[sdiy] HF-VCO / counter / memory / DAC ?

[Phillip Harbison]

Michael Zacherl wrote:
> Hi, based on the NCO thread, I have to ask: did a HF-VCO +
> counter + memory + DAC combination go out of fashion?
> I understand it’s not really feasible for a commercial
> product, but for DIY?

This describes my senior project in 1980-81. It was a high
frequency clock feeding a bank or 4 74LS161 counters which
I could program to get 256x the frequency I wanted. The
output fed into 2 more '161s which generated an 8-bit
address for a (for the time) 1024x8 static RAM which fed a
fast (for the time) 8-bit DAC. That DAC fed a multiplying
8-bit DAC that I used as digitally controlled amplifier.

The worked and got me an A, but there was one problem. To
be able to generate 256x any top octave frequency from a
single clock I would need a ridiculously fast clock and
ECL logic both of which were way outside my budget.

I was not familiar with phase-locked loops in those days.
If I built this again now, I would use the 4x '161s to
generate the fundamental frequency then use a PLL to
multiple that by 256. I plan to try it someday. I'm not
sure a PLL can lock fast enough to not sound like it is
constantly in "glide mode". I'm not even sure there are
PLLs with that kind of range.

Another possibility would be to have duplicated the
clock + 4x 161's a dozen times then have a multiplexer
that chooses which to use. Each clock/divider block
would still produce 256x the fundamental, but only for
one of the twelve notes on the scale. It might even be
possible to vary the counters to do pitch bending. I
suppose it would also be possible to have 12 clocks that
are shared between all voices and only one divider with
a mux to choose one of 12 clocks. That would be cheaper
than 12x clocks for every voice.

I will wait for everyone to tell me how crazy this is.
What can I say. I was 21 years old at the time. I had
also fallen into the depths of the digital abyss. ;)

I still want to tinker with the PLLs.

-- 
Phil Harbison