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

[Martin Braun]

On 07/17/2014 01:05 AM, Stefan Oltmanns wrote:
> 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?

No, that would still be a real signal. In your nomenclature, if ω=10Hz *
2\pi, then you get a complex signal (10 Hz being what you said was the
center frequency of the battery signal).

> 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).

To understand what's going on, I recommend you generate an FSK signal in
GRC and then use quadrature_demod and see what happens.

> 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?

Hm, this sounds too much like guesswork. Do you have a spec sheet or
something describing the modulation in more detail?


M



> 
> 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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