1200 and 9600 bps Packet RX
Ever wondered what that hissing packet sounds mean? wire your radio speaker to the PC soundcard input, run Soundcard Packet and see whats going on. that worked for the ISS too. (reading only, no TX)

download Soundcard Packet RX

Technical Stuff:

1200 BPS Packet switches between two audio frequencies, 1200 and 2200 hz.
The smallest "unit" of constant frequency (one bit) is 1000/1200 = 0.83 ms. That is ONE period at 1200hz, nearly 2 periods at 2200hz.

Supposing we sample the audio input with 12000 samples/sec one bit will be 10 samples long. Calculating the DFT (FFT) of those 10 samples will tell us if the 1200hz or the 2400hz component is bigger. Bad news is that the 2200 hz signal doesent match those 10 samples, 2400 hz would be much simpler to decode. The next approximation is 2182 hz, needing 11 samples for DFT.

To get an good estimation we have to calculate 2 DFT's (10 and 11 samples), but luckily only one "bin" per DFT:
-bin 1 for 1200hz, 10 sample DFT
-bin 2 for 2200hz, 11 sample DFT
In other words we calculate only 2 complex correlations.

That is the DSP code of "PacketTRX".
"Soundcard Packet" is slightly different. It uses one FFT with 16 samples length, 11025 Samples/sec, and compares the 2 bins nearest to 1200/2200 hz.

from me here
packet.zip