AFSK Encoding and Decoding in Ruby

Question

I'm looking to write an 'Audio Frequency Shift Keying' (AFSK) encoder and decoder that could be used to transmit and receive data using radio/sound waves.

Bell 202 is the AFSK standard I'm trying to work from it encodes binary data into two tones 1200hz and 2200hz. These two tones stand in for the 1 and 0 in binary.

I'm trying to use win32-sound within ruby to generate those tones, which it does. The issue is that to switch between one tone and move to the other it takes about 1 second to complete which is far too slow for the 1200 baud speed that bell 202 is capable of operating at.

My question is: Is there any other method to create these tones and switch between them faster? Also while I'm here is there a gem that would allow me to decode those tones back into binary data?

Answer 1

I tried to write an AFSK modulator/demodulator in Ruby a while ago. I got a working prototype that could write and read WAV files but came nowhere near real time performance (I optimised it up to about 8x real time if I recall correctly).

Some tips:

• Ruby + Audio pretty much sucks, you probably won't find many Gems for demodulation
• For me, only a pure C solution (with a small Objective-C wrapper so I could use it in an iPhone app) could provide the necessary performance boost
• Eliminating discontinuities in the encoded signal by using Continuous Phase FSK (CPFSK), helps a lot with the signal quality
• I used a coherent demodulator followed by a state machine which worked out pretty well https://www.kth.se/polopoly_fs/1.141538!/Menu/general/column-content/attachment/lec9.pdf