uditory 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. By Fourier's theorem any signal may be represented as the sum of a series of sine waves, so the signal used is completely general.
An example, of a system for demonstrating the present invention is shown in FIG. 13, in which an audio synthesizer 1302, a Hewlett-Packard Multifunction Synthesizer model 8904A, is controlled by a computer 1301, Hewlett-Packard model 330M, to generate a monaural audio signal that is fed to the inputs 1303, 1304 of two channels of an audio delay line 1305, Eventide Precision Delay model PD860. From delay line 1305 the right channel signal passes to a switchable inverter 1306 and left and right signals then pass through respective variable attentuators 1307, 1308 and hence to two power amplifiers 1309, 1310 driving left and right loudspeakers 1311, 1312, respectively.
Synthesizer 1302 produces smoothly gated sine wave bursts of any desired test frequency 1401, using an envelope as shown in FIG. 14. The sine wave is gated on using a first linear ramp 1402 of 20 ms duration, dwells at constant amplitude 1403 for 45 ms, and is then gated off using a second linear ramp 1404 of 20 ms duration. Bursts are repeated at intervals 1405 of about 1-5 second.
In addition, using the system of FIG. 13 and the waveform of FIG. 14, the present invention can build up a transfer function over the audio spectrum by adjusting the time delay in delay line 1305 and the amplitude by attentuators 1307, 1308. A listener would make the adjustment, listen to the sound placement and determine if it was in the right location If so, the next frequency interval would be examined. If not, then further adjustments are made and the listening process repeated. In this way the transfer function over the audio spectrum can be built-up.
FIG. 15 is a table of practical data to be used to form a transfer function suitable to allow reproduction of auditory images well off the direction of the loudspeakers for several sine wave frequencies. This table might be developed just as explained above, by trial and error listening. All 