Date : Wed, 07 Apr 1999 13:02:20 +0100
From : "Christopher J Thornley" <C.J.Thornley@...>
Subject: Re: BBC tape to MP3 and back to Beeb
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I have a variation of the circuit which uses two opamps and one as a schmitt
trigger
This is equivalent to the circuit in a c64 cassette recorder, which turns a sine
wave or shaped signal into a square wave clearing up most of the signal
I have ordered by interlibrary loan various circuit for spectrums and c64 which
clean up signal and may of them use the internal circuitry from a C64 cassette
recorder.
These signal where then fed straight into a TTL input i.e. a edge connector.
Spectrums tended only to use the diode circuit I included to decode the signal.
This operates like a poor man multi channel analyser and send a one for signal
above a value and nothing for signals below the key is to sample from it at a
high rate and due to data redundancy i.e a lot of 0000 or 1111 work out where
signal are and possibly correct errors.
Most computers i.e. BBC, Apple etc. either generate the signal from a Uart which
is intern amplified. Or from a TTL pin in case of the Spectrum, C64 amplified
and recorded to tape.
i.e. in and Elektor 1985 article which had a circuit on using a domestic
cassette recorder with a C64 used the schmitt trigger circuit.
I have looked at some signal from some commercial games and these are perfect
square waves. Compared to experiments of signals which have saved to tape which
are distorted square wave i.e. asymmetric curved triangle waves
If any ones interested I will scan these article in and OCR them when they all
arrive from interlibrary loan.
As far as im aware no computer used FSK decoders either because they where
expensive or not compatible with the signal formats i.e the signal where not
perfect sine or square wave and a lot of compnets where probally required to
clean them up
Does anybody know if there where any variations in tape format i.e. fast
loaders, copy protection where ever used on the BBC as the only system I know of
is the Danish Basicode program which was supposed to allow basic to be passed
between computers by using a standard tone.
Chris
"Robert Schmidt" <robert@...> on 29/03/99 18:07:51
To: Christopher J Thornley/CMS/University of Salford/GB
cc: bbc-micro@...
Subject: Re: [BBC-Micro] BBC tape to MP3 and back to Beeb
Considering the low baud rate of the BBC tape signal, there's lots of
redundant information in a straight sample of a tape. Of course, the
risks in using MP3 are:
1) the initial sampling rate (when making an uncompressed WAV, for
example)
2) the compression factor chosen in your MP3 compressor.
I think Chris Richardson posted reasonably safe figures for this.
That said, tape is a lossy medium anyway. It was amazing how bad tapes
could sound, while still loading flawlessly on the Beeb... :)
I'm sure your circuit is fine, but does it handle noise and spikes?
They gave me a hard time when I wrote my BBC tape decoder program. (I
wrote it without any knowledge of signal processing... :-)
Christopher J Thornley wrote:
>
> Would there not be data loss? MP3 is lossles compression i.e an aprox
> representation of a signal. So you have to set the data rate correctly. Other
> wise you could lose all data.
--
Robert Schmidt <robert@... > Software Engineer
Technoguide AS: http://www.technoguide.no
Tel: +47 22510463 Fax: +47 22734749
Private: http://www.nvg.unit.no/~rsc
Tel: +47 22606076
jeg lukker et oye og ser halvt
jeg lukker begge og ser alt -- seigmen
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