[linux-dvb] [RFC] SNR units in tuners

Manu Abraham abraham.manu at gmail.com
Mon Oct 27 17:40:40 CET 2008


Andy Walls wrote:
> On Mon, 2008-10-27 at 15:03 +0100, Georg Acher wrote:
>> On Mon, Oct 27, 2008 at 10:37:52AM +0000, Morgan Tørvolt wrote:
>>  
>>> As I used to work with satellite signals on an earth station, and was
>>> responsible for the development of measurement techniques, I thought I
>>> should join in here for some hopefully revelaing info.
>>>
>>> I am guessing here of course, but I believe that there is no real SNR
>>> measurement in any of the tuners available for computers.


True, most demodulators do provide a CNR from which SNR can be
calculated out. CNR is measured from the early stages, while SNR is
measured out from the final stages.


>> My guess is that it is possible. Actually, it is quite easy for QPSK ;-) You
>> only need to calculate the distance of the IQ-value from the ideal symbol
>> center ( (sqrt(0.5),sqrt(0.5)) or whatever) after the
>> rotator/retiming-block.
>>
> 
> Isn't that just Error Vector Magnitude (EVM)?

Not only that, by the time a driver reads that through a slow bus such
as I2C (of course not to be forgotten about other delays: inherent to
demodulator internals) though it can be used to evaluate SNR on the
demodulator IP core, the events being at 2 distinct points of time as it
is not RT wrt the driver, i guess it makes no sense.


Regards,
Manu



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