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Surround processors should
not come after the preamp
Surround processors should
not come before the preamp
Surround processors should
not be in a tape loop
Using Surround
Sound Processors
The PRE preamplifier incorporates a special surround sound processor mode
allowing it to integrate the highest performance audio with surround sound—
that is, dual-purpose music and movie systems. In order to better understand
the value of this design, it is essential to understand a bit about the nature of a
Dolby Pro Logic Surround
The Dolby Stereo
®
system encodes four discrete channels into a two-channel
matrix during the production of the movie soundtrack. This two-channel signal
is compatible with normal stereo (and even monophonic) playback. With the
proper decoding during playback, however, it is possible to recover the origi-
nal four channels from the two which are present on the laserdisc or on the
hifi videotape. These channels are Left, Center, and Right in the front, and a
single Surround channel for the sides and rear of the audience.
Dolby Pro-Logic decoders incorporate a form of Dolby noise reduction similar
to the Dolby B one finds in cassette decks. This noise reduction circuitry is
level-sensitive. That is, Dolby noise reduction intentionally treats strong signals
differently than weak signals. In order to operate correctly , the signal strength
of the source must be "calibrated" to the expectations of the Dolby noise re-
duction circuitry . (It is for this reason that one finds "Record Calibration" fea-
tures on better-quality cassette decks.) It is therefore inappropriate to feed
a surround sound decoder with the variable output of a preamplifier .
Were you to do so, every change of the volume control on the preamplifier
would cause the Dolby circuitry to mistrack. In extreme cases, severe distor-
tion can result as the Dolby circuitry overloads.
The next logical alternative might be to use the Pro-Logic decoder ahead of
the preamplifier, sending its Left and Right outputs through the preamplifier as
a selectable Source. Sending the Right and Left Outputs from a surround
sound decoder to a pair of inputs on a conventional preamplifier is
also inappropriate, since any change of the preamp's volume control would
then throw the carefully calibrated output levels of the decoder out of adjust-
ment, changing the volume of the Left and Right speakers while leaving the
Center and Surround speakers unaffected. One could attempt to restore the
proper balance by marking a "calibrated" point on the preamplifier's volume
control and then using only the Pro-Logic decoder to adjust the volume of the
system, but this method is both crude and imprecise, yielding inconsistent per-
formance at best.
The last remaining alternative would seem to be to insert the decoder "within"
a preamplifier by hooking it up in a tape loop. Unfortunately, this setup re-
turns the Left and Right outputs of the decoder to the preamplifier, where they
can be inappropriately changed without changing the Center and Surround
outputs. It is therefore inappropriate to place surround sound decod-
ers in a tape loop.
™
decoder.
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