[linux-dvb] Please see if you can help: Twinhan VP 1025 DVB-SPCICard

Steven Toth stoth at hauppauge.com
Tue Aug 15 21:29:12 CEST 2006


>> I don't fully understand the real electrical/RF implications of circular 
>> left/right vs linear horizontal/vertical other than to say that it's 
>> related to how the signal is sent (polarised) from the satellite (it's 
>> polarisation) and that you have to have a matching LNB of the same 
>> polarisation. In some cases I've been able to receive circular right with a 
>> H/V LNB but that was a rare exception, circular left never worked. I'd say 
>> buy a linear H/V LNB if your goals are to tune to H/V feeds.
>>
>> I'm not suggesting this is your only issue but it's certainly one of the 
>> most important. If you have access to a spectrum analyzer when you could 
>> see very quickly the L-band representation to see whether the LNB is 
>> down-converting correctly, or whether you have a tuner related issue.
>>
>> Steve
>>
>>     
> Thanks, Steve.
> I was getting confused because it appears that even if you specify R or L in 
> channels.conf file, the response from szap still only shows V or H.
> Other documentation that I found says that circular right is activated on 
> the LNB with 13 volts, the same as linear vertical would be activated.  
> Circular left is activated with 18 volts the same way linear horizontal 
> would be.
>   

> Does that sound correct?
>   
Yes. dvbtune (via the standard linux API) really uses V/H to set 13/18. 
R and L basically mean the same thing (13/18). The electronics on the 
LNB however (triggered by 13/18) deal with the polarisations 
differently. If you trying to receive a V/H signal on a R/L LNB this is 
going to be a problem. You need to match a R/L LNB with a R/L polarised 
satellite feed. It's an easy test, re-align your dish to 119deg where 
you have regular DVB feeds running on R/L polarisations - matching your 
DSS circular LNB.

(I did a whole bunch of tests with a spectrum anaylzer and rotating 
circular lnbs, no effect what-so-ever on the downconverted carrier. V/H 
on the other hand, well, it started to bleed energy from the opposite 
polarisation).

In the end we bought a quad LNB for the lab in New York, able to put out 
Linear and Circular polarisations on a pair of cables. The LNB 
specifically handles polarisations for V/H, R/L.


> I am now starting to think that possibly the signal I'm trying to tune to is 
> too weak.  I am only using a small 18 inch DirecTV dish.  Any insights on 
> this?
>   
I used a 18 inch dish for DVB-S work at ome on 119 degress, I'd suggest 
you give it a try.

Regards,

Steve



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