Date   : Mon, 12 Feb 1996 15:18:20 +0000 (GMT)
From   : Mark Cooke <mpc@...>
Subject: Re: Decoding BBC tapes...

On Mon, 12 Feb 1996, Ole Stauning wrote:

> 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-samplespr. sec. (make it a
> variable). I would love to program this, but i am long away from home and
> my BBC :-(

Nice - what you can do to avoid noise is to ignore zero crossings withing 
a certain time frame of the initial one:

           ,--------.            .--------
                      `--------'

          ^         ^           ^
          0 crossing
 
           XXXXXX    XXXXXX      XXXXXX
         Ignore regions.

The ignore region should be (probably) just smaller than the shortest 
time between zero crossings that you expect to see. This should add noise 
resilience, as you ignore the times when the waveform is closest to 0.

Just my tuppence worth as I don't have much free time these days. It's 
nice to see the list volume going up again with people working on emulators.
One of these days, I'll blow the dust of BPC and try to breath some life 
into the assembly code again. I wouldn't hold my breath though...

Mark

Mark Cooke                             The views expressed above are mine
Systems Programmer                     and do not reflect in any way the
University Of Birmingham               current policy of my employers.