[linux-dvb] DVB API update

Manu Abraham abraham.manu at gmail.com
Tue Sep 18 04:25:02 CEST 2007


Johannes Stezenbach wrote:
> 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)
> 

As an example, BER is not just a dump of some registers. It depends on
the symbols being acquired, averaged etc. If you have a lower symbol
rate, you have a larger acquisition time frame, also lower the symbols,
the larger the number of snapshots required for proper averaging.

Also when you start a measurement, all of it has to be ideally at the
same snapshot of time, eventhough you can't get the exact same snapshot,
you must be somewhere quite near.

Just take a look over here:
http://www.dsprelated.com/showmessage/33285/1.php

No wonder it is so hopeless.

HTH
Manu



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