 of images. The extra elements are shown in FIG. 19, in which left and right signals 1701, 1702 may be supplied from the outputs 1688, 1689 respectively of the signal processor of FIG. 16. In each channel a delay 1703, 1704 respectively is inserted, and the output signals from the delays 1703, 1704 become the sound processor outputs 1705, 1706.
The delays introduced into the channels by this additional equipment are independent of frequency. They may thus each be completely characterized by a single real number. Let the left channel delay be t(1), and the right channel delay t(r). As in the above case, only the differential between the delays is significant, and we can completely control the equipment by specifying the difference between the delays. In implementation, we will add a fixed delay to each channel to ensure that at least no negative delay is required to achieve the required differential. Defining a differential delay t(d) as:
If t(d) is zero, the effects produced will be essentially unaffected by the additional equipment. If t(d) is positive, the center of the listening area will be displaced laterally to the right along dimension (e) of FIG. 3. A positive value of t(d) will correspond to a positive value of (e), signifying rightward displacement. Similarly, a leftward displacement, corresponding to a negative value of (e), may be obtained by a negative value of t(d). By this method the entire listening area, in which listeners perceive the illusion, may be projected laterally to any point between or beyond the loudspeakers. It is readily possible for dimension (e) to exceed half of dimension (s), and good results have been obtained out to extreme shifts at which dimension (e) is 83% of dimension (s). This may not be the limit of the technique, but represents the limit of current experimentation.
Two ordinary, spaced-apart loudspeakers can produce a sound image that appears to the listener to be emanating from a location other than the actual location of the loudspeakers. The sound signals are processed according to this invention before they are reproduced so that no special playback equipment is required. Although two loudspeakers are required the sound produced is not the same as conventional stereophonic, left and right, sound however, stereo signals can be processed and improved according to this invention. The inventive sound processing involves dividing each monaural or single channel signal into two signals and then adjusting the differential phase and amplitude of the two channel signals on a frequency dependent basis in accordance with an empirically derived transfer function. The results of this is processing is that the apparent sound source location can be placed as desired, provided that the transfer function is properly derived. Each transfer function has an empirically derived phase and amplitude adjustment that is built-up for each predetermined frequency interval over the entire audio spectrum and provides for a separate sound source location. By providing a suitable number of different transfer functions and selecting them accordingly the sound source can appear to the listener to move. The transfer function can be implemented by analog circuit components or the monaural signal can be digitalized and digital filters and the like employed.