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

Luc Brosens dvb at bsc-bvba.be
Thu Nov 22 18:15:23 CET 2007



Oliver Endriss wrote:
> Luc Brosens wrote:
>> Hi,
>>
>> side note :
>> the problems in my previous post "KNC1 TV-Station S, revision 0x1894, doesn't tune", were related to the PCI-slots of the motherboard I used
>> rebuilt the machine around a new motherboard, both KNC1's are now recognized and able to tune
>> lesson learnt : check the hardware before complaining about the software ...
> 
> Hm.
> 
>> why does inserting and accessing a CAM reduce the signal and SNR levels ? (even if no descrambling is needed, as for BBC World)
>> how can this be solved ?
>> anyone out there having the same problems ?
> 
> I guess that the CAM increases the noise on the power supply planes of
> the card. This might affect the tuner. ;-(
> 
> CU
> Oliver
> 
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

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  82% | snr  52% | ber 13583 | unc 0 |
status S     | signal  62% | snr  49% | ber 13375 | unc 0 |
status S     | signal  79% | snr  53% | ber 13563 | unc 0 |
status S     | signal  80% | snr  49% | ber 13387 | unc 0 |
status S     | signal  71% | snr  52% | ber 13524 | unc 0 |
status S     | signal  63% | snr  49% | ber 13565 | unc 0 |
status S     | signal  81% | snr  52% | ber 13125 | unc 0 |
status S     | signal  63% | snr  49% | ber 13548 | unc 0 |
status S     | signal  37% | snr  49% | ber 13420 | unc 0 |
status S     | signal  62% | snr  49% | ber 13429 | unc 0 |
status SC    | signal  37% | snr  49% | ber 13627 | unc 0 |
status S     | signal  60% | snr  50% | ber 12929 | 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  62% | snr  49% | ber 13280 | unc 0 |
status S     | signal  67% | snr  50% | ber 13254 | unc 0 |
status S     | signal  64% | snr  51% | ber 13167 | unc 0 |
status S     | signal  62% | snr  49% | ber 13333 | unc 0 |
status S     | signal  81% | snr  53% | ber 13535 | unc 0 |
status S     | signal  62% | snr  49% | ber 14173 | unc 0 |
status SC    | signal  52% | snr  50% | ber 13901 | unc 0 |
status S     | signal  63% | snr  49% | ber 13795 | unc 0 |
status S     | signal  83% | snr  53% | ber 13907 | unc 0 |
status S     | signal  62% | snr  49% | ber 13942 | unc 0 |
status S     | signal  48% | snr  49% | ber 14070 | unc 0 |
status SC    | signal  62% | snr  50% | ber 13688 | unc 0 |
status S     | signal  31% | snr  49% | ber 13815 | unc 0 |
status SC    | signal  56% | snr  49% | ber 13550 | unc 0 |
status S     | signal  62% | snr  49% | ber 14052 | unc 0 |
status S     | signal  32% | snr  49% | ber 14079 | unc 0 |
status S     | signal  62% | snr  49% | ber 13345 | 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  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  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  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  79% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
status SCVYL | signal  80% | snr  84% | ber 0 | unc 0 | FE_HAS_LOCK
status SCVYL | signal  78% | 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  80% | 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 |
status S     | signal  62% | snr  49% | ber 13984 | unc 0 |
status S     | signal  62% | snr  50% | ber 13548 | unc 0 |
status S     | signal  57% | snr  51% | ber 13530 | unc 0 |
status SC    | signal  39% | snr  48% | ber 14016 | unc 0 |
status S     | signal  57% | snr  49% | ber 13375 | unc 0 |
status SC    | signal  31% | snr  49% | ber 13870 | unc 0 |
status S     | signal  62% | snr  49% | ber 13359 | unc 0 |
status S     | signal  72% | snr  51% | ber 13155 | unc 0 |
status S     | signal  81% | snr  51% | ber 13120 | unc 0 |
status S     | signal  75% | snr  49% | ber 13528 | unc 0 |
status S     | signal  81% | snr  52% | ber 13161 | unc 0 |
status S     | signal  63% | snr  49% | ber 13402 | unc 0 |
status S     | signal  82% | snr  53% | ber 13166 | unc 0 |
status S     | signal  63% | snr  49% | ber 13357 | unc 0 |
status S     | signal  83% | snr  53% | ber 12724 | unc 0 |
status SC    | signal  60% | snr  49% | ber 13034 | unc 0 |
status S     | signal  82% | snr  52% | ber 13347 | unc 0 |
status S     | signal  63% | snr  49% | ber 13640 | unc 0 |
status S     | signal  67% | snr  49% | ber 13273 | unc 0 |
status S     | signal  62% | snr  50% | ber 14039 | 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)

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



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