 
summed by the digital summation device 50, along with the unprocessed 
output of the A/D converter from line 1, while the right output channel of 
each of the four HRTFs filter networks is summed by the digital summation 
device 52, along with the unprocessed output of the A/D converter from 
line 6. The summed output for the left and right channels are then passed 
through separate digital to analog (D/A) converters 82 and 84. 
The parameters of each part of the circuit illustrated in FIG. 2, such as 
time delays, gains, and HRTF filters, are designed in order to meet 
specific psychoacoustic criteria described below. 
The overall gain of the four delayed sounds are made louder than the direct 
sound through the use of the digital multiply or gain devices 62, 64, 66 
and 68. Each of the four delayed sounds is boosted by six dB. This was 
determined empirically as the best value for achieving the 
pseudo-stereophonic effect with a range of different sound materials, such 
as speech and music. This multiply could also be set by the user via knob 
setting so that at a minimum setting, the pseudo-stereophony would be 
effectively by-passed and at a maximum setting, of about 12dB, an 
exaggerated effect could be produced. For systems not allowing operator 
interaction, about 6dB would be the optimal fixed value. 
With respect to setting the time delays, each delay arrives at the 
loudspeakers differentially in time. The following temporal dimensions 
were taken into account for the setting of each value of delay: (1) the 
initial time gap between the undelayed sound and the first delay; (2) the 
time between each successive delay; and (3) the time of the final delay, 
which is a function of the first two parameters. An important 
psychoacoustic consideration is that the signals remain below the level of 
echo disturbance. Thus, the initial time delay gap, rather than the timing 
of the final delay, was found to be crucial in this regard. The acceptable 
time delay is also a function of the gain of the delay. For the gains used 
herein, a range of between 15 and 25 milliseconds (ms) was found useful 
for the initial time delay gap. 
The duration between each successive delay was selected to be within a 
range so that each delay would not be heard as a separate sound. For the 
circuit described herein, a range of 5 to 10 ms between each successive 
delay was found to be ideal. 
In order for head related transfer function to create a sensation of 
increased auditory spaciousness, 30 ms was determined as the minimum value 
for the timing of the final delay. This is in line with research into the 
effect of early reflections in concert halls. 
Based on these considerations, four sets of time delays were determined for 
minimum and maximum of delay produced by the delay devices 54, 56, 58 and 
60: 
______________________________________ 
Delay Delay Delay Delay 
54 56 58 60 
______________________________________ 
A: 15 20 25 30 
B: 15 25 35 45 
C: 25 30 35 40 
D: 25 35 45 55 
______________________________________ 
The first sets A and B use a 15 ms initial time delay, and sets C and D use 
a 25 ms time delay. Intermediate values could also be used, depending on 
the effect desired. For creating an effective sensation of stereo, A and B 
work better for speech but not as well for music as do C and D. This is 
because the threshold for echo disturbance for music is higher than 
compared to speech. A user interface would allow setting the initial time 
gap with a knob between 15 and 25 ms and the gap between successive 
reflections between 5 and 10 ms. 
There are three considerations in the implementation of the HRTF filters 
used: the angle of incidence synthesized for the particular delay; the 
sequence of arrival; and the phase and frequency response to the filters. 
The angles of incidence used are illustrated in FIG. 8. These angles are 
based on measurements made at 0.degree. elevation ("ear level"). These 
angles are chosen because of symmetry (90-270 and 240-120), location to 
the sides (to produce a stereo effect, filters measured at the locations 
of a virtual stereo speaker produces the best results), and the angles are 
maximally incoherent with regard to the unfiltered monaural source. The 
sequence is arranged so the sound delays alternate twice from left, then 
right. This is done so as not to weight the sound image towards one 
particular side. 
The FIR filters used according to the present invention are created with a 
filter design program, based on an algorithm described by McClellan, Parks 
and Rabiner in their article "FIR Linear Phase Filter Design Program" (in 
Programs for Digital Signal Processing (1979; New York, IEEE Press)). This 
is a commonly used algorithm available in several filter design software 
packages. Each filter uses a 75 tap symmetrical arrangement of 
coefficients, resulting in a linear phase response. Linear phase filters 
are desirable in that they do not distort the time response of the 
waveform. If all filters used are of linear phase, as with the HRTF filter 
network of the present invention, then there is no chance of the combining 
of the output signals in such a way that their time responses interact 
constructively or non-constructively. 
The pseudo-stereophony effect of the present invention can be implemented 
into hardware using off-the-shelf integrated circuit chips, such as a 
Motorola 56001 DSP chip. All of the processing described with respect to 
FIG. 2 can be accomplished with these or equivalent DSP chips. 
In the aforementioned embodiment of pseudo-stereophony, only four HRTF 
filter networks were used. However, a larger number of delay-gain-HRTF 
filter networks could be added in an alternative embodiment. Fewer or 
greater number of coefficients used to represent the HRTFs used in FIR 
filters 78 and 80 could also be part of an alternative embodiment. 
Other embodiments of the invention may or may not provide a user interface 
for gain level and time delays, as mentioned above. 
The results of the present invention provide advantages over the prior art. 
For example, the signal from each channel sounds less "colored" (i.e., 
timbrally altered) than with previous methods. Also, a pseudo-stereo image 
can be had from a wide range of listening positions, as a result of the 
complex frequency response interaction of the filters. Previous methods 
require a more fixed listening position. The two output channels of the 
present invention can also be mixed to monaural (one channel) without 
disturbing coloration effects that result from phase cancellation. 
Moreover, the sound image has an increased dimension of spaciousness. This 
results from the resultant decorrelation of the two output signals. The 
present invention also allows multiple inputs with differential frequency 
responses to be more easily distinguished from one another, compared to 
monaural listening. 
Numerous modifications and adaptations of the present invention will be 
apparent to those so skilled in the art and thus, it is intended by the 
following claims to cover all such modifications and adaptations which 
fall within the true spirit and scope of the invention.