sition, 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. Some or all of them may also be applied to the inputs of a storage system 805. This system is capable of storing complete processed stereo audio signals, and of replaying them simultaneously to appear at outputs 806. Typically this storage system amy have different numbers of input channel pairs and output channel pairs. A plurality of outputs 806 from the storage system are applied to further inputs of stereo mixer 804. Stereo mixer 804 sums all left inputs to produce left output 807, and all right inputs to produce right output 808, possibly modifying the amplitude of each input before summing. No interaction or coupling of left and right channels takes place in the mixer.
A human operator 809 may control operation of the system via human interface means 810 to specify the desired image position to be assigned to each input channel.
It may be particularly advantageous to implement signal processors 802 digitally, so that no limitation is placed on the position, trajectory, or speed of motion of an image. These digital sound processors that provide the necessary differential adjustment of phase and amplitude on a frequency dependent basis will be explained in more detail below. In such a digital implementation it may not always be economic to provide for signal processing to occur in real time, though such operation is entirely feasible. If real-time signal processing is not provided, outputs 803 would be connected to storage system 805, which would be capable of slow recording and real-time replay. Conversely, if an adequate number of real-time signal processors 802 are provided, storage system 805 may be omitted.
In FIG. 9, operator 901 controls mixing console 902 equipped with left and right stereo monitor loudspeakers 903, 904. Although stability of the final processed image is good to a loudspeaker spacing (s) as low as 0.2 m, it is preferable for the mixing operator to be provided with loudspeakers placed at least 0.5 m apart. With such spacing, accurate image placement is more readily achieved. A compute