 its counterpart crosstalk cancellation both rely on the geometry of the loudspeakers and listening area and so must be individually adjusted for each case.
It is clear that attempted refinements of the stereo system have not produced great improvement in the systems now in widespread use for entertainment. Real listeners like to sit at ease, move or turn their heads, and place their loudspeakers to suit the convenience of room layout and to fit in with other furniture.
Thus, it is an object of the present invention to provide a method and apparatus for processing an audio signal so that when it is reproduced over two audio transducers the apparent location of the sound source can be suitably controlled, so that it seems to the listener that the location of the sound source is separated from the location of the transducers or speakers.
The present invention is based on the discovery that audio reproduction of a monaural using two independent channels and two loudspeakers can produce highly localized images of great clarity in different positions. Observation of this phenomenon by the inventors, under specialized conditions in a recording studio, led to systematic investigations of the conditions required to produce this audio illusion. Some years of work have produced a substantial understanding of the effect, and the ability to reproduce it consistently and at will.
According to the present invention, an auditory illusion is produced that is characterized by placing a sound source anywhere in the three-dimensional space surrounding the listener, without constraints imposed by loudspeaker positions. Multiple images, of independent sources and in independent positions, without known limit to their number, may be reproduced simultaneously using the same two channels. Reproduction requires no more than two independent channels and two loudspeakers and separation distance or rotation of the loudspeakers may be varied within broad limits without destroying the illusion. Rotation of the listener's head in any plane, for example to "look at" the image, does not disturb the image.
The processing of audio signals in accordance with the present invention is characterized by processing a single channel audio signal to produce a two-channel signal wherein the differential phase and amplitude between the two signals is adjusted on a frequency dependent basis over the entire audio spectrum. This processing is carried out by dividing the monaural input signal into two signals and then passing one or both of such signals through a transfer function whose amplitude and phase are, in general, non-uniform functions of frequency. The transfer function may involve signal inversion and frequency-dependent delay. Furthermore, to the bet knowledge of the inventors the transfer functions used in the inventive processing are not derivable from any presently known theory. They must be characterized by empirical means. Each processing transfer function places an image in a single position which is determined by the characteristics of the transfer function. Thus, sound source position is uniquely determined by the transmission function.
For a given position there may exist a number of different transfer functions, each of which will suffice to place the image generally at the specified position.
If a moving image is required, it may be produced by smoothly changing from one transfer function to another in succession. Thus, a suitably flexible implementation of the process need not be confined to the production of static images.
Audio signals processed according to the present invention may be reproduced directly after processing, or be recorded by conventional stereo recording techniques on various media such as optical disc, magnetic tape, phono record or optical sound track, or transmitted by any conventional stereo transmission technique such as radio or cable, without any adverse effects on the auditory image provided by the invention.
The imaging process of the present invention may be also applied recursively. For example, if each channel of a