Re: [Discuss-gnuradio] Demodulating digital signal (FSK?) from audio

[Martin Braun]

On 07/16/2014 03:08 PM, Marcus Müller wrote:
> this doesn't look like FSK, because then the amplitude of the
> oscillations shouldn't change (only their frequency).
> If I had to guess, it would be on-off-keying, and you could simply
> detect that by squaring the signal, and using the integrate block on
> that, with a integration length amounting to your symbol duration in
> samples, which might be a little hard to guess from the signal you
> posted, but maybe you know the symbol rate from elsewhere, or can
> determine it by comparing signals from different battery states?

The dips might also be between bursts -- it does look a bit like FSK,
but hard to say.
Stefan: If you mix this down to zero, your signal will be complex anyway
(radio signals are also always real, but we don't care :D ). Then you
can put it into a quadrature_demod_cf.
Question is, how do you synchronise? Maybe you can use those dips to do
that... Or maybe the symbol timing is well defined, then it's easier.

M



> 
> Greetings,
> Marcus
> 
> On 16.07.2014 14:51, Stefan Oltmanns wrote:
>> Hello,
> 
>> I would like to write an application that checks the battery status
>> of wireless microphones. The battery status is transmitted as a
>> very low frequency (below 10 Hz) signal that is mixed in the normal
>> audio. I was able to filter the signal out of the demodulated audio
>> and display it (see image). AFAIK this modulation is called FSK. 
>> The signal that is shown there should decode to data-blocks
>> containing "11100000000" or something like that, are there any
>> blocks in GnuRadio that can do that? Because the signal is derived
>> from audio it is not complex but normal float, all GnuRadio
>> demodulators seem to work only on complex data. Can somebody please
>> help me?
> 
>> Best regards, Stefan
> 
> 
> 
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