Date   : Thu, 12 Apr 2001 22:55:46 +0100
From   : Dewi Williams <mauay@...>
Subject: Re: storing BBC tape programs on minidisc?

Thomas Harte wrote:
>Of course, before anyone answers in general terms I'd have them remember
>that the BBC tape encoding (well the Electron does anyway, so I'd assume . .
>.) uses a sine wave, not a square one like every other micro ever invented,
>but I wonder : would the compression applied when saving on every day, run
>of the mill minidiscs prevent a BBC being able to decode its saved files?
>And what about if they were instead stored in mp3 format? Has anyone tried
>any of these combinations?

I don't know the details of what form of compression a minidisc
utilises, but I know that MP3 uses a Discrete Cosine Transformation(DCT)
which is based on the Fourier series technology, what this means is that
it uses a series of (Co)Sine waves of varying frequency to build up the
wave, starting with the overall shape adding the more subtle points as
it goes. This is why it is so well suited for sound compression, voices
and instruments produce some form of distorted Sine wave.

The fact that the BBC tape encoding uses a Sine wave should mean that
MP3 can better represent the waveform than if it were a square wave,
which from the original E-mail a Spectrum would use, trying to compress
a square wave would mean that the compressed version would overshoot the
original data at the end of the stalk. (The technical term for this is
Gibbs phenomenon, and occurs anywhere the waveform is not
differentiable.) But I suspect that the low bitrate of a Spectrum tape
would mean that this does not present a problem.

And you never believe people that say that modern Maths has some
relevance.... :))

Jiim Jonez.