Madrigal Imaging N38 - Manual (Page 28)

28
Using Surround Sound
Processors
The Nº38 Preamplifier incorporates a special surround sound processor
mode which makes it uniquely capable of integrating 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™ decoder.
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 original
four channels from the
two which are present on the laserdisc or on the hi-fi videotape. These
channels are Left, Center, and Right in the front, and a single Surround
channel for the sides and rear of the audience. In order to recover all
four channels, it is necessary to have a stereo source (laserdisc, Hi-Fi
videotape and stereo TV being the most common) and an
appropriate decoder.
Dolby Pro-Logic decoders incorporate a form of Dolby noise reduction
similar to the Dolby B one finds in cassette decks. This form of noise
reduction 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 reduction circuitry. (It is for this reason
that one finds “Record Calibration” features 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 distortion 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 adjustment, 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 performance at best.
Surround sound
processors should not
come after the preamp
Surround sound
processors should not
come before the preamp