s, respectively, that are recorded using conventional stereo recording equipment 1207. If replayed on conventional stereo replay equipment 1208, driving right and left loudspeakers 1209, 1210, respectively, with the signals originating from microphones 1205, 1206, conventional stereo images 1211, 1212, 1213 corresponding respectively to sources 1201, 1202, 1203 will be perceived by a listener 1214 in a second room 1215. These images will be at positions that are projections onto the line joining loudspeakers 1209, 1210 of the lateral positions of the sources relative to microphones 1205, 1206.
If the two pairs of stereo signals are processed and combined as detailed above using sound processor 1216, and reproduced by conventional stereo playback equipment 1217 on right and left loudspeakers 1218, 1219 in a third room 1220, crisp spatially localized images of the sound sources are apparent to listener 1226 at positions unrelated to the actual positions of loudspeakers 1218, 1219. Let us suppose that the processing was such as to form an image of the original right channel signal at position 1224, and an image of the original left channel signal at 1225. Each of these images behaves as if it were truly a loudspeaker; we may think of the images as "virtual loudspeakers"
A transfer function in which both differential amplitude and phase of a two-channel signal are adjusted on a frequency dependent basis across the entire audio band is required to project an image of a monaural audio signal to a given position. For general applications to specify each such response, the amplitude and phase differential at intervals not exceeding 40 Hz must be specified independently for each of the two channels over the entire audio spectrum, for best image stability and coherence. For applications not requiring high quality and sound image placement the frequency intervals may be expanded. Hence specification of such a response requires about 1000 real numbers (or equivalently, 500 complex ones). Differences for human perception of auditory spatial location are somewhat indefinite, being based on subjective measurement, but in a true three-dimensional space more than 1000 distinct positions are resolvable by an average listener Exhaustive characterization of all responses for all possible positions therefore constitutes a vast body of data, comprising in all more than on million real numbers, the collection of which is in progress.
It should be noted that the transfer function in the sound processor according to this invention, which provides the differential adjustment between the two channels, is build up piece-by-piece by trail and error testing over the audio spectrum for each 40 Hz interval. Moreover, as will be explained below, each transfer function in the sound processor locates the sound relative to two spaced-apart transducers at only one location, that is, one azimuth, height, and depth.
In practice, however, we need not represent all transfer function responses explicitly, as mirror-image symmetry generally exists between the right and left channels. If the responses modifying the channels are interchanged, the image azimuth angle (a) is inverted, whilst the altitude (b) and range (r) remain unchanged.
It is possible to demonstrate the inventive process and the auditory illusion using conventional equipment and by using simplified signals. If a burst of a sine wave at a known frequency is gated smoothly on and off at relatively long intervals, a very narrow band of the frequency domain is occupied by the resulting signal. Effectively, this signal will sample the required response at a single frequency. Hence the required responses, that is, the transfer functions, reduce to simple control of differential amplitude and phase (or delay) between the left and right channels on a frequency dependent basis. Thus, it will be appreciated that the transfer function for a specific sound placement can be built up empirically by making differential phase and amplitude adjustments for each selected frequency interval over the audio spectrum. 