[linux-dvb] [Proposal] Meaningful reporting of SNR

Michael Krufky mkrufky at linuxtv.org
Wed Apr 12 19:13:07 CEST 2006


Mac Michaels wrote:
> On Wednesday 12 April 2006 09:57 am, Robert Schlabbach 
> wrote:
>   
>> From: "Rusty Scott" <rustys at ieee.org>
>>
>>     
>>> 1)  That it be encouraged to report SNR in dB whenever
>>> possible.
>>>       
>> I'm afraid most hardware is incapable of producing such
>> measurements. Instead, I'd propose scaling all signal
>> measurements to _percentages_.
>>     
>
> SNR (Signal to Noise Ratio) and Signal Strength are 
> different measurements of the RF signal. 
>
> Signal Strength is difficult to compute for a particular 
> card. It is the actual level (quantity) of the signal 
> appearing on the RF input connector of the tuner for a 
> particular TV Channel. There are both internal (RF 
> frontend, AGC design, etc.) and external (antenna gain, 
> booster amplifiers) that may affect this value. In theory a 
> weak signal with a lot of noise will have the same strength 
> as a weak signal with little noise. In the first case the 
> signal may not be useable whereas it may be acceptable in 
> the second case.
>
> SNR is probably more useful as it is more indicative of the 
> quality of the signal received rather than just the 
> quantity. SNR is a ratio in dB. Increasing positive values 
> indicate that the signal is stronger than the noise. 
> Negative values indicate that the noise is stronger than 
> the signal. SNR is less affected by internal and external 
> influences because additional amplification/attenuation 
> usually affect the signal and the noise equally.
>
> I am not familiar with DVB chips. They may provide signal 
> strength directly. The chips supported by the lgdt330x 
> driver provide SNR directly. The specification gives the 
> exact formula to compute SNR in dB for any value read from 
> the chip.
>
> SNR is not in the "Video for Linux Two API Specification 
> Draft 0.13". I have no problem reporting SNR in dB or as a 
> percentage of full scale. Either way the value returned is 
> useful for aiming an antenna or indicating cable connection 
> problems. I propose adding some more details to the "Video 
> for Linux Two API Specification":
>   
[proposal snipped]

Mac-

This topic does not apply to V4L2 --- We are talking about the DVB 
subsystem.  Yes, we all know that a single tuner is shared between the 
analog and digital functionality in hybrid devices, but this is 
irrelevant to this discussion.  The DVB API is the issue at hand.  V4L2 
has nothing to do with it.

Cheers,

Michael Krufky





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