Intro. 5 Timing model
Systems, Video and Audio all have a timing model in which the end-to-end delay from the signal input to an encoder to
the signal output from a decoder is a constant. This delay is the sum of encoding, encoder buffering, multiplexing,
communication or storage, demultiplexing, decoder buffering, decoding, and presentation delays. As part of this timing
model all video pictures and audio samples are presented exactly once, unless specifically coded to the contrary, and the
inter-picture interval and audio sample rate are the same at the decoder as at the encoder. The system stream coding
contains timing information which can be used to implement systems which embody constant end-to-end delay. It is
possible to implement decoders which do not follow this model exactly; however, in such cases it is the decoder’s
responsibility to perform in an acceptable manner. The timing is embodied in the normative specifications of this
Recommendation | International Standard, which must be adhered to by all valid bit streams, regardless of the means of
creating them.
All timing is defined in terms of a common system clock, referred to as a System Time Clock. In the Program Stream
this clock may have an exactly specified ratio to the video or audio sample clocks, or it may have an operating frequency
which differs slightly from the exact ratio while still providing precise end-to-end timing and clock recovery.
In the Transport Stream the system clock frequency is constrained to have the exactly specified ratio to the audio and
video sample clocks at all times; the effect of this constraint is to simplify sample rate recovery in decoders.
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