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channel components in Lt are not identical to the ones in Rt
as a result of a channel balance error, center information will
come out of the surround channel in the form of unwanted
crosstalk.

3. THE DOLBY SURROUND DECODER

This leads us to the original Dolby Surround decoder. The
block diagram in Fig.1 5-2 shows how the decoder works.
Except for level and channel balance corrections, the Lt
input signal passes unmodified and becomes the left output.
The Rt input signal likewise becomes the right output. Lt and
Rt also carry the center signal, so it will be heard as a
"phantom" image between the left and right speakers, and
sounds mixed anywhere across the stereo soundstage will be
presented in their proper perspective. The center speaker is
thus shown as optional since it is not needed to reproduce the
center signal.

The L-R stage in the decoder will detect the surround signal
by taking the difference of Lt and Rt, then passing it through
a 7 kHz low-pass filter, a delay line, and complementary
Dolby noise reduction. The surround signal will also be
reproduced by the left and right speakers, but it will be heard
out-of-phase which will diffuse the image.
Since the heart of the decoding process is a simple L-R
difference amplifier, it is referred to generically as a "passive"
decoder. This is to distinguish it from decoders using active
processes to enhance separation which are known as "active"
decoders.

Fig. 15-2 Passive surround decoder block diagram

4. DSP CIRCUIT

A surround component (L-R) is extracted from L, R audio
signals coming through the AV SW in the matrix circuit as
shown in Fig. 15-3. The surround component enters the DSP
circuit through the LPF.
The signal is A/D converted, delayed by an arbitrary time of
0~100 msec (every 3.2 msec) by digital process and then D/
A converted and outputs from the DSP IC. The DSP IC
develops two outputs; (LO) for FRONT (LO) and (RO) for
REAR and each output is controlled by the microcomputer
for each surround mode. The output signal (LO) for FRONT
is added and subtracted with the input signal in a matrix
circuit and output from the front speaker in passing through
the audio processor and main amplifiers.
At the same time, the output signal (RO) for REAR is fe?? to
the Dolby NR circuit, but switched to "Dolby surround"
mode, and then output from the rear speaker in passing
through audio processors and rear main amplifiers.
In this case, the DSP stands for not only a simple digital
surround processor but also a digital surround field processor.
That is, it works to give a simple surround effect but to give
effect as if the listener can feel reality suitable for the
programs. For example, it aims to give the listeners a reality
matching to each program they are enjoying in their home
listening room so that they can obtain reality of big concert
hall or feel as if they are watching a move at a reserved seat
in a movie theater.