Date   : Fri, 26 Nov 2010 20:26:55 +0100
From   : rick@... (Rick Murray)
Subject: Electron Ferranti ULA reverse engineering progress

On 26/11/2010 05:14, Alan Williams wrote:

> Having looked at the output years ago on an oscilloscope I suspect a 2
> bit D/A converter is used to generate the output waveform.  I couldn't
> speculate on the in put though.

Speculation: Band-pass filter on the two freqs, presence of signal 
triggers a positive state for either logic one or logic zero, which is 
then clocked out as a stream of bits... That's my thought. It wouldn't 
need to be terribly complicated as the input is fairly well defined. 
Just a bit of leeway on the freqs and clocking in order to cope 
gracefully with the minor variations in tape speed.

[OT]
I tried to test this once with two "identical" tape players and C90 
tapes. I measured around 20 seconds difference (1/3 of a minute in 45), 
but I'm not entirely sure there wasn't a factor of tape length as the 
retest was playing the tapes back after swapping them around and the 
difference was greater. Such are the joys of analogue systems. ;-)


Best wishes,

Rick.

-- 
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