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

[Stefan Oltmanns]

Am 17.07.2014 10:04, schrieb 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 think the center is 4 Hz, the signal is below 10 Hz

> 
>> 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.

That´s a good idea, but I have a stupid question: How do I generate a
FSK signal? There are lots of modulators available, but no FSK.

>> 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?

No, it´s a proprietary protocol and I´m reverse-engineering it (I can
actually read it when I look at the signal, what I have to do is make
the software read it). All I have is circuit diagrams (service manuals)
of microphones and receivers. There is no detailed description of the
battery status signal (and of course no source of the µC code), no more
than it´s a low frequency signal in the range around 5 Hz with a nominal
FM deviation of 2 kHz.

I was able to remove everything below 200u of the signal, convert it to
complex and then quadrature_demod generates something quite nice. Is
that how the demodulated signal is supposed to look like? AFAIK it´s now
pulse position modulation.
But it´s still an "analog" signal, how can I make a real digital signal?

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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grc_screenshot.png
Description: PNG image

battery_demod2_mc.png
Description: PNG image

battery_demod2_varicap.png
Description: PNG image