[sdiy] Linear response VCOs?
Michael E Caloroso
mec.forumreader at gmail.com
Mon Mar 23 17:46:40 CET 2026
> I believe that reading multiple notes from a keyboard for polyphonic
voice assignment requires a CPU. At the very least it requires matrix
wiring of the keys, some sort of digital scan,
> and a method to deliver a unique pitch CV to a selected voice. That's
probably extremely difficult without a CPU unless the voice assignment
algorithm is baked into the logic design.
1937 - Warbo Formant Organ by Christian Warnke and Harald Bode
1971 - US 3,610,799 Allen Organ
1976 - US 3,969,968 Melville Clark/David Luce
1976 - US 3,986,423 David Rossum/Tom Oberheim
1977 - US 4,041,825 Armand Pascetta
1974 - Emu 4050 polyphonic keyboard controller
1975 - Oberheim Four voice FVS, Eight voice EVS
1975 - Yamaha GX-1
1977 - Yamaha CS series polyphonics
All of these are polyphonic scanning methodologies that do not use a CPU.
During the early 1970s, CPUs were cost prohibitive. Building a discrete
computer for the digital scanning and CV generation was more cost
effective. The Rossum/Oberheim is the most efficient design, which was
licensed for the Oberheim polyphonics.
Discrete computer design is something of a lost art, obsoleted by CPUs and
embedded systems. I studied discrete computers/microprocessors as an
emphasis when I was in college.
If you thought polyphonic keyboard scanners were hard to understand, try to
make sense of the discrete computer in the vintage (not reissue) Buchla 248
MARF!
MC
On Sun, Mar 22, 2026 at 9:15 PM brianw <brianw at audiobanshee.com> wrote:
> Thanks for mentioning the Pro-One DAC, Olav, because I had not looked at
> these details before (and I own one!)
>
> Which aspects of the Pro-One DAC design do you consider to be clever?
>
>
> Choosing an 8-bit DAC, but only engaging the upper 6 bits?
>
> Stealing the 6-column 8021 Port 0 outputs from the keyboard matrix to
> double as DAC parallel data inputs?
>
> Calibrating a laser-calibrated DAC, where each step used is normally 40
> mV, so that it actually has 41.667 mV (one quarter step)? *
>
> Placing a CPU between the keyboard scan and the VCO input, so that
> sequencing and transposing are simple to achieve?
>
>
> [Further Ramblings]
>
> I believe that reading multiple notes from a keyboard for polyphonic voice
> assignment requires a CPU. At the very least it requires matrix wiring of
> the keys, some sort of digital scan, and a method to deliver a unique pitch
> CV to a selected voice. That's probably extremely difficult without a CPU
> unless the voice assignment algorithm is baked into the logic design. I
> think that explains why most polyphonic synths use both a CPU and a DAC,
> even though nothing requires that they (CPU & DAC) can't each be of benefit
> on their own.
>
> I'm excluding duo-phonic keyboard wirings that can read both a
> low-note-priority and a high-note-priority CV from the same set of keys for
> a two-voice architecture; and I'm excluding full-polyphony keyboards where
> each key has a dedicated voice. Those designs do not require a CPU, of
> course.
>
> The Pro-One is a bit of an exception, here, since it's monophonic but
> still has a CPU to read the keyboard. One side effect of this is the ease
> with which a sequencer with transposition can be implemented. I suspect
> that the fact that the Prophet 5 was designed (in 1978) before the Pro-One
> (in 1981) meant that they were already familiar with using a CPU to scan a
> keyboard, so the fact that it wasn't necessary for a monophonic keyboard
> was moot. They just used the technology that they already knew, and enjoyed
> the advantages that come with that design. In fact, I recall that Dave
> Smith came up with the idea to use a CPU at a time (1975 for the Sequential
> Circuits Model 800) when the synth industry was not doing that.
>
> Brian
>
>
> * Note that the AD558 DAC incorporated laser-trimmed resistors. Normally,
> the Vout, Vout-sense, and Vout-select pins are all shorted together to
> produce exactly 10 mV per step in full 8-bit mode. By placing a resistor
> and trim pot in series between Vout and Vout-sense, I assume that the
> Pro-One tweaks this to 10.41667 mV. Then, using only the upper 6 bits of
> the input, this can output in steps of 41.667 mV, up to 2.635 V total. I
> haven't figured out the gain of the 3280 + TL082 op-amp pair, but they
> might double that voltage to the expected half step resolution.
>
>
> On Mar 22, 2026, at 3:44 PM, Olav Kvern wrote:
> > The Sequential Pro-One is another mono synth with a DAC. I still think
> that the way it's done is clever.
> >
> > Thanks,
> >
> > Ole
> >
> > On 3/22/26 7:12 AM, Michael E Caloroso wrote:
> >> > Correct me if I'm wrong but old synths using DAC for CV were all
> >> > polyphonic.
> >> If it was programmable using solid state memory, it had DAC with MUX/
> S&H. Not limited to just polyphonics.
> >> Oberheim OB-1 monophonic was programmable and used a DAC with MUX/S&H
> for CV. Released in 1977.
> >> Moog Source was another one, released in 1980.
