[mythtv] What units of SNR is returned by FE_GET_SNR

Mac Michaels wmichaels1 at earthlink.net
Mon Jun 6 18:05:32 UTC 2005

I send this to both linux-dvb and myth-dev lists because SNR 
is generated by a linux-dvb driver and displayed by the 
mythtv application while doing a tuning scan.

I have searched the DVB documentation for the units of 
signal-to-noise ratio and found only that is is an int16_t. 
Not much help. SNR is usually specified in db and defined 

SNR = 20 * log10(signal / noise)

The spec for my hardware is similar, but different:

SNR = 10 * log10(14400 / [value read from dmod chip])

All this makes sense and produces numbers in the range of 
+43db to -13db. for my demod chip. Thus my driver returns 
values in db that easily fit in an int16_t. The value is 
positive when the signal is greater that the noise. A value 
of 0 means that the signal is equal to the noise. The value 
is negative when the noise is higher than the signal.

mythtv reports the SNR as a percent value. Percent of what?
I see no way to report the max and min SNR from the driver 
so that a percentage can be calculated.

mythtv displays near 0% for a good SNR of say 18db. It 
displays 100% for a negative SNR of say -13db. Since the 
data type is int16_t why is a negative value displayed as 
greater than a positive one?

-- Mac

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