Transmission
DTTV is transmitted on radio frequencies through terrestrial space in the same way as standard analog television, with the primary difference being the use of multiplex transmitters to allow reception of multiple channels on a single frequency range (such as a UHF or VHF channel) known as subchannels.
The amount of data that can be transmitted (and therefore the number of channels) is directly affected by channel capacity and the modulation method of the channel. The modulation method in DVB-T is COFDM with either 64 or 16-state Quadrature Amplitude Modulation (QAM). In general, a 64QAM channel is capable of transmitting a greater bit rate, but is more susceptible to interference. 16 and 64QAM constellations can be combined in a single multiplex, providing a controllable degradation for more important program streams. This is called hierarchical modulation.
New developments in video compression have resulted in the H.264/MPEG-4 AVC standard which enable three high-definition television services to be coded into a 24 Mbit/s European terrestrial transmission channel.
The DVB-T standard is not used for terrestrial digital television in North America. Instead, the ATSC standard calls for 8VSB modulation, which has similar characteristics to the vestigial sideband modulation used for analogue television. This provides considerably more immunity to interference, but is not immune — as DVB-T is — to multipath distortion and also does not provide for single-frequency network operation (which is in any case not relevant in the United States).
Both systems use the MPEG transport stream and H.262/MPEG-2 Part 2 video codec specified in MPEG-2; they differ significantly in how related services (such as multichannel audio, captions, and program guides) are encoded.
Read more about this topic: Digital Terrestrial Television