[sdiy] Autotuning CEM3340/V3340 oscillators

[paula at synth.net]

Tom,

  for 008 we used an analog devices 14bit DAC, which is beautiful, but is 
SMD and quite pricey (but SUPER linear and stable).

  If you want more bit depth/resolution, why not use two DACs and have 
one driving the Vref of the other?
A single MCP4922 should do the trick (available in Through hole too), 12 
bit, dual channel external Vref for each DAC.

Paula


On 2017-09-20 13:19, Tom Wiltshire wrote:
> Hi all,
> 
> A little while ago I was asking about setting the lowest frequency on
> a V3340 oscillator, because I was struggling a bit. In the end, it
> seemed to be only prototype woes, and the board has been fine since.
> 
> Since then, I’ve been experimenting with autotuning the VCO using a
> PIC (a 16F1778), and I thought some of you might be interested to hear
> a progress report.
> I have the PIC receiving a MIDI input and producing control voltages
> for the VCO via both channels of a MCP4822 dual 12-bit DAC. The pulse
> output of the VCO drives a NPN transistor to pull down one of the
> PIC’s input pins to act as a tuning input.
> 
> Tuning the VCO isn’t too difficult, except at the very low end (I’ll
> come to that in a mo). I’m using the typical “successive
> approximation” method, which is described nicely in the Prophet T8
> service manual. In short, you set the highest bit of the DAC, then
> measure the waveform period. If the waveform period is too short, you
> clear the bit, and otherwise you leave it set. Then you move to the
> next bit down, and set that and repeat the test. When you get down to
> the lowest bit, you have the DAC value that gives you the nearest
> frequency below your required frequency. Note any errors are always on
> the low side because of the algorithm.
> 
> There have been some quirks. One is that I’m running the PIC at 32MHz,
> and consequently have 32MHz or 8MHz available as 16-bit timer clock
> frequencies. Even with prescale settings of /2, /4, or /8, this is too
> fast to measure frequencies below 16Hz. I got around this by keeping
> track of timer overflow, which effectively gives me a 17-bit counter,
> which is *just* long enough for the job. To be honest, no-one will
> notice tuning errors way down there anyway - it’s on the border of
> sub-audio.
> 
> Another quirk is using a cheap SPI DAC with only 12-bit resolution.
> There are some nice TI 16-bit DACs available in 8-pin DIP, but they
> cost £10/unit, unlike the one I’m using. Obviously I could go SMD, but
> I’m avoiding that.
> Twelve bits isn’t enough to produce a good CV over the full MIDI note
> range. 4096 / 128 notes = 32 sub-semitone steps, or about 3 cents -
> not audible, but borderline. So I’d thought to produce an fairly
> general “Octave CV” on one channel and a more detailed “Note CV” on
> the other, which would cover only a part of the full range - say four
> octaves (4096/48 = 85 steps, much better). This seems to work pretty
> well, as far as it goes. I can’t decide how wide to have the Note CV.
> Would it make sense for it to only cover a single octave? That’s the
> most efficient in some ways, but by making it wider, I can do larger
> pitch shifts without having to change the Octave CV. That makes
> updates quicker and also removes the risk of a glitch at the moment
> the octave switches. It’s “six of one and half a dozen of the other”,
> as we say in English.
> 
> Another problem is the SPI interface. Since I have to send two bytes
> to program a 12-bit variable into the DAC (so four bytes for the two
> CVs) there is a limit to how fast the DAC can be updated. You can then
> reduce that theoretical maximum another chunk to allow for actual
> calculation time. This is a problem for something like MIDI pitch
> bend, where the data needs filtering (e.g. more calculation time) and
> also needs sending out frequently enough no steps are audible. Adding
> smoothing to the analog CV would be one solution, but if the steps in
> the Pitch Bend are heavily smoothed by analog filtering, then no rapid
> changes in Note CV are possible. Unless you change things around and
> use a separate channel for Pitch Bend CV, with the Bend CV filtered
> and the Note CV not - but then that’s three DAC channels for one note!
> Not very efficient!
> The advantage of that is that *only* the Pitch Bend CV has to update
> rapidly. The Note and Octave CVs can change only when a new note is
> played - much simpler.
> 
> Finding the best way to do this with low-cost and easily-available
> chips is proving not entirely straightforward. There are a lot of
> possible ways to arrange things in terms of the range of the various
> CV channels and the distribution of different data to the CV channels,
> and I’m experimenting to see what gives a decent result.
> 
> Hope this is of interest,
> Tom
> 
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
> 
> 
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