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Signal to noise reported by kernel for HD-5500
PostPosted: Sat Apr 05, 2008 9:32 am Reply with quote
tld
 
Joined: 05 Apr 2008
Posts: 2




The signal to noise reported by the kernel's dvb driver for my HD-5500 cards (being used for OTA DTV) seems impossibly low.

If I use the femon program from the linuxtv dvb-apps package (in my case media-tv/linuxtv-dvb-apps under gentoo) I get the following:

Code:
femon -a 2   
FE: LG Electronics LGDT3303 VSB/QAM Frontend (ATSC)
status SCVYL | signal f642 | snr 21ab | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal f642 | snr 21ab | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal f949 | snr 2215 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal f642 | snr 21ab | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal f949 | snr 2215 | ber 00000000 | unc 00000000 | FE_HAS_LOCK


...which, when showing in 'human readable' format is:

Code:
femon -a 2 -h
FE: LG Electronics LGDT3303 VSB/QAM Frontend (ATSC)
status SCVYL | signal  96% | snr  13% | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal  98% | snr  13% | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal  94% | snr  12% | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal  97% | snr  13% | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal  93% | snr  12% | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status SCVYL | signal  97% | snr  13% | ber 00000000 | unc 00000000 | FE_HAS_LOCK


On my MythTV frontend, if I use the F7 key to bring up the signal monitor (which represents the s/n in dB) it shows about 3.9dB.

I'm on a very recent kernel (2.6.24-gentoo-r4) so my video4linux drivers are certainly recent.

The above are numbers I get when my reception on that station is flawless. If the s/n were correct, I can't believe I'd even get a lock. Does anyone understand know if the dvb drivers just simply reports that incorrectly from the HD-5500? Thanks in advance.

Tom
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PostPosted: Sat Apr 05, 2008 10:38 am Reply with quote
xyzzy
 
Joined: 12 Feb 2006
Posts: 225




Divide the SNR value by 256 to get the approximate SNR in dB.
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PostPosted: Sun Apr 06, 2008 8:23 am Reply with quote
tld
 
Joined: 05 Apr 2008
Posts: 2




Thanks! So you're saying if I take the raw number reported, for example the hex value 21ab divided by 256: 8619/256 = 33.7dB?

Certainly makes for a much more believable number.

Out of curiosity how did you find that out?

Thanks again.

Tom
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Signal to noise reported by kernel for HD-5500
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