Data processing: speech signal processing – linguistics – language – Audio signal bandwidth compression or expansion
Reexamination Certificate
1999-06-29
2002-10-15
Dorvil, Richemond (Department: 2654)
Data processing: speech signal processing, linguistics, language
Audio signal bandwidth compression or expansion
C704S270000
Reexamination Certificate
active
06466913
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method of determining a sound localization filter for approximation of a head related transfer function, and also relates to a sound localization control system incorporating the sound localization filter.
(2) Description of the Related Art
A technique of sound localization is known. In this method, a pair of microphones are provided at the positions of the two ears of a dummy head to record the original sound emitted from a sound source in a first space where the dummy head is arranged. The reproduced sound, obtained by reproducing the recorded sound, is supplied to a pair of headphone speakers provided at the positions of the two ears of a listener. By using this method, the listener can hear the reproduced sound as if the source of that sound was located, in a second space where the listener stays, at the same position as that of the actual sound source in the first space. This technique is called the sound localization.
Japanese Laid-Open Patent Application No.2-298200 discloses a technique of sound localization control which uses either an analog filter or a digital FIR (finite impulse response) filter. In the method of the above publication, the amplitude and the phase of binaural signals are controlled through signal processing so as to control the sound localization. The original sound emitted from the sound source is analyzed in the frequency domain, and the frequency-dependent amplitude difference and phase difference are applied through signal processing to the binaural signals of right and left channels which are supplied to the headphone speakers of the listener. By using the method of the above publication, the localized position of a simulated sound source within the second space relative to the position of the listener can be shifted to a desired position through the signal processing. In other words, the sound localization can be controlled by using the method of the above publication.
In order to realize the sound localization control, a sound localization filter must be adapted for approximation of a head related transfer function. FIG.
1
A and
FIG. 1B
are diagrams for explaining a head related transfer function used for the sound localization control.
FIG. 1A
shows a binaural system having a dummy head provided in a first space. In the system of
FIG. 1A
, a pair of microphones of the R (right) and L (left) channels are provided at the positions of the two ears of the dummy head to record the original sound emitted from a sound source in the first space where the dummy head is arranged. The reproduced sound, obtained by reproducing the recorded sound, is supplied to a pair of headphone speakers of the R and L channels provided at the positions of the two ears of a listener in a second space.
FIG. 1B
shows a binaural system including a pair of sound localization (S/L) filters
101
and
102
. The S/L filters
101
and
102
are provided between the microphones of the first space and the speakers of the second space for approximation of right-channel and left-channel head related transfer functions HRTF-R and HRTF-L. The system of
FIG. 1B
simulates the functions of the system of
FIG. 1A
by using the S/L filters
101
and
102
.
In the system of
FIG. 1B
, the original monaural signals originated by the actual sound source in the first space are processed through the S/L filters
101
and
102
so as to shift the localized position of the simulated sound source within the second space relative to the position of the listener, to a desired position. In order to realize the sound localization control, measurements of the frequency characteristics of the head related transfer functions (the HRTF-R and HRTF-L) for each of a set of predetermined direction angles about the front position of the listener are needed. In the system of
FIG. 1B
, a plurality of sets of filter coefficients of the S/L filters
101
and
102
which represent the measured characteristics for all the predetermined direction angles are retained in a memory, and one of the sets of filter coefficients is selected according to the desired direction angle for the localized position, so as to apply the selected coefficients to the S/L filters
101
and
102
.
“A Study on Clustering Method of Sound Localization Transfer Function” of the Institute of Electronics, Information And Communication Engineers (IEICE), EA9301 (1993.4), by S. Shimada and others, teaches a method of determining the sound localization transfer function by measurement of the impulse response of a digital filter to white noise generated in a given environment.
FIG. 2
shows measurements of frequency characteristics of a head related transfer function with respect to a set of predetermined direction angles about the front position of a listener. In
FIG. 2
, the curve of 0° indicates the measured frequency characteristics for the front position of the listener, and the curves of 0° through 120° indicate the measured frequency characteristics for the set of predetermined direction angles 0° through 120°.
A sound localization (S/L) filter is realized by storing a plurality of sets of filter coefficients of a digital filter, which represent the measured filter characteristics, such as those of
FIG. 2
, for all the predetermined direction angles in a memory of a sound localization control system. One of the sets of filter coefficients stored in the memory is selected according to the desired direction angle for the localized position, so as to apply the selected coefficients to the digital filter. Hence, the sound localization control is possible by using the sound localization control system having the digital filter.
However, in a conventional sound localization control system having a digital filter, the sets of filter coefficients stored in the memory of the system are fixed to the measurements of the frequency characteristics of the digital filter in the given environment. It is impossible for the conventional sound localization control system to freely change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners.
Japanese Laid-Open Patent Application No. 5-252598 discloses a sound localization control system using a digital FIR (finite impulse response) filter. In the system of the above publication, a set of vectors of filter coefficients of the digital filter which represent typical filter characteristics, including the impulse responses of spatial transfer functions and the transfer functions of headphones, are obtained by using a clustering method of vector quantization, and such vectors of filter coefficients are stored in a database. However, the filter coefficients depend on the environments and the listeners used for the measurement, and it is difficult to change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners.
Further, the sound localization control system of the above-mentioned publication requires a large size of the hardware including the FIR filter and the database, and requires a computational complexity of signal processing. On the other hand, a digital IIR (infinite impulse response) filter can have a small size of the hardware with the coefficient memory, and makes it possible to easily change the stored filter coefficients so as to suit the filter characteristics to various environments or the individual listeners. However, a technique which designs a digital IIR filter for approximation of a transfer function with complex frequency characteristics, such as those of
FIG. 2
, is not yet established. In addition, it is desirable that the digital IIR filter is efficient in achieving the sound localization control. Generally, it is difficult to achieve complex frequency characteristics of a head related transfer function with a digital IIR filter, and a digital IIR filter is likely to become unstable due to limit cycle oscillation.
It has been reported that, when design
Kasuga Masao
Yasuda Seigou
Cooper & Dunham LLP
Dorvil Richemond
Opsasnick Michael N.
Ricoh & Company, Ltd.
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