Panasonic EURO 4 Chassis, Technical Manual

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69
23. Encoding and Decoding Concepts
23.1. Dolby Surround Encoding
If we look at the Dolby matrix encoder we can see that
the encoder accepts four separate input signals; left,
centre, right and surround (L, C, R, S) and creates two
final outputs, left-total and right total (Lt and Rt).
The L and R inputs go straight to the Lt and Rt outputs
without undergoing any type of processing. The C
input is divided equally between Lt and Rt with a 3dB
level reduction. The S input is also divided equally
between Lt and Rt, but it first undergoes three
additional processing steps:
1. Frequency bandlimiting from 100Hz to 7KHz
2. Encoding with a modified form of Dolby B-type
noise reduction
3. Plus and minus 90
E
phase shift applied to
create a 180
E
phase differential between the
components feeding Lt and Rt
It is clear that there is no loss of separation between
the left and right signals, they remain completely
independent. Not so obvious is that there is also no
theoretical loss of separation between the centre and
surround signals.
Since the surround signal is recovered by taking the
difference between the Lt and Rt, the identical centre
channel component in Lt and Rt will be exactly
cancelled in the surround output.
Likewise since the centre channel is derived from the
sum of Lt and Rt, the equal and opposite surround
channel components will cancel each other in the
centre output.
The ability for this cancellation technique to maintain
high separation between centre and surround signals
requires the amplitude and phase characteristics of
the two transmission channels to be as close as
possible.
For instance if the centre channel
components in Lt are not identical to the ones in Rt,
channel imbalance would occur resulting in centre
channel information coming out of the surround
channel in the form of unwanted cross-talk.
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