Dolby Labs were clever enough not to try encoding four channels into two.
They only encoded one, the rear channel. This is why it works.
The rear information is phase encoded into the L and R channels so that
it is 90
degrees ahead of the L channel and lacks 90 degrees of the R channel.
The rear channel information phase difference between the L and R
The front center channel isn't actually a separate channel at all. The
Dolby Stereo decoder wasn't very good at this, but the Pro Logic decoder
is very clever.
It outputs to the center speaker only such information that is equal in
level and phase
between the L and the R channels. It is the mono information of the
It isn't a separate channel because the decoder outputs to the center
speaker also all
mono information that is included in the stereo signal that you put into
the L and R
channels. However, that doesn't matter, because the center is the point
mono information actually should come from! In fact the Pro Logic
partly the directional distortion that listeners sitting off-axis have.
The decoder also feeds all antiphase content that was fed into the L and
into the rear speakers, but in practise the amount of antiphase
components in music
or sound effects is very small.
Dolby Stereo encoding is very robust. You can even swap the L and the R
but the Pro Logic decoder still decodes C and R signals into the correct
If you do the same with any matrixed quad recordings, the decoding
doesn't work right.