Oscillator Calibration Guide


The steps below will guide you through the calibration process for both Mangrove and Three Sisters.

Mangrove (pre-2023) and Three Sisters (pre-2024)

You will need:

Overview

The goal is to turn a trimpot accessed through the back of the module such that the frequency of the oscillator when 2V is sent to the v/octave input is exactly four times the frequency when 0V is sent in to the oscillators v/octave input. In other words, to make sure that the oscillator is exactly two octaves above the 0V frequency when 2V is sent in. It does not matter what you choose to tune your oscillator to for the 0V frequency, as long as you make sure to set the 2V frequency to be four times the 0V one.

Procedure

  1. Turn your system on and wait at least 10 minutes for the module to warm up.
  2. Send the output of your oscillator to the input of your tuning device (use the SQUARE output for Mangrove, or turn the QUALITY of Three Sisters to max and use the LOW output).
  3. Select a base frequency which will correspond to 0V. The choice is arbitrary. If you choose 100Hz for 0V, then the goal is to get 2V into the oscillator generating a 400Hz oscillation. If you prefer to use a chromatic scale, then choosing C2 for 0V would mean that your goal is to get 2V to generate C4.
  4. Send 0V from your voltage source into the v/octave input of your oscillator (PITCH input on Mangrove, FREQ input for Three Sisters). Don't just remove the cable for 0V, actually send 0v from your source to the oscillator!
  5. With 0V going into your oscillator, now use the frequency control knobs to tune it to your base frequency; in this case, tune it to 100Hz. On Mangrove, this means using the PITCH (coarse) and FINE knobs, while on Three Sisters you will use the FREQ knob to tune it to your base frequency.
  6. Now send 2V in to your oscillator.
  7. Check the frequency of the oscillation. If it is precisely 4 times your base frequency (in this case, 400Hz), then you are done. If not, proceed to step 8.
  8. If it is flat (e.g. below 400 Hz), turn the trimpot on the rear of the module a few degrees CW for Mangrove or a few degrees CCW for Three Sisters. If it is sharp, turn the trimpot a few degrees CCW for Mangrove or a few degrees CW for Three Sisters. Please note: you are *not* trying to tune it to 400Hz! You are instead changing the *scaling* of the input voltage range, and after turning need to repeat this process!
  9. Repeat Steps 4-9.

Keep in mind you'll likely need to repeat steps 4-9, ten or more times to have the module well tuned.

Mangrove2: 2023

Here are some brief notes on how to trim the new Mangrove (+PPM), in case you're having trouble in your system. In general, avoid touching the "ramp" trimmers as we have a special testing system that gets them maximally in-tune before leaving the workshop. The volt-per-octave trimmers may need to be adjusted though, as not all sequencers treat 1V the same.

There is a detailed video planned that walks through the process, but I'm leaving this here in the interim.

Note: All trimmers are 5-turn pots. This means it takes 5 full rotations of the screwdriver from one end to the other, but also means there's no "stop" where you can feel the limit of the range. Always best to listen to the output while turning to make sure things are changing.

First thing to do is trim the 2 volt-per-octave ("v8") trimmers. These are done as listed above:

* core oscillator is "pitch-v8", easiest using SQUARE output, and patch voltage to PITCH input. Best results if PITCH is around 11:00.

* impulse oscillator is "impulse-v8". use FORMANT output, and FORMANT input. set PPM switch to "cycle", Formant knob to 12:00, and BARREL to 12:00. Air around 1:00 gives the cleanest wave for frequency measurement.

After those are done, you can trim the BARREL response. This is actually quite difficult, and we built a special rig to get it as accurate as possible.

These are important when placing the impulse oscillator in "cycle" mode. As the BARREL control is rotated / modulated, the pitch will fluctuate up/down substantially if not calibrated. This is not an exact science, but you can get pretty close (within 30cents across the range) with some effort.

The setup is:

* Formant at 12:00, Barrel at 12:00, Air at 1:00
* PPM mode to "cycle"
* Listen to Formant (and optionally mult it to a tuner)
* Attach a bipolar, triangle LFO to BARREL (+/-5V)
* Make sure the LFO has an indicator (or patch to one) so you know when it's positive or negative.

If you're using a tuner, you'll want the LFO slow (2~5 seconds), but if you're tuning by ear (usually faster & easier) you can go faster (1/2 ~ 1second).

Below, "centre" means LFO at 0V, "low" means LFO at -5V, and "high" means LFO at +5V. "edges" refers to both "low" and "high"

The calibration process goes:

*If both edges are *sharper* than centre
    - turn "ramp-depth" clockwise, until high is same pitch as centre

*If both edges are *flatter* than centre
    - turn "ramp-depth" anti-clockwise, until high is same pitch as centre

Once high and centre are aligned:

*If bottom is *flatter* than centre
    - turn "ramp-balance" anti-clockwise, until edges are the same pitch as each other

After completing the 2nd stage, if you moved the trim at all, the centre & edges will be out again, so you need to repeat the process. Carry on repeating until you're turning the trimmers less than 45degrees each time as this is about as accurate as you can get.

After calibrating the circuit, you'll hopefully have some new ideas for how to use the detune to your benefit...

 

Three Sisters2: 2024

Centre OS (Centre OffSet)

The CENTRE channel is composed of 2 filters in series; highpass into lowpass. In "crossover" mode, the frequencies of both filters are controlled together by FREQ, and SPAN pushes the two halves in opposite direction. In "formant" mode, both halves are controlled the same way by FREQ, but the SPAN control has no impact on the CENTRE channel; SPAN only affects HIGH and LOW channels in "formant" mode.
Due to parts variance, there is typically a small difference in the real cutoff frequencies of those halves when fed an identical control voltage, which is where the "Centre OS" (Centre OffSet) control comes in. It adds a small offset voltage to only the first half of the CENTRE channel. As such, you can make sure the two halves are perfectly aligned. This is important to make avoid beat frequencies when the filter is self-oscillating, but it also plays a small role in setting the amplitude of the channel in "formant" mode.


Edge OS (Edge OffSet)

With the second version of Three Sisters, we set out to allow all three filters to track volt-per-octave accurately. The V8 trims make sure each one individually tracks well.
 
However, when self-oscillating in "formant" mode, we want the possibility for all of the filters to be aligned together at the same frequency. Due to parts variance, there is typically some difference in the real frequency given the same control voltage.
 
To calibrate each filter's volt-per-octave accuracy, first align the LOW and HIGH filters with the SPAN control, then use the "Edge OS" (Edge OffSet, which offsets the two filters on the "edges", that is, not the Centre), to push LOW and HIGH into alignment with the natural frequency of the CENTRE channel.
 
Some background on the circuit's design
Rather than add an offset trim for each filter, or using one channel as a reference for two offset trimmers, we decided on an optimization! Since SPAN pushes LOW and HIGH in opposite directions, it is always possible to align those 2 channels together by adjusting the SPAN control. But when LOW and HIGH are aligned, CENTRE will often be at a slightly different frequency. The simple solution would be to add a trim that shifts the CENTRE channel (both halves), but for implementation reasons, it was easier to add a small offset to control both LOW and HIGH at the same time.