[linux-dvb] CAM inserted/used reduces signal and SNR ?

Manu Abraham abraham.manu at gmail.com
Thu Nov 22 21:11:30 CET 2007


Luc Brosens wrote:
> 
> so I changed gnutv to only initialise the CAM after obtaining FE_HAS_LOCK
> apparantly, initialising the CAM is enough to loose the tuning lock, not to return afterwards
> so this is not a solution
> 

This won't help. The CPU on the CAM is running irrespective whether 
you are trying to initialize it or not. In fact the CPU usage is much lower 
at initialization time.

> here's a run with femon monitoring the tuning status :
> 
> I start of with the CAM in the slot, and try to gnutv-tune to BBC World (which is not scrambled, so need for the CAM)
> mythtv at silverstar:~/dvb-apps> femon -a 1 -H
> FE: ST STV0299 DVB-S (DVBS)
> status S     | signal  63% | snr  49% | ber 14077 | unc 0 |
> status S     | signal  79% | snr  51% | ber 13311 | unc 0 |
> status S     | signal  69% | snr  50% | ber 13776 | unc 0 |
> status S     | signal  31% | snr  49% | ber 13116 | unc 0 |
> status S     | signal  54% | snr  49% | ber 13085 | unc 0 |
> status S     | signal  75% | snr  49% | ber 13509 | unc 0 |
> status S     | signal  80% | snr  50% | ber 13017 | unc 0 |
> status S     | signal  21% | snr  41% | ber 0 | unc 0 |
> 
> no FE_HAS_LOCK achievable, both signal levels and snr are too low
> so I pull out the CAM (and restart gnutv)
> the lock is immediate :
> 
> status SCVYL | signal  80% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
> status SCVYL | signal  79% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
> status SCVYL | signal  79% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
> status SCVYL | signal  78% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
> 
> signal is around 80%, snr a consistent 84%
> I re-insert the CAM, which is detected by gnutv, and the CAM is initialised
> tuning lock is lost at once :
> 
> status S     | signal  62% | snr  50% | ber 13585 | unc 0 |
> status S     | signal  31% | snr  49% | ber 13498 | unc 0 |
> status S     | signal  82% | snr  49% | ber 13540 | unc 0 |
> even though the signal level drops, it's probably the drop in snr that's causing trouble
> 
> both cards have the same problem, so it doesn't appear that they're defective (or they both happen to have the same fault, which is unlikely)
>

It looks to me that something is wrong with the frontend drivers (someplace) 
which causes a phase distortions, alongwith that in that "tender" state the 
digital noise from the CA module CPU's adds to it. The "tenderness" can be
possibly attributed to a wrong device setup.

You might like to check with your windows installation whether you see the 
same issue. Most likely, i guess not. If you see the same with windows 99% 
chance is that that whatever you might do, digital noise is affecting your 
demodulator and the tuner.

> question time :
>> how much of a chance do I have of solving this if I somehow increase the signal strength ? would that increase the snr too ? (I could shorten the
> cables, even if it means relocating the PC)
>> without the CAM, I have a signal strength of 80%. Is that considered a good, strong signal ? (my settop box is happy with it)
>> I asked in a previous post if it's possible to capture scrambled to disk, and descramble later. This is what my settop box does. Is this even
> theoretically possible (sending data to the CAM from a file instead of from the tuner) ? Anybody know of  a utility that supports this ?
> 
> and now I'm off installing W2K on the damn thing, to see if that works ...
> 
> Luc


Manu




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