[sdiy] VCO autotune for poly analog modular

[Amos]

As an afterthought, I think you should call it the "Sequential
Quintophonic Analog New Timbral Orchestra" or SQuANTO. :-)

On 4/3/07, Amos <controlvoltage at gmail.com> wrote:
> Yes you can do this.
>
> Possible disadvantages/challenges:  You'll either need (A) a processor
> burly enough to interpolate a curve between your sampled note values +
> stored offsets, or (B) a LUT containing offset values for every note
> at every octave setting of each oscillator in your system.  The former
> demands more CPU horsepower than you might otherwise need, and the
> latter means a heinously-long calibration routine.
>
> Also, you may need to experiment to find a waveform that is "easy to
> count" from the processor's POV.  Not doing this may cause your
> routine to time out or hang.
>
> Good luck!
>
> -Amos
>
> On 4/3/07, Magnus Danielson <cfmd at bredband.net> wrote:
> > From: mrmike <mrmike at clickbang.com>
> > Subject: [sdiy] VCO autotune for poly analog modular
> > Date: Tue, 03 Apr 2007 10:00:15 -0500
> > Message-ID: <46126BFF.5010907 at clickbang.com>
> >
> > Mike,
> >
> > > Yes, I'm insane.
> > > I'm trying to build a polyphonic analog modular based on five ASM-1s.
> > > Think Obie 4 voice, rather than Prophet-5, as a design style.  I would,
> > > however, like to be able to set an accurate single voice across all five
> > > modules. Basically, all five panels could have their own controls, but a
> > > switch on panel one will disable the front panel controls  on panels 2-5
> > > and slave them to the controls on panel one.  In this mode,  an autotune
> > > function would be useful.  Has anyone ever implemented one of these in a
> > > DIY polysynth?
> > >
> > > I was thinking of having the tune function compare VCO freq at a given
> > > CV voltage at several points across the 0-5V range for each VCO, then
> > > create a lookup table with a compensation map for each VCO.  As each CV
> > > for each voice comes in, its corresponding spot in the table is found
> > > and the tuning offset voltage is added to the initial CV and then sent
> > > to the VCO. Is this a dopey idea?
> > >
> > > Thoughts welcome!
> >
> > Now I get the ASM-1 question on a separate thread! :-)
> >
> > Look at the Oberheim OB-8 (service manual on my webpage).
> >
> > It uses a single Z80 processor, a 14 bit DAC and an Intel 8253 PIC. Two of
> > the three 16 bit counters is used for the auto-tune feature.
> >
> > What the logic do is to setup the VCAs for each oscillator such that during
> > auto-tune it scans through each VCO and listen to the output mix with the
> > 8253. It compares the VCO to the CPU clock, which is a crystal oscillator.
> > Without checking the details, I beleive that the CPU clock is divided down
> > to create the counter gate-time and a certain number of oscillator cycles is
> > measured. It is fairly obvious how to elaborate on that. It is easy to get
> > sufficient resolution, I know alot about that.
> >
> > In the other end you need an oscillator model which you need to correct.
> > The most simplest model is to just have CV offset and scale being corrected.
> > This is easy enought. You can take just a few samples (two will do) and you
> > will get both offset and scale errors corrected. A more elaborate scheme would
> > also do high frequency compensation to handle errors in the high frequency
> > compensation already there.
> >
> > Once you have measured your compensation parameters, you use them to offset
> > and scale your signals.
> >
> > The cool thing about the OB-8 VCO trimming is that you have two LEDs and those
> > tell you which way to turn the single trimmer on the VCO. When both LEDs are
> > on. After that the auto-tuning does the rest.
> >
> > Unfortunatly this is not done for the VCFs. Sigh.
> >
> > Hitting the auto-tune on the OB-8 certainly makes things muuuch more in tune.
> > I even find it "too perfect" for some stuff so I don't turn it or actually
> > joggles the power-switch to un-tune it (There should be a feature to disable
> > tuneing!). It is sufficient and not too complex. You should be able to do
> > something similar.
> >
> > Build an high-freq tuning support would be nice.
> >
> > Cheers,
> > Magnus
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