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

[Stefan Oltmanns]

Thank you very much for the fast answers!
The signal I attached in the last message is generated by a µC and then
a lowpass is applied (not just RC, but two op-amps). In another mic type
the signal is generated by changing the reference frequency (a varicap
controlled by the µC is connected to the main crystal).

What exactly does mix to zero mean? From what I understood so far,
I=RF*cos(ωt) and Q=RF*sin(ωt), when I set ω=0 => I=RF and Q=0 is that
what you mean?

I changed the filter settings and now the dip between two blocks is more
precise. I attached two signals generated by the different microphone
types using the same protocol. I tried the quadrature_demod, but result
especially for the varicap-mic seems not be useful (also tried changing
the only parameter).
I think what I have to do is measure the distance between two peaks
(only positive, threshold 200u):
Distance more than 0,5s -> New Block starts
Distance between 0,3s and 0,4s -> 1
Distance between 0,15s and 0,25s -> 0
else -> reset (delete buffer and wait for new block start)
Can this be done with GnuRadio?

Best regards
Stefan

Am 16.07.2014 15:43, schrieb Marcus Müller:
> Ahh-- my mistake, I was assuming the "dips" were something like one
> symbol, the other being the continous wave with the 400u amplitude, and
> completely missed the differences in period on the non-dippy signal...
> The lower halfwaves of the lower-frequency oscillations look a little
> strange; maybe this signal was generated by RC-lowpassing a PWM signal?
> 
> 
> On 16.07.2014 15:18, Martin Braun wrote:
>> 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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battery_mc.png
Description: PNG image

battery_varicap.png
Description: PNG image