[linux-dvb] How do I turn the signal/noise, siganl strength into meaningful values?

Mac Michaels wmichaels1 at earthlink.net
Mon Jun 20 16:13:06 CEST 2005


I asked a similar question and was told that there is no 
specification. It is what ever the driver writer decides to 
return.

In my question concerned FE_READ_SNR. My driver computes the 
SNR in db and returns that. Since the data type was signed 
I assumed that was the expected value. I return negative 
numbers for no signal and small positive numbers i.e. 18+ 
for reasonable signals. The actual SNR value is computed 
from raw data supplied by the demod chip.

Unfortunately in this case mythtv treats the signed int as 
an unsigned int and displays it as a percentage. This good 
SNR values from my driver appear as very weak values and 
negative values peg the meter at 100%.

The v4l2 specification might be amended to include more 
information in the info struct. Applications can then know 
the units of the various signal conditions reported by the 
driver. This could be decibels, tenths of decibels, 
microvolts, raw data, percent full scale, errors per 
second, etc.

-- Mac.  

On Monday 20 June 2005 06:13 am, Barry Scott wrote:
> Using the FE_READ_SIGNAL_STRENGTH, FE_READ_SNR and
> FE_READ_BER I get numbers but need to understand how to
> turn them into useful to humans values.
>
> I'm using AverMedia 771 DVB-T card and Pinnacle PCTV-SAT
> on 2.6.11 for FC3.
>
> First observation is that FE_READ_SIGNAL_STRENGTH values
> seen to be smaller for larger signals.
> Second observation use that FE_READ_SNR always duplicates
> bits 0-7 in 8-15, why?
>
> What are the units what is the scale?
>
> Barry
>
>
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