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RE: [Discuss-gnuradio] PMR data decode

From: Matthew Burnham
Subject: RE: [Discuss-gnuradio] PMR data decode
Date: Tue, 9 Jul 2002 16:32:53 +0100

Okay, I've been playing around today. Have managed to get a simple data
slicer working and got some plots which I'll pop on the web in case
anyone's interested (or can tell me the difference between the signal
types). They're at http://www.btinternet.com/~mat_burnham/. 2sine.png
looks to me like a sine wave, but I don't think I've got enough
bandwidth to sample it properly.

I think my main problem is my lack of understanding of radio
modulation... (but I've been looking for documents on the web to solve
that). From what I've seen, I'm not quite sure why the
FreqXlatingFIRfilter is necessary, as the signal is already narrowband
as it has been downconverted within the receiver before going to the
soundcard (other than the filtering).

I'm don't fully understand what Quadrature demodulation is doing, but
will read up on this. I can understand Filtering, but what is
decimation? Other than the middle bits of the chain, I understand the
rest as follows:

AudioSource - input from discriminator output on scanner into soundcard
              analogue --> floating point
Quadrature Demod
Data Slicing - floating point --> binary data (1 = +ve, 0 = -ve for
Output       - binary --> file (e.g.)

Ian Wraith mentioned that some people have been feeding the I & Q data
on each channel of their sound card's stereo line input. What are the
benefits of this - does it allow decoding of some signals that are not
possible through simply connecting the discriminator to the soundcard?

> -----Original Message-----
> From: Ettus, Matt [mailto:address@hidden 
> Sent: 08 July 2002 23:04
> To: 'Matthew Burnham'; address@hidden
> Subject: RE: [Discuss-gnuradio] PMR data decode
> > Is anyone working on deocding of data found in the PMR 
> bands (e.g. MDT 
> > terminals, pagers, etc.).
> I'm not familiar with those specs, but if you send more info, 
> and possibly some audio samples (assuming this signal is 
> narrower than 20 kHz BW), it might be pretty easy to decode. 
> > I'm quite impressed with GNU Radio so far
> > having got a nice display of the waveforms using the audio scope 
> > example. I'm not sure how to proceed in decoding in terms of the 
> > modules to link together. I envision something along these lines:
> It depends largely on what type of modulation it uses.  
> Assuming FSK, the chain would most likely be
> AudioSource
> FreqXlatingFIRfilter
> Quadraure Demod
> Filtering/Decimation
> Data Slicing
> Output
> > This example is for data where the bits are manchester encoded in 
> > amplitude when viewed on the scope. (I originally believed 
> this to be 
> > FSK, but now I'm not sure as I've seen some other data signals that 
> > seem to be a sinewave at two discrete frequencies, which would 
> > necessitate further FSK decoding first. Anyway, that's a 
> side issue on 
> > my lack of understanding of FSK modulation.)
> > 
> > I've got my head around a lot of the architecture having 
> read through 
> > the entire mailing list archive and the (limited) 
> documentation, but 
> > I'm not sure how I would go about getting from an audio input to a 
> > data output. I connected a VrAudioSource to a VrFileSink, but I'm
> > not sure of
> > the format of the data in the file
> If I understand your question correctly, the data file output 
> is pretty much raw binary.  It could be bytes, words, floats, 
> complex floats, etc.
> Matt

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