DVB-T COFDM needs EQ like any other digital system because of the hostile nature of the terrestrial transmission path. For every symbol transmitted using DVB-T in excess of 1 in 12 carriers is a bpsk pilot. The scattered pilots shift 3 carrier positions every symbol to give a regular pilot pattern where-by over a 4 symbol timeframe there is an equalisation estimate available for 1 in 3 carriers. This combined with the long symbol time ~1 ms (8k) gives quite a reasonable estimate of the current channel performance (via interpolation), allowing equalisation of all the data carriers being demodulated through the IFFT. Note: there is a complete channel estimate available every 4 data symbols. This is one of the major differences between 8-VSB and DVB-T Neil Pickford Australia Quoting Henry Baker <hbaker1@xxxxxxxxxxxx>: > I have a very dim understanding of COFDM. > > I understand that it involves splitting up the > bandwidth into thousands of independent narrow > channels, and that FFT's can be used to efficiently > do a lot of the processing. > > The question is the following: if the channel is dispersive, > meaning that the delays are frequency-dependent, then a > COFDM whose channels are narrow enough shouldn't care at all, > since it is unlikely that a single narrow channel would be > wide enough to disperse the frequencies within the channel. > > Many equalizers are trying to recover the proper shape of > a signal, but if the signal is very narrow-band, the signal > is always a good approximation to a sine wave, no matter > what happens to it in the channel. > > So why would a COFDM ever need equalization? > > (I realize that multipath could involve polarization and > cancellation, but this merely kills the capacity of one > or more of the individual narrow-band channels, which should > be taken care of by FEC, not equalization.) > > > > ---------------------------------------------------------------------- > You can UNSUBSCRIBE from the OpenDTV list in two ways: > > - Using the UNSUBSCRIBE command in your user configuration settings at > FreeLists.org > > - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word > unsubscribe in the subject line. > ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.