Re: Capturing, not avoiding, EM interference

The reasoning about the little noise on the cheap mike and lots on the
expensive one are right on. As are the suggestion of a coil (or phone
coil) to avoid picking up audio noises.

The tales of deliberately picking up interference from computer
cicuitry and playing it through a speaker are also true. The difficulty
is that most computer circuitry operates at much higher than audio
frequencies. But nevertheless, in the old days of the Intel 8080,
Motorola 6800, RCA 1802, and 6502 -CPUs, running at maybe 1MHz (no typo
that!), many a fellow with too much time on his hands and some memory
of high school band or something, did some programming for just this
purpose. Timing loops which demodulated to something hearable mostly,
and inevitably in raw assembler, often hand entered into RAM.

The most impressive demo I ever heard personally was a rendition of
'She'll be Coming 'round the Mountain', performed by a Southwest
Technical Products Corp 6800 with the top cover off. Played right
though a cheap AM radio next to the computer. Little soul to the music,
but the performer could go all night, and in this case did until the
rest of the folks present begged and pleaded...

This was a pretty good machine. An actual linear power supply, a
backplane motherboard, with a dedicated I/O bus, CPU card, and a
glorious 4 or 8 KB of static RAM. Maybe $1500 or so in kit, no
terminal, printer, graphics, modem, ... The extra 4KB RAM card was
something like $400 or $500. Lots of fun putting one together, though.
You could get a cassette tape interface (Kansas City Standard format --
so sloooow, if as reliable as the cassette machine itself) for them for
about $100 in kit, or a paper tape reader/punch (also not a speed
demon). Later, SWTPC offered a kind of peculiar dot matrix printer
(wide calculator paper tape only!) for perhaps $150. They did sell a
kit of Don Lancaster TV Typewriter for not much, so you could get a
serial video terminal if you wanted. 50 or 60 columns of fuzzy fixed
width characters (depending on the quality of the TV), and maybe 28
lines or so if memory serves. A lot better priced than the $2000 dumb
terminals of the time though.

But I'm rambling...

.

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