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 of these images were found to be stable and repeatable in all three listening rooms detailed in FIG. 5m, for a broad range of listener head attitudes including directly facing the image, and for a variety of listeners.
We may generalize the placement of narrowband signals, detailed above, in such a manner as to permit broadband signals, representing complicated sources such as speech and music, to be imaged. If the differential amplitudes and phase shifts for the two channels that are derived from a single input signal are specified for all frequencies though the audio band, the complete transfer function is specified. In practice, we need only explicitly specify the differential amplitudes and delays for a number of frequencies in the band of interest. Amplitudes and delays at any intermediate frequency, between those specified, may then be found by interpolation. If the frequencies at which the response is specified are not too widely spaced, and taking into account the smoothness or rate of change of the true response represented, the method of interpolation is not too critical.
In the table of FIG. 15, the amplitudes and delays are applied to the signal in each channel and this is shown generally in FIG. 16 in which a separate sound processor 1500, 1501 is provided. The single channel audio signal is fed in at 1502 and fed to both sound processors 1500, 1501 where the amplitude and phase are adjusted on a frequency dependent basis so that the differential at the left and right channel outputs 1503, 1504, respectively, is the correct amount that was empirically determined, as explained above. The control parameters fed in on line 1505 change the differential phase and amplitude adjustment so that the sound image can be at a different, desired location. For example, in a digital implementation the sound processors could be finite impulse response (FIR) filters whose coefficients are varied by the control parameter signal to provide different effective transfer functions.
The system of FIG. 16 can be simplified,