Oscillator Calibration Guide
The steps below will guide you through the calibration process for both Mangrove and Three Sisters (or any other analog oscillator).
You will need:
- A tool to measure the frequency of the oscillator (a tuner or oscilloscope)
- An accurate voltage reference (if possible, use the module that will drive the PITCH input of your oscillator most often)
- A small flathead screwdriver.
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.
- Turn your system on and wait at least 10 minutes for the module to warm up.
- 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).
- 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.
- 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!
- 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.
- Now send 2V in to your oscillator.
- 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.
- 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. NB: 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!
- 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.
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...