Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Reexamination Certificate
1999-05-05
2004-03-02
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
C381S001000, C381S061000
Reexamination Certificate
active
06700980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to a device and method for synthesizing a virtual sound source.
2. Discussion of Related Art
In stereophonic sound reproduction, the objective is to transmit a realistic sound image to the listener by means of two sound channels. In conventional stereo reproduction, the direction of incidence of the sound is determined by the amplitude and phase ratios of the sound signal on different channels. Thereby the direction perceived by the listener as the direction from which the sound is coming, is always in the area between the loudspeakers or in the direction of either of the loudspeakers.
The conventional stereo effect achieved by two loudspeakers is limited, especially when the loudspeakers of the left and right channel are close to one another, as in a television set or a portable stereophonic radio cassette recorder, for example. When both loudspeakers are almost in the same direction with respect to the listener, there are no very distinct differences in the perceived sound direction.
The increase of multimedia applications that followed the growth of the computation capacity of personal computers has increased the need for a more advanced sound reproduction than the conventional stereo reproduction, which would be able to offer the listener a more realistic three-dimensional sound environment than before. A well known method to expand the capability of a sound reproduction system to represent sound direction is the use of several sound channels and loudspeakers, which is familiar from cinemas, for example.
Man perceives the direction of the incoming sound mainly by means of interaural time differences (ITD) and interaural level differences (ILD). In a two-channel sound reproduction system, it is in principle possible to simulate all the directions of the sound by changing the above mentioned factors. In this way, it is possible to create an impression that the sound comes from a direction outside the pair of loudspeakers.
In order to create the desired differences in the desired ITDs and ILDs of the sounds, so called HRTF (Head Related Transfer Function) filters are used in this method. HRTF filters mean transfer functions specified by measurement or calculation, which describe the filtering of a sound coming from a certain direction, mostly due to the effect of the shape of the head and external ear. By means of HRTF filters, it is possible to create an artificial sound image of a virtual sound source in stereophonic loudspeaker reproduction, if crosstalk from each loudspeaker to the opposite ear is taken into account in calculation.
FIG. 1
shows the known first filter system
10
for implementing a sound image based on at least one virtual sound source. The first filter system
10
consists of a first filter block
17
, which contains four parallel filters
11
,
12
,
13
and
14
, by means of which the signals Xl and Xr brought to the system are filtered in order to create a spatial effect, and two summing devices,
15
and
16
. Both channels include two filters, one of which functions as a HRTF filter
11
;
14
, and the other as a crosstalk cancellation filter
12
;
13
.
If the sound sources are placed symmetrically around the listening position, a corresponding system can be implemented more efficiently by another filter arrangement
20
shown in FIG.
2
. In this implementation, the filters
11
,
12
,
13
and
14
have been replaced by a first
24
and a second spatial filter
25
, whereby the expansion can be implemented with only two filters. When the objective is to use a system in which the properties of the filters
24
,
25
can be adjusted separately, the filters
24
,
25
can be connected to a separate filter control circuit
28
, by means of which the filtering of the signals can be changed in order to change the sound image.
A problem in the methods described above is constituted by the HRTF filters' complicated phase and frequency response properties. In stereophonic sound reproduction this is not a problem, because the desired spatial effect is achieved by these properties. If the signals being processed also contain monophonic signal components, the filters cause harmful distortions, because the hearing direction of the monophonic signal component need not be changed. In systems like this, the monophonic signal sounds colored. In principle, the distortion of the monophonic signal component could be corrected by adding one more filter stage to the system output, but this in turn would distort the desired spatial effect.
In this patent application, monophony means coherence between the signals of at least two channels. In a two-channel system, this means that coherence can be perceived in the signals of both channels. In a system with more channels, the monophony must be defined separately for each channel pair. Thus it is possible that the sound image contains multiple monophonic signals simultaneously.
Correspondingly, the stereophony of a signal means the portion of a signal of at least two channels between which there is no coherence. According to the above definition, it is possible that the signal consists partly of a monophonic and partly of a stereophonic signal.
FIG. 3
depicts a third filter arrangement
30
according to the patent application FI 962181, in which a third filter
31
has been added to the second filter block
21
according to
FIG. 2
, the delay properties of which filter correspond to the spatial filters
24
and
25
. The second filter block
21
, the third filter
31
added to it and the summing devices
36
and
37
together constitute the third filter block
34
. In the solution according to the reference publication, sum and difference signals are calculated from the signals coming to the system in the device
32
. The strength of the sum signal received is changed with amplifiers
33
. The signal after the amplifiers
33
is used as an approximation of the monophonic signal contained by the channels. This approximation of the monophonic signal is subtracted from the signals of both channels, whereby essentially only a stereophonic signal remains in each channel. After this, the stereophonic signal is led to the second filter block
21
in order to produce a spatial effect, and the monophonic signal is led via the third filter
31
past the second filter block
21
to be summed back to the signals coming from the outputs of the second filter block
21
.
The solution according to the patent specification FI-962181 does not entirely eliminate the colorization of the monophonic signal. In addition, a preadjusted constant value is used in this solution to reinforce the sum signal that approximates to the monophonic signal, whereby it is assumed that the ratio of monophonic and stereophonic signals remains constant. In reality, the ratios between stereophonic and monophonic signal components can vary considerably in a typical music recording, for example, which in a system based on that solution causes incomplete filtering, which is perceived as discrepancies and errors in the sound image produced.
SUMMARY OF INVENTION
It is the objective of this invention to achieve a new method and device for synthesizing a virtual sound source, by which the problems of the prior art described above can be eliminated.
In a method according to a first aspect of the invention, a virtual sound source is synthesized in a system which includes at least a right and a left channel for transmitting signals, and a filter block containing at least one filter and amplifier, through which the signals are conducted, is connected to the channels.
According to the first aspect of the invention, the stereophony of the signals fed to the filter system is determined by means of a mono/stereo estimator. According to this estimation, amplification coefficients are specified for the signals received from each filter, on the basis of which coefficients the signals received from filters are amplified.
In one embodiment of the method according to the invention, the stereophony of the sign
Hämäläinen Matti Sakari
Holm Jukka
Isen Forester W.
McChesney Elizabeth
Nokia Display Products Oy
Ware Fressola Van Der Sluys & Adolphson LLP
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