[linux-dvb] General approach to scanning for DVB-C and DVB-S

Robert Schlabbach robert_s at gmx.net
Fri Sep 1 01:52:32 CEST 2006


From: "Philip Boucherat" <philip.boucherat at ntlworld.com>
> Just a general question about scanning algorithms for DVB-C and DVB-S ...
> scanning seems quite a difficult problem for DVB-C as for each possible
> frequency there are any number of possible symbol rates, and several
> possible qam values. I can see from the eutelsat web site that there
> seem to be a limited number of symbol rates for DVB-S, but I can't find
> any such published information about DVB-C.

Actually it is the other way around. Symbol rates for DVB-S vary widely, as
the frequency bandwidths of transponders do. In contrast, the selection of
frequency bandwidths in cable networks is limited to 6, 7 or 8MHz, and I
have yet to hear from any network using DVB-C on any channel raster other
than 8MHz.

Now with an 8MHz channel, the specified Nyquist roll-off of 0,15 limits the
symbol rate to a maximum of 6956 kbaud. Other symbol rates I know of are
6900 kbaud (leaves some frequency margin between channels) and 6875 kbaud
(a simple division of the 27500 kbaud of the DVB-S transponders which are
fed into the cable network).

As to modulation types, I haven't heard of any others than QAM-64, QAM-128
and QAM-256 being used.

So in a "blind scan", try the symbol rates 6875/6900/6956 kbaud with
QAM-64/128/256. As to the frequencies, your best bet is probably to try the
centre frequencies of the standard UHF channels 21 through 69, i.e. 474,
482, 490, 498 ... 858MHz.

I think that would at least work on all cable networks in Germany. As for
the UK, you will have to see - and report your results back here, please :)

Best Regards,
--
Robert Schlabbach
e-mail: robert_s at gmx.net
Berlin, Germany




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