Klavis CalTrans

Great little module and easy to use. Just set the lowest note you will use on one of 4 VCOs, aim for it to be the same note, then plug the output of the oscillator to the calibration in and the output of the desired channel back to the same VCO. Say out 1 to VCO 1 volt/oct input and when you select channel 1 from the flashing channels the channel will be swept through the octaves correcting for any tracking errors, this takes about a minute or 2. The calibration light will stop flashing. Now when you connect a keyboard or sequencer to channel 1 input of the above example the out goes to the VCO and now your tracking will be accurate and you can then octave shift easily with the encoder and also change semitones as well. The manual is quite good and explains most things well. The 4 lights at the top tell you when you are pitch shifting and you can quickly reset each channel to the base frequency with reference to the flashing lights and they will stop flashing to show you are at base frequency. You cant afford to be without this module for tonal music. Have fun.

The CalTrans corrects tracking very well (I got a 1-2 cents error in 6 octaves for my Pittsburg Taiga), increases the tuning range for modules that don't have a wide range by default, and corrects inaccurate CV outputs of controllers (like Keystep Pro) by quantizing them.
The only problem is that its calibration input seems to have a low sample rate and poor (or no) aliasing filtering. It is very sensitive to distortion and noise, so the signal must be quiet and filtered for calibration.
It's not obvious from the documentation, you expect to just plug in the cable and calibrate anything, but that's not the case with this module, I even thought my module didn't work.
Also, the CalTrans could be more useful if it used calibration input not only for calibration (you calibrate once and never use it after), e.g. for turning portamento on and off on the gate, but it doesn't.

This really saved me as far as my VCO tracking is concerned. I now have very reliable tracking for 4 VCOs across a 5 octave keyboard. But you absolutely must go to the Klavis site, download the latest firmware & follow the manual exactly. I calibrated my 4 oscillators & the CV output from a Pittsburgh Modular SV-1 to better than my expectations. Well enough that I would be able to rely on tracking in a live situation.