Re: [Discuss-gnuradio] Decoding 2FSK Compensating for carrier jitter/skewing (CFO)

[Cinaed Simson]

On 07/09/2017 12:15 PM, Andy Walls wrote:
> On Sat, 2017-07-08 at 21:38 -0400, Andy Walls wrote:
>>> Date: Fri, 7 Jul 2017 19:50:55 +0300
>>> From: HLL
>>> Hi all,
>>>
>>> I'm relatively new to DSP and gnuradio but I tried tons of stuff
>>> and
>>> I couldn't decode a fairly simple FSK data.
>>> baudrate seems around 600-700 bps and fsk deviation is less then
>>> 3k.
>>
>>
>> Hmmm.  I took a look at your signal and tried building a coherent 2-
>> FSK 
>> demodulator.  Under the assumption that it was straight 2-FSK, the
>> signaling tones looked to be at +/- 1200 Hz when properly centered.

Just to be clear - doesn't 2FSK have 4 tones? I'm not sure what you mean
by signaling tones.

I just looked at it in inspectrum and there are 4 visually identical
structures - and I would guess the fact there are 4 structures is just a
coincidence.

They're roughly 1.45 seconds long with blank spacers varying from 116 ms
to 200 ms.

I must be doing something wrong.

-- Cinaed


>> The
>> fastest bit rate appeared to be 1880 bits/sec.
>>
>> But in reality that doesn't work.  I could never get good symbol
>> timing
>> recovery as the "FSK" signal appeared to have two different baud
>> rates.
>>
>> After some reflection, the signal you have appears to actually be
>> AFSK
>> inside of FM.  Zoom out a little on the output of the quad demod, and
>> your eye can see the two tones.  The two tones appear to be at 350 Hz
>> and 940 Hz.  The tones are unusual in that they are square wave tones
>> vs. sine wave.
>>
>> I haven't worked you the baud rate yet.  I'll hack away at it more
>> tomorrow. 
> 
> It definitely is AFSK in an FM transmission.
> 
> The baseband baud rate appears to be a 233 bits/second.
> 
> The beginning of every packet starts out with the following bits:
> 
> 000000000000000000000000 100 01111110
> 
> I keyed off of those last 8 bits above (01111110), since they looked
> like an HDLC flag character to me.
> 
> The payload bytes of the 4 packets in the file (after that 01111110 bit
> pattern) are:
> 
> 6884485b066e7505647e875a4ac70c74447474477e47e5f85c47065f44be79a74e67a6a452bfffffffff
> 68c4485b062c7565e67227564ac74e74247478477447e5f85ce7065f44be75a74867a6ec52dfffffffff
> 68c4485a278e6f856e7006706a470c7fa4f67c057c47e7f06f47447f66ae7d867048874243dfffffffff
> 68c4485a278e6f85ec70a67a6267cc7fa4f678257647e7726fe7447f662c7d8670488700437fffffffff
> 
> The ff's at the end are the space after the end of the packet.
> 
> 
> Try the attached GRC file.
> 
> For it to work, you will need to build and install:
> 
> 1. A relatively recent GNURadio
> 
> 2. The gr-nwr OOT module found here:
> https://github.com/awalls-cx18/gr-nwr
> You'll have to write your own PyBOMBS recipe file, if using PyBOMBS to
> build GNURadio and OOT modules.
> 
> 3. The gr-reveng OOT module found here:
> https://github.com/tkuester/gr-reveng
> 
> Regards,
> Andy
> 
> 
> 
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