<div dir="ltr">> 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,<div>> 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.<span class="gmail-Apple-converted-space"> </span><br></div><div><span class="gmail-Apple-converted-space"><br></span></div><div><span class="gmail-Apple-converted-space"><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1937 - Warbo Formant Organ by Christian Warnke and Harald Bode</span><br></span></div><div><span class="gmail-Apple-converted-space"><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1971 - US 3,610,799 Allen Organ</span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px"><br></span></span></div><div><span class="gmail-Apple-converted-space"><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1976 - US </span></span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">3,969,968 Melville Clark/David Luce</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1976 - US</span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px"> 3,986,423 David Rossum/Tom Oberheim</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1977 - </span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">US 4,04</span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1,825 Armand </span><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">Pascetta</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1974 - Emu 4050 polyphonic keyboard controller</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1975 - Oberheim Four voice FVS, Eight voice EVS</span><br></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1975 - Yamaha GX-1</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">1977 - Yamaha CS series polyphonics</span></div><div><br></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">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.</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px"><br></span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">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.</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px"><br></span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">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!</span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px"><br></span></div><div><span style="color:rgb(53,60,65);font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";font-size:14px">MC</span></div></div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Sun, Mar 22, 2026 at 9:15 PM brianw <<a href="mailto:brianw@audiobanshee.com">brianw@audiobanshee.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">Thanks for mentioning the Pro-One DAC, Olav, because I had not looked at these details before (and I own one!)<br>
<br>
Which aspects of the Pro-One DAC design do you consider to be clever?<br>
<br>
<br>
Choosing an 8-bit DAC, but only engaging the upper 6 bits?<br>
<br>
Stealing the 6-column 8021 Port 0 outputs from the keyboard matrix to double as DAC parallel data inputs?<br>
<br>
Calibrating a laser-calibrated DAC, where each step used is normally 40 mV, so that it actually has 41.667 mV (one quarter step)? *<br>
<br>
Placing a CPU between the keyboard scan and the VCO input, so that sequencing and transposing are simple to achieve?<br>
<br>
<br>
[Further Ramblings]<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Brian<br>
<br>
<br>
* 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.<br>
<br>
<br>
On Mar 22, 2026, at 3:44 PM, Olav Kvern wrote:<br>
> The Sequential Pro-One is another mono synth with a DAC. I still think that the way it's done is clever.<br>
> <br>
> Thanks,<br>
> <br>
> Ole<br>
> <br>
> On 3/22/26 7:12 AM, Michael E Caloroso wrote:<br>
>> > Correct me if I'm wrong but old synths using DAC for CV were all<br>
>> > polyphonic.<br>
>> If it was programmable using solid state memory, it had DAC with MUX/ S&H. Not limited to just polyphonics.<br>
>> Oberheim OB-1 monophonic was programmable and used a DAC with MUX/S&H for CV. Released in 1977.<br>
>> Moog Source was another one, released in 1980.<br>
>> MC<br>
>> On Thu, Mar 19, 2026 at 7:14 AM Roman Sowa wrote:<br>
>> Correct me if I'm wrong but old synths using DAC for CV were all<br>
>> polyhonic. That means a lot of CV sources needed. So they used S&H and<br>
>> muxed DAC. To have PWM with fast enough response to feed MUX and<br>
>> S$H, it<br>
>> would have to run at enormouse frequency, not suitable to affordable<br>
>> technology back then. And putting separate counter as PWM generator for<br>
>> every CV is much more expensive, and takes more space than DAC-MUX-S&H.<br>
>> Back then if you wanted a timer, you got 8253 offering 3 timers in one<br>
>> package, and I'm not even sure if it had PWM mode at all.<br>
