Date   : Mon, 12 Feb 1996 11:02:06 GMT
From   : David Alan Gilbert <gilbertd@...>
Subject: Re: Decoding BBC tapes...

John Sullivan wrote:

 > I think the BBC's tape format should be fairly easy to decode. It uses a
 > really basic FSK (ie, serial data with one frequency to represent 0 and
 > another to represent 1, probably both fairly crude squarewaves), so if you
 > count zero-crossings of the waveform over a shortish time period, you will
 > probably get two distinct values which correspond to the two frequencies
 > used to record the serial tape data-stream. You can therefore quite easily
 > convert the WAV file into a serial bitstream. 

I had a crack at this a couple of months back; I have two ways of
undoing FSK; one is zero crossing counting and the other is measuring
the slope of the zero crossings.  The slope measurement is being used
in a program I'm writing to decode weather fax; its nearly working
(bar some Linux kernel bugs...).
The bit stream I got out never looked quite right; sometimes I could
see the 42's at the start of blocks but more often I couldn't.
However the tape recorder is a bit warbly so that might need fixing first!

Cheers,

Dave

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