> >> MC
> >> On Thu, Mar 19, 2026 at 7:14 AM Roman Sowa wrote:
> >> Correct me if I'm wrong but old synths using DAC for CV were all
> >> polyhonic. That means a lot of CV sources needed. So they used S&H
> and
> >> muxed DAC. To have PWM with fast enough response to feed MUX and
> >> S$H, it
> >> would have to run at enormouse frequency, not suitable to affordable
> >> technology back then. And putting separate counter as PWM generator
> for
> >> every CV is much more expensive, and takes more space than
> DAC-MUX-S&H.
> >> Back then if you wanted a timer, you got 8253 offering 3 timers in
> one
> >> package, and I'm not even sure if it had PWM mode at all.
> >> Roman
> >> W dniu 2026-03-18 o 21:39, Mike Bryant pisze:
> >> > Does anybody know why these old synths didn't use PWM/PDM
> >> techniques ?
> >> >
> >> > LS-TTL or CMOS feeding a comparator into an analogue integrator
> >> gave 12
> >> > bits performance at audio frequencies even in the 70s so CVs good
> >> enough
> >> > for tuning would have been easy.
> >> >
> >>
> ------------------------------------------------------------------------
> >> > From: brianw
> >> > *Sent:* 18 March 2026 19:02
> >> > The Prophet 5, Rev 1 and Rev 2, use a 7-bit DAC made from
> hand-picked
> >> > resistors. There is a note in the Service Manual that you should
> *not*
> >> > replace these resistors because of the challenge of matching a
> new one
> >> > to the network. 1 LSB is calibrated to 1/12 V (0.0833 V) for easy
> use in
> >> > 1V/8va scaling. CV ranges from 0 V to 10.583 V (127/12), but the
> Prophet
> >> > 5 only uses the lower 6 bits for pitch, limiting the range to
> 5.333 V
> >> > maximum and thus 5 octaves. All CV were 7-bit, but the pitch
> combined
> >> > coarse and fine with the scale of the DAC changed so that there
> were 64
> >> > steps in the coarse range plus another 128 steps in the fine
> range. This
> >> > wasn't quite as accurate as a 13-bit DAC, but still quite
> accurate for
> >> > the time.
> >> >
> >> > The Prophet 5 Rev 3 simply used a 16-bit DAC, but maintained the
> >> > firmware design with 7 bits per CV, so the pitch did not enjoy a
> full
> >> > 16-bit precision. The 13-bit pitch values still have 16-bit
> accuracy,
> >> > though, just not 65536 steps of precision.
> >> >
> >> > One thing to note, Mark, is that a 6-bit DAC has an LSB that's
> 1.56% of
> >> > the total range, so 1% resistors would be quite awful. Then
> there's the
> >> > fact that a 1% error in the MSB could throw the whole binary
> scale off
> >> > enough that the values are not monotonic (i.e. an increase in the
> code
> >> > could actually cause a decrease in voltage!). A 7-bit DAC has the
> LSB at
> >> > 0.78% so you definitely need better than 1% precision. These
> >> > manufacturers were not making a custom resistor array so much as
> >> > hand-selecting individual resistors that were matched well across
> the
> >> > whole group.
> >> >
> >> > Today, not only are 1% resistors more readily available than they
> were
> >> > in the seventies, but you can even get 0.1% tolerance resistors
> at a
> >> > reasonable. Still, that doesn't even get you to a full 9-bit DAC.
> This
> >> > illustrates how impressive DAC chip technology is. One of the
> fasted DAC
> >> > chips I've designed with can run at a sample rate of 125 MHz
> (yeah, MHz,
> >> > not kHz) based on current switching rather than voltage, but it
> stops at
> >> > 14-bit precision because the smallest current is only 0.0061% of
> the
> >> > largest, and it's difficult to be precise enough at such a large
> scale
> >> > factor. Larger DAC precision requires a different technique than
> >> > binary-weighted digits. Fortunately, there are many ways to
> implement a DAC.
> >> >
> >> > Brian
> >> >
> >> >
> >> > On Mar 18, 2026, at 4:34 AM, Tom Wiltshire wrote:
> >> >> Roland had form for this. SH-101 uses a simple DAC built from a
> few resistors too.
> >> >>
> >> >> Like Roman said, it doesn't really make sense nowadays when DACs
> are cheap, but it was worth it then.
> >> >>
> >> >> Tom
> >> >>
> >> >> On 18 Mar 2026, at 11:31, mark verbos wrote:
> >> >>> Like a TR-909.
> >> >>> But, surely it is cheaper to use 1% resistors rather than a
> custom resistor array made.
> >> >>>
> >> >>> Mark
> >> >>>
> >> >>> On Mar 17, 2026, at 18:44, David Manley wrote:
> >> >>>> It's interesting to see how PAiA's John Simonton solved some
> these issues in the 1970's by having a custom laser trimmed resistor
> network built for their 6-bit "Equally Tempered DAC" to be used with linear
> VCOs. See the bottom of the schematic on page 18, the resistor values are
> on the last page.
> >> >>>>
> >> >>>> https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf
> >> <https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf>
> >> > <https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf
> >> <https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf>>
> >> >>>>
> >> >>>> As is typical for PAiA a very low cost solution: build your
> >> own DAC with a few components.
> >> >>>>
> >> >>>> -Dave
> >> >
>
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