Digital Television - A Practical Guide to Video and Audio Compression - page 432

transport stream, just switching from one local stream to another seems to take longer than
it should on some viewing equipment. The amount of delay depends on the buffering
techniques and whether the receiver is smart enough to do some work for you before you
decide to change channels.
Sony is developing high-definition products with as many as 7 tuners. These will all
decode simultaneously so that switching from one stream to another merely connects the
output to the display instead of some kind of sink or null device. Having extra tuners
allows multiple simultaneous recordings.

27.3.4 Statistical Multiplexing

When the bit rate is fixed, it introduces some inefficiency to your coding system. The video
may not always require that amount of bit rate. Reserving that capacity is wasteful when
several streams are multiplexed together.
Statistical multiplexing is a more advanced variant of the CBR/VBR approach. It
assumes that at any instant in time, some streams are encoding very efficiently, yielding
high compression ratios and requiring low bit rates. Others may be bursting at high bit
rates.

The theory works as long as the channels are bursting at different times. This does
not work when you simulcast the same channel several times (perhaps with regional vari-
ants on some kind of opt-out basis for an hour each day).
Statistical multiplexing allows the streams to breathe in and out so that bit rate is
constant overall. Each individual stream is able to consume more bit rate while the others
are momentarily starved to provide the extra capacity. This technique is used in broadcast
systems where multiple compressed video streams are packed into programs and then
into transport streams.
Figure 27-2 shows the bit rates for two programs whose bit rate demands peak at dif-
ferent times. Conveniently in this case, one peaks when the other dips.
So when Channel 1 is bursting to a high-bit-rate requirement, Channels 2, 3, 4, and 5
are probably broadcasting something far less complex and requiring fewer bits. Figure 27-3

Program 1

Program 2

Figure 27-2 Multiple channels.