Quote Robert Eidschun:
As mentioned in my previous reply, it seems to me that I should just have Dolby E audio on any version of my project intended for projection.
At least that way you would have absolute control of what comes out of what speaker. But you will need to clarify the picture/dolby E sync requirements beforehand.
But if I really wanted to stick with a two-channel PCM mix, then could I get 'round the problem that you mention above by mixing all of the music (which is stereo) to mono and then placing the result on each of the two tracks?
Sure... if you want pure phantom centre mono music. Seems a waste to me!
Does this mean that if there was only one channel of audio, then that lone channel was routed only to speakers that were up front and centered, so that all of the audio came out of them and no audio came from any place else?
Yes... for a 35mm mono SVA soundtrack film -- which was what most of them were in the provincial cinemas.
Before the introduction of Dolby Stereo (now more often referred to as Pro-Logic) which happened worldwide in 1975/76, 35mm release films generally had either a mono optical soundtrack, or a four-channel magnetic stripe (left, centre, right and effects).
70mm film generally had a six channel magnetic stripe (plus mono optical) giving left, inner left, centre, inner right, right and effects.
Magnetic stripes on films were very expensive to add and wore out quickly, which is why they were only used in the premier theatres.
Dolby revolutionised the industry by first introducing Dolby A (later SR) noise reduction to the optical soundtrack. This provided such an improvement in signal-noise that they could then afford to halve the track width whilst maintaining an acceptable signal-noise ratio. That allowed the printing of two audio tracks on the film -- which is easy, cheap and reliable.
But the film industry wanted at least four channels. The centre front was essential, stereo across the front was important, and the enveloping effects channel was nice.
So Dolby modified some of the ideas that were kicking around at the time to do with phase-amplitude matrix quadrophonic systems, and that lead to the Pro-Logic technology and the introduction of Dolby Stereo, providing the industry with the familiar LCRS format. (The first Star Wars film being one of the earliest really well known adopters).
However, because of the de-matrixing process, it is inherently a compromised system that does weird things under certain circumstances, which is why the mix has to be monitored through the complete encode-decode chain to ensure that any funnies are caught and corrected before the final print.
Dolby Pro-Logic does what will appear to be unpredictable things to conventional stereo material not intended for pro-Logic reproduction.
Later digital incarnations of the Pro-Logic system (such as Dolby's PLII, but there are others) manage to handle conventional stsreo material a little better, but it can never be as reliable as a discrete channel system... which is why Dolby Digital was introduced to the film industry, and then Dolby E to the TV production industry.
Technical Editor, Sound On Sound