[linux-dvb] DVB API update

Johannes Stezenbach js at linuxtv.org
Mon Sep 17 18:02:50 CEST 2007


On Sat, Sep 15, 2007, Wolfgang Wegner wrote:
> 
> - dvb_fe_type: DVB-S2 is missing and I personally would also like
>   to see ASI here...

See my other mail, IMHO we should add the ASI defines at
the same time we merge a driver which uses it.

> - frontend status:
>   - BER is lacking a proper definition (to which base is it calculated?)
>   - signal strength: same problem, what are the ranges?
>   - snr: again, no base and ranges given

The original Nokia DVB API spec had proper definitions, but
Holger Waechtler and me decided to drop them in favour of
"read hw register and scale to range 0..0xffff". The reasons
for this were:
- for some hw we didn't have info to implement it properly
- for those hw where we had info we saw it wasn't worth the
  effort -- I believe the implementation in the demod firmware
  is just such a coarse approximation that it doesn't
  even make a difference if you take the logarithm or not
  (provided you scale back into the 16bit range)
  (of course you get different numbers, but none of them
  maps any better to what you'd expect)

Someone once told me: Marketing requirement is that the
signal quality bar is in the upper third when the picture
is good, and in the lower third when the picture is bad.
Everthing else is irrelevant and the numbers can be pure fantasy.
And that's what I believe you get from most demods, and how
it is implemented in most commercial receivers.
But, well, that was a few years back and it was crappy hw, maybe
the situation improved.

I'm not opposed to it if you want to improve the situation,
but IMHO you shouldn't put your expectations too high...


HTH,
Johannes



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