CCs: JMC - at end

> Interesting part is that this is simply an ADC operation, if you could
> have chance to increase the gain of the signal coming from =A0internal
> microphone in hardware then there wouldn't be any need for an
> extension to read response from the stripe. I know I was close to
> doing this, without any extensions I remember I was able to partially
> get track data from the stripe using the internal microphone.

That is probably doable with a single track (and I in fact suggested
that in another forum earlier today) but if you want to read and
transfer all 3 tracks present on some cards for some applications it
would be "harder" [tm]. Still notionally doable, but assuming you need
a 0/1 decision point for each track, you then need to be able to deal
with 8 levels and tone encode them in some manner - whether phase or
frequency shifting or AM or ... . I think somewhere in the middle of
that you go and get a 20 cent microcontroller (100k volume) and use
that to make life simpler and more flexible.

SX had the advantage of being fassssssst (for its day). Not many other
advantages though :-). "Cleverest" data reading task I've done (and
almost my first) was arguably reading a ? 7 track mag tape data deck
which inputs perhaps 2k word blocks in a burst. Had to use a  4 MHz
6802 (1 uS cycle) - one of only two processors sensibly available at
that stage. Other was 8080. (Uphill both ways, bottom of a lake ...).
There is no way that the processor can loop fast enough to keep up
with the data burst using "normal" code.  So steal its subroutine &
IRQ hardware stack pointer, turn off IRQs and go to. It was dragged
out of this utterly minimalist wait-for-clock, read, push, loop ...
loop by some end of block hardware exception which I now forget.
Probably used NMI and a tape-transport ready line. Worked. Read NZ
Post Office international toll exchange tapes for some years until
newer solutions arrived. (Real reading of tapes was done using a
mainframe. This system allowed in-exchange reading for urgent call
trace etc). Thinks. Hmm - end of 1977 - only ~ 34 years ago :-). Did
you have microprocessors back in those days, daddy? Only just, son,
only just. Wow. 34 years :-).  [[CC JMC was my boss]].


           Russell

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