>> Roman<br>
>> W dniu 2026-03-18 o 21:39, Mike Bryant pisze:<br>
>> > Does anybody know why these old synths didn't use PWM/PDM<br>
>> techniques ?<br>
>> ><br>
>> > LS-TTL or CMOS feeding a comparator into an analogue integrator<br>
>> gave 12<br>
>> > bits performance at audio frequencies even in the 70s so CVs good<br>
>> enough<br>
>> > for tuning would have been easy.<br>
>> ><br>
>> ------------------------------------------------------------------------<br>
>> > From: brianw<br>
>> > *Sent:* 18 March 2026 19:02<br>
>> > The Prophet 5, Rev 1 and Rev 2, use a 7-bit DAC made from hand-picked<br>
>> > resistors. There is a note in the Service Manual that you should *not*<br>
>> > replace these resistors because of the challenge of matching a new one<br>
>> > to the network. 1 LSB is calibrated to 1/12 V (0.0833 V) for easy use in<br>
>> > 1V/8va scaling. CV ranges from 0 V to 10.583 V (127/12), but the Prophet<br>
>> > 5 only uses the lower 6 bits for pitch, limiting the range to 5.333 V<br>
>> > maximum and thus 5 octaves. All CV were 7-bit, but the pitch combined<br>
>> > coarse and fine with the scale of the DAC changed so that there were 64<br>
>> > steps in the coarse range plus another 128 steps in the fine range. This<br>
>> > wasn't quite as accurate as a 13-bit DAC, but still quite accurate for<br>
>> > the time.<br>
>> ><br>
>> > The Prophet 5 Rev 3 simply used a 16-bit DAC, but maintained the<br>
>> > firmware design with 7 bits per CV, so the pitch did not enjoy a full<br>
>> > 16-bit precision. The 13-bit pitch values still have 16-bit accuracy,<br>
>> > though, just not 65536 steps of precision.<br>
>> ><br>
>> > One thing to note, Mark, is that a 6-bit DAC has an LSB that's 1.56% of<br>
>> > the total range, so 1% resistors would be quite awful. Then there's the<br>
>> > fact that a 1% error in the MSB could throw the whole binary scale off<br>
>> > enough that the values are not monotonic (i.e. an increase in the code<br>
>> > could actually cause a decrease in voltage!). A 7-bit DAC has the LSB at<br>
>> > 0.78% so you definitely need better than 1% precision. These<br>
>> > manufacturers were not making a custom resistor array so much as<br>
>> > hand-selecting individual resistors that were matched well across the<br>
>> > whole group.<br>
>> ><br>
>> > Today, not only are 1% resistors more readily available than they were<br>
>> > in the seventies, but you can even get 0.1% tolerance resistors at a<br>
>> > reasonable. Still, that doesn't even get you to a full 9-bit DAC. This<br>
>> > illustrates how impressive DAC chip technology is. One of the fasted DAC<br>
>> > chips I've designed with can run at a sample rate of 125 MHz (yeah, MHz,<br>
>> > not kHz) based on current switching rather than voltage, but it stops at<br>
>> > 14-bit precision because the smallest current is only 0.0061% of the<br>
>> > largest, and it's difficult to be precise enough at such a large scale<br>
>> > factor. Larger DAC precision requires a different technique than<br>
>> > binary-weighted digits. Fortunately, there are many ways to implement a DAC.<br>
>> ><br>
>> > Brian<br>
>> ><br>
>> ><br>
>> > On Mar 18, 2026, at 4:34 AM, Tom Wiltshire wrote:<br>
>> >> Roland had form for this. SH-101 uses a simple DAC built from a few resistors too.<br>
>> >><br>
>> >> Like Roman said, it doesn't really make sense nowadays when DACs are cheap, but it was worth it then.<br>
>> >><br>
>> >> Tom<br>
>> >><br>
>> >> On 18 Mar 2026, at 11:31, mark verbos wrote:<br>
>> >>> Like a TR-909.<br>
>> >>> But, surely it is cheaper to use 1% resistors rather than a custom resistor array made.<br>
>> >>><br>
>> >>> Mark<br>
>> >>><br>
>> >>> On Mar 17, 2026, at 18:44, David Manley wrote:<br>
>> >>>> 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.<br>
>> >>>><br>
>> >>>> <a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf" rel="noreferrer" target="_blank">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a><br>
>> <<a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf" rel="noreferrer" target="_blank">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a>><br>
>> > <<a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf" rel="noreferrer" target="_blank">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a><br>
>> <<a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf" rel="noreferrer" target="_blank">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a>>><br>
>> >>>><br>
>> >>>> As is typical for PAiA a very low cost solution: build your<br>
>> own DAC with a few components.<br>
>> >>>><br>
>> >>>> -Dave<br>
>> ><br>
<br>
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