[linux-dvb] Re: [vdr] Re: Siemens dvb-c and HTV

[Dennis Noordsij <dennis.noordsij@movial.fi>]

> QAM128 is a not a symmetrical modulation. This makes it a difficult mode to
> operate. The hardware has to have a very "clean" signal path and the
> software has to be tailor to this type of applications. Sometimes it's
> easier to do QAM256 than QAM128. I'm surprise that people are actually
> using QAM128.
>
> Can you get any performance information from the card. For example the
> uncorrected packets, BER, SNR, etc. This could provide some information on
> how bad the situation is and if there is hope of getting this working. I
> remember working with the ST cable part and spending a lot of time messing
> around with some undocumented registers to get it working at QAM256. We
> never got the QAM128 fully functional and I remember that at B2C2 we took
> it out of the specification.


This is from memory as I don't have cable anymore and use the cards as tv-out 
cards only.

Typically the signal is very strong in these appartment buildings, signal 
strength indicator is maxed out, though SNR doesn't look that bad. (You'll  
regret accidently touching the end of the antenna cable!).

Initially ves1820 (demodulator) register settings were changed from spec to 
get it to work at all with QAM128, but later on it was found that changing 
the PWM value (which is calibrated in the factory and stored in an eeprom) 
can make huge changes in reception quality. (Someone else please fill in 
details for where the PWM is used exactly :-)
My BER dropped from 100,000 to 20,000 by just guessing a new (lucky) PWM 
value.

Eventually (as a consolidation to Finnish QAM128 victims :-) I did get a 
result where reception was such that there were no uncorrected block errors 
during normal viewing. It did take some voodoo with regard to the combination 
of cables, splitters and amplifiers. I wouldn't trust it to record 3 channels 
simultaniously though (not QAM128 anyway)

One you do get it working, and notice that for example SubTV is out of audio/
video sync most of the time, EPG's are truncated, etc, rest comfortably 
knowing that is SubTVs and Digias fault and not the cards:-)

Now I use a nova-t, and it is outstanding. 

Cheers,
Dennis



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