From: Graham Toal <gtoal@an-teallach.com>
Date: Fri, 20 Aug 93 08:01:03 PDT
To: cypherpunks@toad.com
Subject: building a sound sampler for cryptophone application...
Message-ID: <9308201356.AA07181@an-teallach.com>
MIME-Version: 1.0
Content-Type: text/plain


My old uncle - a retired electrical engineer - has gone back to
night classes to learn electronics.  His class has to do little
projects that they pick themselves; nothing too advanced yet -
his last one was a musical door-chime...

Anyway, he was asking me for ideas for projects to build, and believe
me I've got dozens :-)  The one that I think is of most value to most
people, if he designs a simple circuit for it that I can publish,
is a cheap sound sampling interface .... which we can happen to
steal for our various cryptophone projects...

Here's how I see the design:  it feeds data into a (probably IBM PC)
parallel port (has to be the bidirectional kind, a plain printer port
clearly won't work), and *it* supplies the timing, ie the PC reads from
the port when the flags say data is ready, and no more data will be
presented to that port until n uSecs later - I foresee it supplying
data at either 8000 bytes/sec or 4000 bytes/sec if the former is too
fast for a PC to handle.

Putting the timing in the sampler frees the PC from a horrendous overhead
in sampling at accurate times, and would make it trivial to feed the
4000Hz samples into code like 'shorten' which would then be shoved down
a v32bis modem quite comfortably.  At a cost *much* less than any
commercial sampler: this thing is built from: a battery; a box; a DAC;
a 7$ microphone; a parallel port driver; a crystal, and a counter.
And that's it.  Dead cheap and easy to build, I hope.

What I'm writing here to ask is where can I get info on what chip to
use to feed data into a PC down the bidirectional parallel port, and
how do you drive the chip and what are the pinouts etc.  I don't expect
anyone to mail me detailed schematics or anything like that, just a
pointer to where to look for them.  (Though if someone *did* have data
sheets, I wouldn't say no to a quick fax :-) )

(fwiw, I used to do electronics as a hobby *years* ago - I once
built a dual-processor micro with dynamic ram, so don't be shy of
mailing me anything grossly technical; I've forgotten most of what
I knew in detail, but I remember enough to steer my uncle in the right
direction, though I won't be doing the actual circuit design myself)

Thanks for your time.  I hope this isn't considered too off-topic... (I
mean, I *could* have posted an incredibly interesting piece about the
Challenger disater instead, eh Eric? ;-))

G
PS Pointers to suitable usenet groups equally appreciated...