[linux-dvb] Re: New UK DVB-T channels testing

[Holger Waechtler <holger@convergence.de>]

Hi,

Klaus Schmidinger wrote:
> Dave Chapman wrote:
> 
>>On Tuesday 08 October 2002 22:19, Klaus Schmidinger wrote:
>>
>>>Are these frequency offsets always +/-167kHz? Will that be the same for
>>>other countries, too?
>>
>>In the UK the offsets (if present) are always +/- 167KHz.  I haven't heard
>>about offsets being used in other countries.

Only +/-166667 offsets are specified in the DVB standard.


>>>I'm asking because I would like to make the frequency for DVB-T channels
>>>in VDR's channels.conf also be in MHz, and if that offset thing is in any
>>>way standardized we could just add another DVB-T specific parameter to
>>>cover the offset (like O0 = 0 kHz, O1 = +167 kHz, O2 = -167kHz).
>>>
>>>I wonder, though, why that offset is necessary, and whether the driver
>>>shouldn't be able to tune to a channel at, say, 490 MHz - 167 kHz
>>>automatically if it is given an inital frequency of 490 MHz. I mean
>>>167 kHz isn't _that_ far off for a transponder with several MHz bandwith.
>>>Or am I missing something important here?
>>
>>No, trying to tune to a frequency of 488.833MHz by approximating to 489MHz
>>doesn't work - I've just tried it.
> 
> 
> That's odd... IIRC with DVB-S you can be off by even 1 or 2 MHz and the driver will
> tune to the transponder correctly, so I really wonder why this isn't possible with
> DVB-T. I mean it's just a mere 166 kHz! Is there no AFC in the DVB-T part of
> the driver?
> 
> Can one of the driver developers comment on this?

That was probably the zigzag scan. It's range is 1000 times wider for 
DVB-S since frequencies are there measured in kHz, not Hz to prevent an 
overflow.

For DVB-C/T we need a finer stepsize since the frequencies there are 
much smaller. If you want to achieve fastest possible tuning you should 
pass the exact frequency.

All demodulators have built-in AFC's too, but their range is sometimes 
pretty limited - and even there you can speed up tuning when you pass 
the exact frequency.


>>I think the "O" option would just make the channels.conf more confusing - it
>>is much simpler just to keep the frequency the way it is, in KHz.
> 
> 
> The problem is that I would like to be able to use the transponder frequency
> as a parameter in a "unique channel ID" (I know, there are people who would
> like that to be the NID, TID whatsoever - there will be a switch to configure that!).
> Anyway, since the "official" frequencies are apparently given as integer MHz values
> (for instance 23 * 8 + 306) and the "offset" is mentioned as an additional parameter,
> see http://www.itc.org.uk/uk_television_sector/reception_advice/digital_trans_guide/show_transmitter.asp?siteID=81.
> So I would prefer _all_ frequencies in channels.conf to be in MHz, so that the numbers 
> can be expressed in 16 bit values. And regarding channels.conf being "confusing": well,
> that's mainly because DVB-T needs so many parameters! I don't know why they have to
> have _that_ many of them, but that's just the way it is. And apparently the "offset"
> is just another of these parameters.

Why don't you want to use 32-bit frequencies?


> Can somebody with DVB-T please run a program that parses the NITs and reports
> the transponder frequencies given there? I'd say if these are given as the "real"
> values (like 488.833MHz), then VDR should store them as such in channels.conf.
> But if they are given as integer MHz values (like 489MHz) then there's no point in
> having them in kHz in channels.conf.

The NIT's report frequencies in 10Hz steps for DVB-T, in 100Hz steps for 
DVB-C and in 10kHz steps for DVB-S.

Holger



-- 
Info:
To unsubscribe send a mail to listar@linuxtv.org with "unsubscribe linux-dvb" as subject.