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Ah okay, the poly requirement makes sense. But we ran PDM DACs at 8MHz using LS-TTL to give better than 16 bit audio for a single channel so you could still feed that to a MUX and S/H.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Roman Sowa <modular@go2.pl><br>
<b>Sent:</b> 19 March 2026 11:10<br>
<b>To:</b> Mike Bryant <mbryant@futurehorizons.com>; synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
<b>Subject:</b> Re: [sdiy] Linear response VCOs?</font>
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<div class="PlainText">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 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>
<br>
Roman<br>
<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 techniques ?<br>
> <br>
> LS-TTL or CMOS feeding a comparator into an analogue integrator gave 12 <br>
> bits performance at audio frequencies even in the 70s so CVs good enough <br>
> for tuning would have been easy.<br>
> ------------------------------------------------------------------------<br>
> *From:* Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of brianw <br>
> <brianw@audiobanshee.com><br>
> *Sent:* 18 March 2026 19:02<br>
> *To:* synth-diy@synth-diy.org <synth-diy@synth-diy.org><br>
> *Subject:* Re: [sdiy] Linear response VCOs?<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">https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf</a>
<br>
> <<a href="https://paia.com/wp-content/uploads/2024/05/8780pgs.pdf">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 own DAC with a few components.<br>
>>>> <br>
>>>> -Dave<br>
> <br>
> <br>
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