Panasonic EURO 4 Chassis, Technical Manual

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23.2. Dolby Pro
.
Logic Decoding
Dolby Surround decoders are designed to decode the
specially encoded audio sound-tracks of Dolby
Surround video productions. The decoders can then
provide
a front to back sound-field which
complements conventional left to right stereo.
Two types of surround decoders are offered, passive
and active.
Passive decoders extract the surround signal
component to supplement the two main front
channels.
Active decoders on the other hand, under the trade
name
Pro
.
Logic, use directional enhancement
techniques and an additional centre channel output
used to improve sound-field localization over a wider
viewing area.
In order to effectively process Dolby Surround
encoded sound tracks the decoders perform several
functions.
The first function of the decoder, which can be
referred to in the block diagrams of the passive and
active decoders below, is input balance control, this
allows the user to change the program balance to
correct for channel balance errors that may exist in
the incoming audio signal, this is vital to ensuring that
the
matrix stage gives optimum decoder
performance.
Next, the input signal is fed via a level control stage,
here the level is matched to the requirements of the
decoder in order to optimize the dynamic range as
well as to obtain proper action of the noise reduction
stage in the surround channel.
The matrix stage then follows, this is the point where
the difference between passive and active decoding
occurs.
Passive decoders use a simple differential amplifier
to derive the L-R signal component or surround
signal. Active decoders use a complex adaptive
matrix stage to derive four output channels with a
higher separation than a passive matrix can achieve.
Two additional functions called the Centre mode
control and the Noise Sequencer are also part of the
active
decoder and will be discussed later.