In the late 60's I remember playing around with a British system that
produced a third channel from a normal stereo record. This back channel
created the ambience of the hall from the differences between the left
and right channels. It was very simple to hook up. All you did was hook
up the new speaker to the positive leads of the left and right
speakers. The record had to have been recorded in a hall to recover the
ambience: records made on mixer boards had no ambience or echo. A
separate amplifier was needed to get the full effect. Some of the
boffins of the time did the math but I don't recall it now. I do recall
getting the effect on Sgt Pepper...good times.