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

Philip Boucherat philip.boucherat at ntlworld.com
Fri Sep 1 02:36:09 CEST 2006


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 at gmx.net>
To: <linux-dvb at linuxtv.org>
Sent: Friday, September 01, 2006 12:52 AM
Subject: Re: [linux-dvb] General approach to scanning for DVB-C and DVB-S

> 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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> linux-dvb at linuxtv.org
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