[Discuss-gnuradio] Decoding CPFSK

[Andrew Rose]

I'm trying to use GNURadio to decode a bit-stream from an FM signal and
could do with some help.

1. Having removed the narrowband-FM modulation from the signal, I'm left
with the following.

2. 1200Hz/1800Hz continuous-phase FSK encoding 1200 bits per second.
i.e. each output bit is either 1 cycle of a 1200Hz wave (1-bit) or 1.5
cycles of an 1800Hz wave (0-bit).  The start of each bit is at a
zero-crossing (although there are obviously zero-crossings which aren't
the start of a bit).

3. I then need to extract the data in bytes from the bit stream (1 start
bit, 8 data bits, 1 stop bit, no parity bits per byte).

I've done step 1 (using a scanner for now - although one day I may use a
USRP for this).  I'm struggling with step 2.  Does GNURadio have a block
(or blocks) for doing this?  Step 3 is trivial.  The only reason I
mention it is because I believe steps 2 & 3 together are a fairly common
method of encoding digital data for radio transmission - and therefore
there might be a single block that does steps 2 & 3 in a single go.

Andrew