r 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 conventional stereo signal is treated as a monophonic signal, and the channels are imaged to two different positions in the listener'space, a complete conventional stereo image along the line joining the positions of the images of the channels will be perceived. In addition, at the time the stereo record or disc is being recorded on multitrack tape, having for example twenty-four channels, each channel can be fed through a transfer function processor so that the recording engineer can locate the various instruments and voices at will to create a specialized sound stage. The result of this is still two-channel audio signals that can be played back on conventional reproducing equipment, but that will contain the inventive auditory imaging capability.
FIG. 1 is a plan view representation of a listening geometry for defining parameters of image location;
FIG. 2 is a side view corresponding to FIG. 1;
FIG. 3 is a plan view representation of a listening geometry for defining parameters of listener location;
FIG. 4 is an elevational view corresponding to FIG. 4;
FIGS. 5a-5k are plan views of respective listening situations with corresponding variations in loudspeaker placement and FIG. 5m is a table of critical dimensions for three listening rooms;
FIG. 6 is a plan view of an image transfer experiment carried out in two isolated rooms;
FIG. 7 is a process block diagram relating the present invention to prior art practice;
FIG. 8 is a schematic in block diagram form of a sound imaging system according to an embodiment of the present invention;
FIG. 9 is a pictorial representation of an operator workstation according to an embodiment of the present invention;
FIG. 10 depicts a computer-graphic perspective display used in controlling the present invention;
FIG. 11 depicts a computer-graphic display of three orthogonal views used in controlling the present invention;
FIG. 12 is a schematic representation of the formation of virtual sound sources by the present invention, showing a plan view of three isolated rooms;
FIG. 13