[linux-dvb] Re: WinTV Nova-CI, DiSEqC and dvbtune

[Holger Waechtler <holger@convergence.de>]

Peter Hettkamp wrote:
> Hello,
> 
> On Fri, Jun 21, 2002 at 12:21:59PM +0200, Johannes Stezenbach wrote:
> 
>>That's why we changed the frontend API in the NEWSTRUCT branch.
>>The FE_READ_SNR and FE_READ_SIGNAL_STRENGTH ioctls are implemented
>>there and simply return values in range 0..0xffff.
>>
> 
> what are the expected semantics of these return values?
> I am just moving the pctv driver to NEWSTRUCT. The pctv hardware does not
> easily supply these values (instead of SNR, it provides a Es/N0 estimation,

that's ok, we used this estimation in some drivers too.


> which corresponds to the SNR via a table. I do not have this table... Signal
> strength might be derived from values in the AGC accumulator, but again
> correspondence to any real world value is still unsolved)

That's the only way to implement this.
(If we would have had any correspondence from these values to the real 
world we would have been able to implement the original Nokia API;)

Just do it this way and scale/offset/clip numbers so that they are 
always in the range [0...0xffff] and a value of 60%-70% means a good 
signal. Unfortunally we don't have any documentation to do this more 
exactly.

Most applications can use this approximation better than an exact dBm 
value anyway. Users want to see a quality bar moving from left to right, 
nothing more. They don't care about absolute numbers.

Holger



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