Lexicon CP-1 PLUS V2.0 Owner's Manual page 61

Theory
and
Design
With conventional surround any
sound comes from at least three direc-
tions.
Simple logic decoders turn down the
left and right speakers during dialog.
This seriously affects music and ef-
fects.
Page 56
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
is even more important in a home decoder than in a professional model,
because the small size of the playback room makes decoding errors more
audible than they are in a theater. The level detection must be very fast, and
the matrix must adapt very quickly or there will be a time lag between the
audibility of a sound and its correct steering. Since phase relationships
determine how the sound is steered, Pro Logic decoding puts unusual
demands on the accuracy of the phase and balance of the input channels.
Other Pro-Logic decoders have a front panel control for adjusting input
balance and for best results a user should carefully adjust this for each
program. But what if the channel balance varies during playback?
The manual balancing procedure does nothing to correct azimuth errors.
During the preparation of the master for a video tape or disc, misalignment
of the playback heads or skewing of the film produce small time differences
between the two channels. Azimuth is poorly controlled in both profes-
sional video recorders and optical film chains. In the final product, which
has been through many generations, it can easily be wrong by 50 microsec-
onds or more, and may vary as the tape or disk is played. At middle and
high frequencies it doesn't take much misalignment to generate large inter-
channel differences in phase, which are just what the decoder uses to do its
steering.
Other Dolby Pro Logic decoders try to deal with this problem by reducing
the treble in the surround, so the out-of-phase sibilants in the film do not
splatter annoyingly from the rear. This does not, however, reduce the
sibilants in the side speakers. Some non-Pro Logic decoders reduce these
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