In the past, complete separation of stereo signals has been accomplished through the utilization of ear phones in which a left channel is applied to the left ear phone and the right channel is applied to the right ear phone. While in 1960 Benjamin Bauer of CBS Laboratories described an effort to make ear phones sound more like loud speakers by simulating the delayed leakage of the left speaker output to the right ear of a listener and vice versa, up until recently, there has been very little effort made to increase stereo separation when utilizing loud speakers.
It will be appreciated that there is a certain amount of mixing of sounds from the left and right speakers at the listener's ears due to the fact that sounds from the left speaker, for instance, will arrive first at the left ear, then at the right ear of a listener. The sounds which arrive at the right ear are mixed with sounds arriving at the right ear from the right channel speaker. Thus the separation which was initially established by virtue of recording techniques is in some sense destroyed since one ear can hear sounds produced by both of the stereo speakers.
While Benjamin Bauer did suggest the aforementioned method to make ear phones sound more like loud speakers, he also suggested the reverse; that it would be possible to cancel stereo mixing when using loud speakers and suggested that this would give loud speakers the separation of head phones.
Subsequent work on stereo loud speaker separation was done in Germany by Damaska et al. In the Damaska et al system, the right signal is delayed by the transit time for a sound to crossd the listener's face to the left ear. This delayed signal is then frequency contoured, inverted and added to the left speaker signal. The correction signal in the left speaker is then used to acoustically cancel the right speaker leakage at the left ear. The same is done between the left and right channels such that acoustic cross-talk between the ears is presumably cancelled.
One of the basic difficulties with delaying the left channel or right channel speaker signals is that there is a broad central region in which there is a "muddying" effect. This effect is due to the utilization of the full left and right channel signals. Note, in the central region no left/right information exists and this signal is essentially a monaural signal. As a result, not only is there frequency distortion, but also volume attenuation. The frequency distortion is due to a comb filtering effect in which the levels of various frequency components are increased or decreased in the central region. Thus the clarity and fidelity of the original recording is lost for central region produced sound.
Moreover, in most recordings the most important sound producing instruments or performers create their sound at center stage. Thus the muddying or frequency distortion of sounds at center stage creates significant problems in a stereo reproduction system when utilizing the aforementioned enhancement technique.