Open as PDF
Simple logic decoders turn down the
left and right speakers during dialog.
This seriously affects music and ef-
speakers, effects are smeared and often unconvincing .
Pro Logic Decoding
A Pro Logic decoder, like the professional Dolby Stereo cinema processor,
both enhances the dialog in the center and removes it from the left and right,
while maintaining as much stereo separation as possible. This is a form of
directional steering. Properly done, steering prevents the dialog from
appearing in the other channels and enhances its plausibility.
The situation is similar with music and sound effects. For example, if the
sound was intended to be in the left, the decoder will remove it from the
center and surround channels. If it was intended to be halfway between left
and center, the Pro Logic decoder presents it equally to the left and center
speakers and removes it from the right and surround channels.
The Pro Logic decoder can give good stereo spread and precise control over
front-to-back perspective. But the real strength of Pro Logic decoding
emerges when music and dialog occur at the same time. When dialog is
present, the center channel information must be removed from the left and
right channels without reducing the spread or loudness of the music.
Pro Logic decoders sense both the direction of the loudest sound and the
difference in level between it and any ambient information. They then use
this information to direct the steering. The accuracy with which this is done
With conventional surround any
sound comes from at least three direc-
Dolby Surround Decoding
When the movie is shown the two Dolby Stereo tracks must be decoded and
separated into the original four. The Dolby Surround decoder does this in
a rather rudimentary way: it supplies a signal to the center channel which
is just the sum of the two input channels. This signal contains the dialog.
However, the left and right signals still contain dialog too, so the dialog is
spread out among the three front speakers. Similarly, the Dolby Surround
decoder takes all out-of-phase signals and sends them to the surround
speakers, while leaving the original out-of-phase components in the left and
right front speakers.
The basic Dolby Surround decoder has high channel separation between
left and right decoded audio, and between center and surround. The
separation between left or right and center, or between left or right and
surround, however, is only a few dB. The simple Dolby Surround decoder
does pretty well with music (although sometimes the center channel is too
loud) but, because any sound will be reproduced in at least three loud-