Fundamentals - The MPEG - page 52

As a practical matter is is usually necessary to keep the bit rate of the multiplex constant. With variable-rate inputs
this is done by creating null packets which are generally called stuffing or packing . The headers of these packets
contain an unique ID which the demultiplexer does not recognize and so these packets are discarded on arrival.
In an MPEG environment, statistical multiplexing can be extremely useful because it allows for the varying difficulty
of real program material. In a multiplex of several television programs, it is unlikely that all the programs will
encounter difficult material simultaneously. When one program encounters a detailed scene or frequent cuts which
are hard to compress, more data rate can be allocated at the allowable expense of the remaining programs which
are handling easy material.
2.23 Timebase correction
One of the strengths of digital technology is the ease with which delay can be provided. Accurate control of delay is
the essence of timebase correction, necessary whenever the instantaneous time of arrival or rate from a data
source does not match the destination. In digital video and audio, the destination will almost always have perfectly
regular timing, namely the sampling rate clock of the final DAC. Timebase correction consists of aligning irregular
data from storage media, transmission channels or compression decoders with that stable reference.
When compression is used, the amount of data resulting from equal units of time will vary. Figure 2.46 shows that if
these data have to be sent at a constant bit rate, a buffer memory will be needed between the encoder and the
channel. The result will be that effectively the picture period varies. Similar buffering will be needed at the decoder.
Timebase correction is used to recover a constant picture rate.
Figure 2.46: Compression results in a variable amount of data for each picture. To send this at a constant bit rate
requires a buffer at encoder and decoder. The result is that the time taken to send each picture varies.
Section 2.16 showed the principles of digital storage elements which can be used for delay purposes. The shift-
register approach and the RAM approach to delay are very similar, as a shift register can be thought of as a
memory whose address increases automatically when clocked. The data rate and the maximum delay determine
the capacity of the RAM required. Figure 2.47 shows that the addressing of the RAM is by a counter that overflows
endlessly from the end of the memory back to the beginning, giving the memory a ring-like structure. The write
address is determined by the incoming data, and the read address is determined by the outgoing data. This means
that the RAM has to be able to read and write at the same time.
Figure 2.47: If the memory address is arranged to come from a counter which overflows, the memory can be made
to appear circular. The write address then rotates endlessly, overwriting previous data once per revolution. The
read address can follow the write address by a variable distance (not exceeding one revolution) and so a variable
delay takes place between reading and writing.
In an MPEG decoder, the exact time of arrival of the data corresponding to a picture can vary, along with the time
taken to decode it. In practice the decoded picture is placed in memory and read out according to a locally re-
created picture rate clock. If the phase of the picture clock is too early, decoding may not be completed before the
memory has to be read. Conversely if the picture clock phase is too late, the memory may overflow. During lock-up,
a decoder has to set the picture phase in such a way that the buffer memories are an average of half-full so that
equal correcting power is available in both directions.