Date   : Sun, 11 Feb 1996 21:15:13 +0000 (GMT)
From   : John Sullivan <js10039@...>
Subject: Re: Decoding BBC tapes...

> On Fri, 9 Feb 1996, Alan Hart wrote:
> > > I'm in the process of writing a program for decoding BBC tapes
into blocks
> > > and assembling blocks into files, by analyzing a WAV recording
of the tape.

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. 

You then need to recover the byte stream from this, which is slightly
harder but you don't have to be too intelligent about it. (Since you know
the bitstream is either 300 or 1200 baud?)

If you *then* want to process the byte stream to get at the file directly,
you could just emulate the beeb's UART with the recovered byte stream as a
data source, and use the OS to decode for you. If you want to do this by
hand I think the tape block format is described in the UG/AUG (my copies
of which are ~160miles away at the moment). 

John

-- 
Go away cat, you make me smile too much.
<a href="http://callisto.girton.cam.ac.uk/users/js10039/">Me!</a>