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[linux-dvb] Re: Differences in DVB-C and DVB-T

Lauri Tischler writes:
 > Gavin Hamill wrote:
 > > Just a quick 'feeler' question here... what are the differences between 
 > > DVB-T and DVB-C ? Is it simply a matter of input frequency, or is there 
 > > more to it than that?
 > Good question, maybe somebody wise can also explain why the two,
 > T and C, are different in the first place.
 > Why do you need two different standards, is it just that some boneheads
 > in the standards committee were unable to get their act together.
 > Or is there a _real_ technical reason.

Both, the frequency of the signal as well as the circumstances of
reception pose different requirements for the type of modulation.

DVB-S, for satellite reception the signal to noise ratio (SNR)
       can be very low ($<10$dB), but there are hardly any echos.

DVB-C, here we have good SNR ($>30$db), but lots of echos.

DVB-T, for terrestial reception we want to have mobile reception
       with simple antennas, so we have to take care of multiple echos,
       interference and signal variations.

Furthermore, there are other constraints on the transmission channels
that come from the legacy of the old analog channels. Those have
certain frequency ranges that will have to remain the same for the
digital channels. Without explanation let's just say that for
terrestial transmission orthogonal frequency division multiplexing
(OFDM) is used for modulation, for cable and satellite transmission
quadrature amplitude modulations (QAM) are used which in the case of
DVB-S is the simplest case of QAM4, i.e. quadrature phase shift keying
(QPSK). For more information see e.g. the book by H. Benoit 1997,
"Digital Television"


| Dr. Marcus O.C. Metzler        |                                   |
|            |        |


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