[sdiy] Quantizer DAC or PWM?

[brianw]

It's possibly mechanical, in the situation where user interface prefers separate coarse (octave) and fine (semitone) controls.

However, it could also be the reverse, where electrical constraints require two controls.

The original Prophet 5, before the common CEM redesign, has a discrete 7-bit DAC made from individual resistors. The resistors were carefully selected to get the desired accuracy. The coarse CV is separate from the fine CV, on different Sample and Hold channels, and they're weighted appropriately before being added together to produce a single pitch CV for each VCO.

The CEM Prophet 5 upgraded the design to a DAC chip, since those were finally available at a reasonable price. It's actually a 16-bit DAC chip, but they only use 14 bits for VCO and the original 7-bit precision for other CV values. A modern design would probably use 16 bits for everything, but Sequential Circuits probably already had 8-bit firmware dealing with the old 7-bit discrete DAC and they didn't want to rewrite the entire thing. Either that, or there isn't enough memory to handle doubling the storage for active and saved patches.

Brian


On Dec 20, 2022, at 4:42 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> But isn't this structure more of an old "mechanical" solution, rather than a microcontroller solution of today? :-)
> 
> /mr
> 
> Den tis 20 dec. 2022 13:15Tom Wiltshire <tom at electricdruid.net> skrev:
>> The Moog Taurus is another example of this structure.
>> 
>> Tom
>> 
>> > On 20 Dec 2022, at 11:50, Roman Sowa <modular at go2.pl> wrote:
>> > 
>> > Separate Octave and semitone resistor ladders with muxes selecting them was the heart of (all?) Yamaha CS-series keyboard controllers.
>> > 
>> > Roman
>> > 
>> > W dniu 2022-12-20 o 12:41, Florian Anwander pisze:
>> >> Hi
>> >> Am 19.12.22 um 22:54 schrieb Didier Leplae via Synth-diy:
>> >>> Ah yes, a separate output for octaves is brilliant!
>> >> btw Scott rider aka Old Crow used a similar trick to achieve Hz/V characteristics with an 8Bit:
>> >> https://github.com/cs80/CS15_SSK/
>> >> He uses a dual channel DAC MCP4922. DAC #1 is providing different reference voltages for each octave, which then are used by the DAC #2 which converts the same twelve note data to a spreading CV the higher the octave / refence voltage from DAC#1 respectively is.
>> >> Florian