Date   : Mon, 12 Feb 1996 15:47:44 +0100
From   : Ole Stauning <os@...>
Subject: Re: Decoding BBC tapes...

> >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. 
> 
> The waveform is pretty sinusoidal.  I found that to reliably detect
> zero-crossings and to reliably distinguish "0" (frequency F) and "1"
> (frequency F/2), a sample rate of 44.1 kHz is "neccessary".  I'm having a
> small problem with noise, and I need a sampling program which does not
> include disturbing clicks when it switches from one buffer to the next.
> 

Maby it is possible to take smaller sound-samples, maby 2400 times pr.
sec. Every
sound-sample should be long enough to enclose two successive zero crossings.
You
only have to measure the time between the two crossings. Of course this
method
would be more sensitive to noise, but then you could try to take more
sound-samples
pr. sec. (make it a variable). I would love to program this, but i am
long away
from home and my BBC :-(

Just an idea!


Regards!

 Ole Stauning. ( any mail to immos@... !!)


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