How does DVB (Digital Television) allow several channels through one frequency?
The digital TV channel numbers don’t correspond to a particular frequency the way they did with analog TV systems. Individual stations by and large have kept the same channel numbers as they had on analog (NTSC) for convenience purposes, to reduce the chance of confusion among consumers. So frequencies are not correlated with channel numbers on ATSC digital broadcast. Again, several sub-channels may be grouped into one channel for convenience purposes: an individual network may have its main programming on channel 8.1, another set of programming on 8.2, and perhaps Spanish-language programming on 8.3.
A DVB-T (or ATSC in the US) transmission sends digital data in an MPEG transport stream. This stream of data (demodulated from a single RF channel) contains many sub-streams multiplexed together. The stream includes control blocks that identify the various sub-streams and allow them to be broken out (demultiplexed) by the receiver. One transport stream can carry multiple digital channels (listed in a Program Association Table), and each digital channel will contain a video stream, one or more audio streams, subtitles, program guide information, etc.
