52
Appendix F – DVB-T Principles
DVB-T uses OFDM (orthogonal frequency division multiplex) modulation. This type of
modulation, which uses a large number of sub-carriers, delivers a robust signal that has
the ability to deal with very severe channel conditions. The DVB-T standard has
technical characteristics that make it a very flexible system: 3 modulation options (QPSK,
16QAM, 64QAM), 5 different FEC rates, 4 Guard Interval options, 2k or 8k carriers
(actually this is 1705 carriers or 6817 carriers), 6, 7 or 8MHz channel bandwidths
One other technical aspect of DVB-T is its capacity for Hierarchical Modulation. Using
this technique, two completely separate data streams are modulated onto a single DVB-T
signal. A "High Priority" (HP) stream is embedded within a "Low Priority" (LP) stream.
Typically, the LP stream is of higher bitrate, but lower robustness than the HP one.
Receivers with "good" reception conditions can receive both streams, while those with
poorer reception conditions may only receive the "High Priority"stream.
After the MPEG transport stream (usually called MUX instead of transport stream) is
demodulated, then the data encoding is the same as DVB-S as explained in Appendix C.
For further information see the DVB-T Standard published as EN 300 744 and ETSI TS
101 154, Specification for the use of Video and Audio Coding in Broadcasting
Applications based on the MPEG-2 Transport Stream.