Re: [Discuss-gnuradio] new to GNUradio: AFSK and codec2 gmsk modem

[Marcus D. Leech]

On 07/28/2013 07:35 PM, Kristoff Bonne wrote:
> Hi,
>
>
> I'm still very new to GNUradio, so please excuse my (perhaps dump) 
> question. I did look around at different places at the web but did not 
> really find an answer:
>
> - My first step with GNUradio was to learn a bit more about using it; 
> so .. do some test.
> I wanted to use GNUradio to determine the width of an AFSK signal; so 
> I started looking for a AFSK modulator / demodulator; to come to the 
> conclussion I didn't find one.
>
> I looked in the GR source, on CGRAN, using several generic search 
> engines without look.
>
>
> Am I missing something here? I would be surprised there is no AFSK 
> modulator / demodulator in GR as it would mean one cannot do 
> packetradio or APRS with gnuradio.
>
>
> - The main reason I am interested in gnuradio is because one of the 
> projects I work on is "c2gsmk", a GMSK-based modem for VHF/UHF using 
> the codec2 vocoder. Gnuradio looks to me like a very interesting 
> platform to do simulation and work on the modem.
> The modem is now rewriten as an API which can operate purely on 
> bitstreams (i.e. all GMSK modulation / demodulation taken out of the 
> code): 56 bits in (40 ms @ 1400 bps codec2), "n" times 96 or 192 Bits 
> out (40 ms @ 2400 or 4800 bps c2gmsk).
>
> I have been browsing the different documents on how to write a block 
> and also looking in some code to find simular examples on how it is 
> done. I have three things I kind-of do not understand.
>
> - How does GNU radio deal with "sessions"? A c2gmsk session is not 
> just a stream of bits; it has the notion of sessions. It has a 
> beginning and a stream also ends; and it can end in a number of 
> different ways.
>
> I'm trying to understand how to fit the idea of "sessions" into GNUradio.
>
> Or am I seeing it all wrong?
>
>
>
> - How does GNUradio deal with information about events in the API.
>
> The c2gmsk API does not only return a bitstream, but also information 
> about events; like a change of state in the statemachine of the API, 
> or an event like "a GMSK sync-pattern has been detected", or "the 
> stream has terminated due to to many out-of-sync frames", or the 
> versionid of the stream received, or statistics of the golay decoding 
> process, etc.
>
>
>
>
> - To get some idea of how a block is written. I have been looking at 
> the codec2 encoder/decoder block. It is interesting as it is also "x 
> bits in, y bits out".
> However, c2gmsk is a bit different: there always is a fixed predefined 
> number of bits in, but the number of bits "out" can vary. Sometimes 
> just send in bits, but do not get anything back (e.g. in the decoder 
> when the stream is not syncronised), sometimes one frame inbound 
> results in one frame outbound, but sometimes one single frame in can 
> result in (say) 5 frames out. (e.g. when encodig the start of a stream).
>
> Is this an issue for gnuradio?
>
>
>
>
> Any information welcome.
>
> Chéééério!
> Kr. Bonne.
>
>
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Just found this, it might help:

https://github.com/dl1ksv/gr-afsk


Also, keep in mind that AFSK is a kind of "double modulation scheme".  
To demodulate it, first demodulate the FM-voice using a narrowband
  FM demodulator, then run that into a suitaby-configured FSK demod.

AFSK was always a "hack" to allow you to send data over narrowband 
FM-voice radios without having to do anything to the radio.  So, they chose
  Bell-202 synchronous modem tones, because they'd "fit" in most 
narrowband FM-voice radios.