[linux-dvb] Re: [OT] Reality - Modulation schemes.

["Bob H \(Jerico/DVB\)" <dvb@orbit.me.uk>]

There are many modulation schemes specified in DVB, but not all are implemented
for DTH systems, I would quote the ETSI papers I've got, but that would involve
standing up and going to my other desk, loging into my old user drive... (you
get the idea how lazy I feel).

Interesting to hear that the new DTH standards won't be implementing RS and VIT
as standard. I don't actually see any problems with 8PSK and certainly don't
understand your 30% not 50% comment, true that 8PSK requires a better carrier
to noise but any good modulator will produce good quality streams. Infact while
most QPSK modulators use a 35% roll-off I think all the 8PSK modulators I have
seen are using 25% roll off. The best performance from 8PSK can be gained by
using 'Turbo' coding, we have tested a sustained lock under 7dB C/N. But a
decent C/N will mostly give you a QEF, Quasi Error Free, resultant data after
error correction.

BTW. Althought the digital carriers received in DTH are often limited to 30MHz
an actual satellite transponder can easily be much larger than this. One of
Intelsat 707's largest transponders (called 65) extends from about 11,458 GHz
to 11,570 GHz which is 112MHz, but the Thor series in contrast use a series of
small 49MBit/sec transponders of good power output for DTH use, originally
broadcasting single analogue carriers.

I hear the plan for terrestrial broadcasts in the UK is a phased shut down of
analogue carriers and then an increase in the power output of the digital
transponders, this will allow a much better carrier to noise (C/N) for each
channel and that would make higher order modulation schemes such as QAM128 more
viable.

For the comment about constellation density I will quickly knock out another
email for the benifit of those who don't know.

Bob


--

Is realities bitrate is too low?



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