Digital Television - A Practical Guide to Video and Audio Compression - page 431

27.3.2 Resilience in Transport Streams

Digital-video broadcasts must deliver the content without dropping frames and at a qual-
ity that is satisfying to watch for long periods. Commercial pressure dictates that video
quality be reduced in favor of broadcasting more channels.
The carriage of the video is quite complex because the transmission medium is
designed to carry multiple channels. In the old days of analog video, each channel would
only be one video signal. Digital TV carries a bundle or multiplex of TV services on
each channel that your receiver is able to tune into. A breakdown of this is shown in
Figure 27-1.
A multiplex may carry 6 video signals with stereo audio plus additional language
channels and other streams of supporting data.
Digital cable is slightly different because although the same multiplexing technique
is used, the cable companies also offer telephone and broadband Internet connections
down the same cable.

27.3.3 Switching to Another Stream

In analog TV sets, this happens instantaneously. There is no buffering required before the
presentation continues. There may be a slight frame roll or bounce as the display syn-
chronizes, but modern TV sets do this very rapidly.
Switching channels on digital TV sets is quite arduous by comparison. Leaving aside
all the issues with tuning and then extracting the program stream from a multiplexed

Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6

Tuner

Demodulate

Multiplex/transponder

P1 P2 EPG P3 P4 Dat

Demultiplex Program stream

V A V A V A V A

Elementary streams

Demultiplex

Video

Audio

Figure 27-1 Multiplexed data.