[linux-dvb] Writing DVB Software - couple questions

Krzysztof Matula km at km.apnet.pl
Thu Feb 24 22:02:39 CET 2005

Chris Warren wrote:
>>[mailto:linux-dvb-bounces at linuxtv.org] On Behalf Of Johannes 
>>Yeah. Some values *are* scaled into 0..0xffff (e.g. snr), but 

That's not enough.
How are they scalled? Linearly? In other way?

>>that still doesn't mean that you get well-defined values. 
>>E.g. some cards will return 0x1234 and others 0x2345 for the 
>>same input signal. I don't know how you want to calibrate 

This seems to be acceptable for signal strength, but not for the other 

> One idea might be in the client software to keep a record of the minimum and
> maximum experienced value and display a percentage/value based on this. So
> you get a scale from 0%-100% for the known limits of the current setup.

Not an good idea either. You still don't know wheter the signal is good 
or bad and haw good or how bad it is.

 From the user/client software point of view the following definitions 
would be appropriate:

signal strength: 0..1
(0 as the hypothetical worst possible signal, and 1 the best)

SNR: exact value in dB (logarithmical scale)

BER: a value representing an average number of errors per bit (the 
current value, not an avarage from the driver start). Usually should be 
0 or well below 1e-6.

It would be most flexible if the values are encoded as floating point 
numbers. Alternatively, some fixed point encoding may be used.


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