The MPEG-2 Codec - A Practical Guide to Video and Audio Compression - page 224

enhancement signal from the background noise but may still be able to discriminate the
base level and therefore decode a moderate-quality picture. This is all designed around
signal-to-noise ratios and is thus called the SNR profile.
Another approach that might be available in the future is for manufacturers to build
economically priced decoders that will only see the base signal. A premium set-top box
might then be offered to deliver a better-quality service.
The spatial scaling is sometimes augmented by temporal scaling to double the frame
rate. A wide range of potential service levels can be offered according to the bit rate available.

11.12 Systems Layer

The MPEG-2 standard introduces the systems layer. This provides mechanisms for com-
bining the video content with audio and captioning. This carriage mechanism is called a
transport stream.
The transport stream multiplexes program streams together. It carries other arbitrary
supporting data streams as well as the programs. The streams can share a common time

Elementary video stream

Video Video Video Video

Programme stream

V A V A V A V A

Elementary video stream

Audio Audio Audio Audio

Programme stream

Prog 1 Prog 1 Prog 1 Prog 1

Programme stream

Prog 2 Prog 2 Prog 2 Prog 2

Programme stream

Prog 3 Prog 3 Prog 3 Prog 3

Multiplex/transponder

Programme stream P1 P2 EPG P3 P4 Dat

Prog 4 Prog 4 Prog 4 Prog 4

EPG data stream

EPG EPG EPG EPG

Interactive data stream

Data Data Data Data

Figure 11-8 Nested streams.