User guide

Operating the Multiplexer using nCompass Control
4.5.2.2 Statistical Multiplexing
The major features of statistical multiplexing are:
• Uses variable bit rate Encoders.
• Employs real-time bit rate management.
• Continuously reallocates bandwidth between Encoders (within a set group).
• Maintains a target picture quality on all channels.
4.5.3 Conventional Multiplexing
4.5.3.1 Limitations of Fixed Bit rate Encoding
In the conventional multiplexing mode, the Encoder bit rate is allocated at the start
of a scheduled period. Each individual Encoder is unable to respond dynamically to
the complexity or speed of change of the picture. Because of this, the picture quality
decreases and the decoded output contains more encoding distortions as the
material becomes more demanding.
4.5.3.2 Allocation of Bit rates
The Operator must take into account the likely or known peaks in picture complexity
to allocate a bit rate which would avoid compression artefacts. The fixed bit rate is,
therefore, set at the level required to satisfy peak demands. This leads to the non-
optimal use of the channel capacity.
4.5.4 Statistical Multiplexing (Reflex)
4.5.4.1 Advantages of Statistical Multiplexing
In the statistical multiplexing (Reflex) mode, a group bit rate is dynamically allocated
in real-time between a set of Encoders (usually four or more).
The Operator assigns a relative quality to each Encoder in a set. This is a measure
of the picture quality expected from each Encoder. A preference can be assigned to
a channel by allocating a higher than average relative quality to the appropriate
Encoder. Excess bit rate from one Encoder can be reallocated to another to keep it
at its picture quality target.
4.5.4.2 Role of the MX8400 Multiplexer in Statistical Multiplexing
The MX8400 requests picture quality information in turn from each Encoder in a set.
Using this information, it then calculates any changes to the apportionment of the
group bit rate to ensure the target relative picture quality for each Encoder is
maintained.
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