[sdiy] 1v /oct with ADC question

Tony Kalomiris weplar at gmail.com
Sat Dec 3 04:09:12 CET 2016


Hi Brian,

A digitally scanned design would have called for more address decoders and
logic for the 5x8 matrix (latches). All 8 outputs of the 'LS138 are used
up. Don't don't if you have looked inside one but it's pretty tight in
there and the little Z80 is pushed to it's limits. At 2 MHz no less.


But as you say you can route it directly to the glide circuit and bypass
the digitization.
Me thinks it's a very cool design, that probably used an existing 37 note
analog keyboard.
Another benefit I imagine is you don't have all these high frequency
scanning signals traveling across the keyboard creating noise.

Kudos to Tony at Encore for his retrofit. However I really like the
arpegiator and internal sequencer!

This is a fun synth to play.

On Fri, Dec 2, 2016 at 7:28 PM, <rsdio at audiobanshee.com> wrote:

> Yes, it does seem a little bizarre since all other CV was disconnected
> from any real analog source. Perhaps they did not want to incur the latency
> of a scanned keyboard and DAC. With the design as they have it, the
> keyboard can directly control the VCO, even if the modulation for the VCO
> that is added comes from a DAC.
>
> I can't tell from the Source schematic - because it's broken across
> several pages and hard to read quickly - but it's theoretically possible
> that the keyboard CV could be applied directly to CV mixers for each VCO
> control input. Octave, Pitch Bend, Modulation, Pitch Envelope and other CV
> sources would be mixed in (added), but those could come from a DAC in some
> cases, as opposed to live, analog CV.
>
> I also wonder whether Sequential Circuits and/or E-mu had a patent on
> keyboard scanning at the time. They certainly invented it. What we take for
> granted now was not always standard.
>
> Brian
>
>
> On Dec 2, 2016, at 3:05 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> > It's a bizarre way of doing it. If you've got a microprocessor, why not
> just scan the keyboard, and then output the voltage to the DAC? (Like the
> Pro-One or SH-101 does) Maybe they ran out of pins.
> >
> > On 2 Dec 2016, at 05:41, Michael E Caloroso <mec.forumreader at gmail.com>
> wrote:
> >> The Moog Source has no ADC.
> >>
> >> The keyboard is analog, but the system is using a comparator with the
> >> keyboard CV at one input and a successive approximation voltage
> >> generated from the DAC at the other input to derive the key note.  The
> >> comparator output is simply a logic input to the CPU.
> >>
> >> That's how Moog got away without using an $$$ ADC.
> >>
> >> MC
> >>
> >> On 12/1/16, Tony K <weplar at gmail.com> wrote:
> >>> Have a look at the Moog Source keyboard circuit. As far as I am aware
> it is
> >>> the only analog keyboard to ADC to DAC again. I love that synth.
> >>>
> >>> Tony K
> >>>
> >>> On Dec 1, 2016, at 1:46 PM, Mike HEQX <mike at heqx.com> wrote:
> >>>> This makes a lot of sense. Never thought of using a DAC after the ADC
> to
> >>>> generate the CV.
> >>>>
> >>>> I would never have been able to make it work otherwise. So now I know
> for
> >>>> the next time.
> >>>>
> >>>> I can't wait to make it play notes for the first time.
> >>>>
> >>>> Thanks to all who responded. Everyone had a different view, and all
> were
> >>>> very valuable.
> >>>>
> >>>> Mike
> >
>
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