Hello,
I guess this topic needs a bit a clarification because it's a bit complex.
VCO 1 and 2 are true exponential VCOs. That means analog circuitry is doing the linear CV (1V/octave) to exponential frequency(double the frequency for each octaves) conversion. This kind of converter always have temperature dependency so it needs to be either compensated, either temperature stabilized. If dependency is not voided properly the slope will be wrong, for example 1 octave will not be 1 octave, but 1 octave and 10 cent.
So if C3 is a perfect C, you will have C4=+10cents, C5=+20cents, C6=+30cents ...
For compensation it is usually a Tempco that is used. A Tempco is a kind of resistor that has a known temperature dift. If you pick a dift value opposite to the VCO dift, it ends up more or less stable. More or less because both have tolerances in their drift value, plus tempco and the VCO components need to be perfectly thermally coupled otherwise they are not at the same temperature.
Using tempco has an advantage and a drawback: they drift very slowly, but drift even when not powered. That means that if a machine stays (even powered off) always at the same room temperature it will be very fast is tune. But if you expose the machine to a temperature change, it will be very long to recover. So it is very difficult to use it in live situations
When using stabilization you use some kind of heater and sensor that ensure the VCO core always runs at a constant temperature much higher that room temperature.
At power up this oven needs few minutes to warm up but when it's locked, it can handle room temperature changes( in a reasonable extend of course).
MatrixBrute VCO1 and VCO2 (and MiniBrute and MicroBrute) are oven based exponential VCOs. That means you have to let them warmup up to 5 minutes. Once locked the slope will be fine (1 octave is a true octave).But then comes the pitch offset issue. When the octaves are right, you now want the notes to be perfect. The offset is what you basically control with fine tune.
Because it is analog, all the components has a little temperature dependency(much lower than VCO core slope), so all the internal CVs have a bit of temperature dependency too. It's definitely not noticeable to control a VCA, but for a VCO pitch we have around 2 cents per °C .
Usually you want to be in perfect tune with other instruments with fine tune at center position. That is the purpose of "AutoTune" in MatrixBrute. It voids the pitch offset to ensure that absolute pitch is right.
So the good way to use the autotune is : wait at leat 5 minutes for the ovens to warm up(using autotune before that will be point less, because the slope will still evolve).
Once ovens are locked (1 octave = 1octave), then you may use auto tune to void any pitch offset.At constant room temperate, you may have to autotune again during the first 30minutes (time to get a stable and homogeneous internal temperature).
if room temperature changes, then autotune again...
By the way we have no background autotune, for example when the VCA is closed, because as we've got external CV inputs for the VCA and VCO we don't even know if the VCA is closed, or if the pitch is being controlled by an external source...
OK but what about VCO3?
This one is a linear VCO, that means it does not do the exponential conversion itself, it is fed with a CV that is already exponential (conversion is made by firmware). No analog expo converter means almost not temperature drift, so at startup it is almost immediately in tune. And it does not need to be autotuned... Magic!
But (because their is always a but
, no expo converter means that this kind of VCO can't do exponential frequency modulation based on linear CV, so you cant' directly control it from an external CV. If you feed it with another VCO on the CV input, you will get linear modulation of the pitch instead of exponential (what you get on matrixBrute in the AudioMod section)
You can still use a microcontroller to sample the external CV input, make it exponential and generate a new expo CV, but then bandwitdh is limited by sample rate, so again no full audio range modulation available...
That is why VCO3 is only a source of AudioMods and never a destination.
