toughy

[WeAreAs1 at aol.com]

Paul Maddox wrote:

<<  I have a wav file of some data from the PPG2.2 (its the factory 
data) and it uses two frequencies, I guess one for 0 and one for 1. 
what I would like is someway of decoding this into a data file. any 
suggestions?>>

It sounds like the data is encoded with FSK (Frequency Shift Keying).  This 
is the method that was used by many older synthsizers (before MIDI sysex 
became the norm) and older computer systems - the data was encoded into an 
audio signal and stored on audio tape.

It would be very easy to build an FSK decoder circuit that would turn the 
high and low frequencies back into binary bits, but getting those bits into a 
format that you can use may be a bit more difficult.  You would need to load 
the bits into some kind of RAM buffer, either on a DIY board, or directly 
into the RAM of your PC via its serial port.  For me, this sort of thing is 
non-trivial, but maybe some other list member has a simple and elegant method 
for doing this (synchronously reading serial data directly into a PC's RAM).

Of course, you'd also have to further distribute the serial stream into 
bytes.  In hardware, this could be done with a serial-to-parallel shift 
register (such as a 74164 or 7491, or the like).  Let's just hope that there 
are no inconvenient "mystery bits" dispersed between those relevant, juicy 
PPG waveform bytes.  In a PC RAM buffer, it'll all end up as bytes anyway - 
you'd just have to shift the data left or right until the bits lined up in 
the bytes the way it was originally intended.

Still, I'm wondering if that data includes any non-waveform data, 
interspersed willy-nilly among the waves.  If so, how do you determine what 
is wave, and what is not?

Let me know if you need help in building an FSK decoder circuit.  I would be 
glad to help out with this.

Michael Bacich