If I read that correctly, it is an admission that you did the same sort
of thing at the same sort of time. Bearing my friend's experiences in
mind, watch out for the men in the white coats! :-)
There was no 'digital pot' circuitry out there, so
people invented their own methodologies. It was altogether a better time
to be an engineer - less regulation, less red tape, you could see what
you were working on. Things didn't work, but that hardly mattered. The
curse of compatability didn't haunt us to the same extent.
On Thu, Feb 19, 2004 at 08:49:19AM -0800, Martin McCormick enlightened
us thusly
> I must respond to some of this message.
>
> When I was in Graduate School in 1977, I did a report that
> Summer on synthetic speech systems. I read a bunch of Bell
> Labs' "Benchmark Papers on Acoustics" and other interesting
> stuff and was going to try to actually build a device that could
> make speech sounds. My knowledge and the technology available
> at the time were not quite up to the task yet, so I ended up
> writing a report on how it is and had been done plus I built an
> analog to digital converter and a digital to analog converter
> that more or less worked.
>
> It was how they worked that, in retrospect, seems really dumb.
>
> What I did was to run a voltage controlled oscillator chip
> (MC4024) in such a way as to allow audio to modulate the
> frequency of the oscillator which swept a range between 1.536
> MHZ and twice that frequency. I then built the beginnings of a
> frequency counter whose reset strobe operated at 6 KHZ.
> 256*6000 = 1.536 MHZ. The counter was supposed to go through
> one complete cycle and be back to 0 for the low end of the
> range. As the oscillator neared 3.072 MHZ, the counter which
> was a pair of 7493's would go through 2 cycles and at 3.072 MHZ
> was at 255 or 0XFF. To get a digital sample, I strobed the
> clock line on a pair of 7475 latches. It really does give you a
> 8-bit digital representation of the signal.
>
> In order to make that signal analog, I used weighted resistors,
> so that was the D/A converter.
>
> You could feed audio in to the VCO and what came out was pretty
> darn bad, but it was recognizable. It would have been a lot
> better with low-pass filtering, but it did work as long as you
> didn't let the oscillator stray above or below its range. When
> it did, the results were catastrophic to say the least as the
> counter would overflow or underflow, causing a completely bogus
> reading.
>
> The successive-approximation-type A/D converters used today have
> it all over this scheme, but it is proof that there are many
> ways to skin a cat. The audio taken from the common point of
> the weighted resistors sounded more like the cat being skinned
> alive since you could hear the 3 kilohertz sampling rate and all
> that aliasing, but my instructor took pity on me, I guess, and I
> passed that course. I truly learned a lot that Summer. At the
> time, I was planning to teech basic electronics in a
> vocational/technical school or junior college so the synthetic
> speech was more of a side issue in my studies, but it was pure
> fun.
>
> Martin McCormick WB5AGZ Stillwater, OK OSU Information Technology
> Division Network Operations Group
>
>
> Declan Moriarty writes:
> >Here's one for the nuttier ideas ever expressed on this forum, but
> >you might love it :-).
> >
> >
> >A guy I know made just such an adjustable digital pot as follows back
> >in 1979. He took his signal, ran it through an 8 bit a/d, inverted
> >all lines, and ran it out through a d/a. Adjustment was provided by
> >tweaking the reference voltage to the d/a. Whether the inversion is
> >needed or not for your application, I don't know. Slap a bit of
> >smoothing (0.22uF) on the output and you have audio quality.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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