[linux-dvb] High number of uncorrected blocks with TT-C1500 andQAM256

Per Dalén perda at oden.cnwab.se
Thu Apr 12 22:52:06 CEST 2007


Kari Salmela wrote:
>> I have had similar experience like You with the same card (TT DVB
> C1500) 
>> regarding QAM64. I discovered that there had been a successful fix for 
>> this done for QAM128 by changing the sweeprate, see 
>> http://www.linuxtv.org/pipermail/linux-dvb/2006-February/008439.html
>> So I made some test to find a working sweeprate for QAM64, see 
>> http://www.linuxtv.org/pipermail/linux-dvb/2006-May/010141.html
>>
>> You might have to the same trip for QAM256?
> 
>> +       case QAM_256:
>> +               delay = 200;
>> +               sweeprate = YOUR_SWEEPRATE;
>> +               break;
>>
>>         default:
>>                 return -EINVAL;
> 
> I am experiencing same problem with QAM 256 .. Tried putting sweeprate =
> 250, but it seemed to make no visible difference. Is there any other way
> to figure out what could be right than just trying different values and
> recompiling?

I'm sorry, but I didn't do my tests in any other way than what You 
describe above. In some of my early tries I also included changes of 
delay parameter also, but when I got the result that I wanted I removed 
the delay changes to see if I could get a minimal patch. You might have 
to include the delay parameter also.
BUT all of this is guessing, I have NO knowledges about the meaning what 
this parameters does. The thing that got me thinking that You have the 
same problems that I had is that the symptoms is similar to mine.

Maybe one of the dvb gurus out there could help us understand the 
underlaying meaning of this parameters and give some tips of good 
thumb-rule values?

> 
> --Kari
> 

/Per

> 
> Petri Helin wrote:
>> Hi,
>>
>> I am not completely sure if this really is connected to the channel
>> using QAM256, but that's what I suspect. I use Technotrend's TT-C1500
>> budget card with appropriate common interface and Conax CAM. This
>> combination works flawlessly on every channel which uses QAM64 or
>> QAM128. Now the Finnish cable company, for which I am subscribed to,
>> started broadcasting Canal+ Film HD some three months ago. This has
>> worked otherwise quite well, but I get constant disturbances/artifacts
>> in the picture. I used to think that it was just because the bit rate
>> was five times the normal (17 Mbit/s compared to 3-4 Mbit/s).
>>
>> But couple of days ago I noticed that this channel is actually the
>> only one I am receiving that uses QAM256. Searching the Internet
>> provided me with couple of forum threads/mailing list posts that
>> suggested that certain DVB-C cards might have problems in receiving
>> QAM256 modulated broadcast, but the hits were mainly on VDRportal and
>> in German and I might have lost something in the translation... In
>> addition to the picture getting garbled every now and then I can
>> notice the problem also from the high values of UNC (uncorrected
>> blocks) and BER (bit error rate). On Canal+ Film HD UNC is between 0
>> and 20 and BER is about 250. On all other channels UNC is 0 and BER
>> close to 0. STR ja SNR are the same on all channels, including Canal+
>> Film HD.
>>
>> Now what I am asking is for any experiences using DVB-C cards to
>> receive QAM256 modulated broadcasts, no matter if HDTV or not.
>>
>> -Petri
>>
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> 
> 
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