ing considerable amounts of equipment, room 2 as a large recording studio almost competely empty, and room 3 was a small experimental room with sound absorbing material on three walls.
For each test the listener was asked to give the perceived image position for two conditions; listener head angle (m) zero, and head turned to face the apparent image position. Each test was repeated with three different listeners. Thus, the image stability was tested in a total of 180 configurations. Each of these 180 configurations used the same input signals to the loudspeakers. In every case the image azimuth angle (a) was perceived as -60 degrees.
In FIG. 6 an image transfer experiment is shown in which a sound image 601 is formed by signals processed according to the present invention, driving loudspeakers 602 and 603 in a first room 604. A dummy head 605, such as shown for instance in German Patent 1 927 401, carries left and right microphones 606 and 607 in its model ears. Electrical signals on lines 608 and 609 from microphones 606, 607 are separately amplified by amplifiers 610 and 611, which drive left and right loudspeakers 612 and 613, respectively, in a second room 614. A listener 615 situated in this second room, which is acoustically isolated from the first room, will perceive a sharp secondary image 616 corresponding to the image 601 in the first room.
An example of the relationship of the inventive sound processor to known systems is shown in FIG. 7, in which one or more multi-track signal sources 701, which may be magnetic tape replay machines, feed a plurality of monophonic signals 702 derived from a plurality of sources to a studio mixing console 703. The console may be used to modify the signals, for instance by changing levels and balancing frequency content, in any desired ways.
A plurality of modified monophonic signals 704 produced by console 703 are connected to the inputs of an image processing system 705 according to the present invention. Within this system each input channel is assigned to an image position, and transfer function processing is applied to produce two-channel signals from each single input signal 704. All of the two-channel signals are mixed to produce a final pair of signals 706, 707, which may then be returned to a mixing console 708. It should be understood that the two-channel signals produced by this invention are not really left and right stereo signals, however, such connotation provides an easy way of referring to these signals. Thus, when all of the two-channel signals are mixed, all of the left signals are combined into one signal and all of the right signals are combined into one signal. In practice, console 703 and console 708 may be separate sections of the same console. Using console facilities, the processed signals may be applied to drive loudspeakers 709, 710 for monitoring purposes. After any required modification and level setting, master stereo signals 711 and 712 are led to master stereo recorder 713, which may be a two-channel magnetic tape recorder. Items subsequent to item 705 are well known in the prior art.
Sound image processing system 705 is shown in more detail in FIG. 8, in which input signals 801 correspond to signals 704 and output signals 807, 808 correspond respectively to signals 711, 712 of FIG. 7. Each monaural input signal 801 is fed to an individual signal processor 802.
These processors 802 operate independently, with no intercoupling of audio signals. Each signal processor operates to produce the two-channel signals having differential phase and amplitude adjusted on a frequency dependent basis. These transfer functions will be explained in detail below. The transfer functions, which may be described in the time domain as real impulse responses or equivalently in the frequency domain as complex frequency responses or amplitude and phase responses, characterize only the desired image position to which the input signal is to be projected.
One or more processed signal pairs 803 produced by the signal processors are applied to the inputs of stereo mixer 804.