Date   : Mon, 12 Feb 1996 20:14:50 +0000 (GMT)
From   : John Sullivan <js10039@...>
Subject: Re: Decoding BBC tapes...

On Mon, 12 Feb 1996, Mark Cooke wrote:

> On Mon, 12 Feb 1996, Ole Stauning wrote:
> Nice - what you can do to avoid noise is to ignore zero crossings withing 
> a certain time frame of the initial one:
>
>[...]

That, I think, stands a good chance of working, but may still suffer from 
noise a bit.

Another method is to use a 'Schmidt trigger' type algorithm. That is, 
quantize the waveform into 5 sections:
 
1-               -----                         -----
2-            /         \                   /
3-    ______/_____________\_______________/_________
4-         /               \             /
5-       /                   \         /
 -    -                         -----

Register a zero crossing only when the waveform goes from region 1 to
3 to 5, or (probably not and/or or you get twice the frequency you
ought to!) 5 to 3 to 1. With the right tweaking of values, this
ensures that even quite large amounts of line noise won't stop you
recovering the data.

John

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