Lexicon CP-1 User Manual download pdf (Page 50)

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Page 46

Simple logic decoders turn down the

left and right speakers during dialog.

This seriously affects music and ef-

fects.

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

Theory

and

Design

With conventional surround any

sound comes from at least three direc-

tions.

placements.

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-

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Lexicon CP-1 User Manual Page 50

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