Robert,
Thanks for your reply. I'm guessing that you are German but the only clue
from the whole thing is that you said "the other way around" instead of "the
other way round". I'm just saying that because you'd be very hard pressed to
find anyone in the UK who speaks more than one or two words of German.
Apologies to all the exceptions! (not counting the 300,000 Germans who live
here!). Anyway, enough of that!
I understand what you are saying, but how do you find this stuff out? Is
there a website somewhere that gives details of the frequency plans of local
cable operators? For instance my local cable provider in Cardiff is NTL, and
even though I know some technical contacts inside NTL they are unable (or
maybe not allowed) to give me any details about frequencies, symbol rates or
anything, and I can't find anything on the internet.
Oh well, it's no use being able to tune into NTL (DVB-C) anyway as it seems
that they encrypt everything, even the channels that would be free to air on
DVB-T. Currently I am using a DekTek modulator card to test DVB-C, and
that's not much of a challenge as then I know what I've just set the
frequency, symbol rate and qam to!
Cheers, Phil
To get back to the point,
----- Original Message -----
From: "Robert Schlabbach" <robert_s@xxxxxxx>
To: <linux-dvb@xxxxxxxxxxx>
Sent: Friday, September 01, 2006 12:52 AM
Subject: Re: General approach to scanning for DVB-C and DVB-S
From: "Philip Boucherat" <philip.boucherat@xxxxxxxxxxxx>
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@xxxxxxx
Berlin, Germany
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