Cross modulation on JP8, JP6 and JX-8P

I was trying to find info about how the JP8 crossmod works last night, but ended up answering a post about it instead. Here is what I wrote:

TL;DR:

JP8 and JP6 crossmod is exponential FM of DCO 1 by DCO 2, JX-8P is amplitude modulation (ring modulation) of DCO 2 by DCO 1 (plus sync if wanted).

Details:

All of this should be fairly easy to figure out from the service manuals for the various synths.

Right now I'm looking at the JP8 service manual. Here, the output from VCO 2 - tapped after the waveform selector - is fed back to the exponential (v/oct) input of VCO 1. There is nothing going into the reference current input of VCO 1 (the circuit around op amp 1A on page 12 of the service manual), so the JP 8 has no linear FM.

The signal from VCO 2 goes through IC18A, which is a BA662 OTA, this acts as a voltage controlled amplifier/attenuator, and this is what is controlled by the panel potentiometer.

The same is the case for the JP-6, although here the signal is fed through a CEM3360 VCA. It is still sent back to the exponential input of VCO 2. Nothing is fed into pin 13 of the CEM3340 VCO so the JP-6 has no linear FM either.

The JX-8P is completely different. It doesn't have VCOs, it has DCOs, meaning the frequency is digitally controlled. It does NOT let the output of DCO 2 control the frequency of DCO 1, neither exponentially nor linearly. Instead, when crossmod is enabled (pos 1 or 2 on the PG-800 switch), the output of DCO 1 controls the amplitude (volume) of DCO 2, which I guess means that the output of DCO 2 is now DCO 1 times DCO 2, or exactly what is described as ring modulation in figure 2 here (given that it follows the gain requirements stated): https://www.keyboardmag.com/gear/on-synthesizers-amplitude-and-ring-modulation

PS: This means that, unlike what OP says about crossmod on the JX-8P, DCO 1 does NOT modulate the FREQUENCY of DCO 2, it modulates the AMPLITUDE.

In the circuit diagram the control for this part of the X-mod is labeled 'Metal', so it is very possible that this has the same effect as 'Metal' on the JX-3p, which is also described as a ring modulator type of sound. Interestingly enough, I cannot find any similar circuit on the JX-3p.

The JX-8P (and JX-3P) does also have a sync input on DCO 1 going to Q18, which is the transistor that resets the sawtooth of DCO 1 (i.e. sets the frequency). Sync comes from the same input that resets DCO 2, so whenever DCO 2 resets, so does DCO 1 (which means this is just normal hard sync I guess).

The post I answered can be read here: https://music.stackexchange.com/questions/57210/what-exactly-is-the-cross-modulation-on-roland-jupiter-and-jx-analog-synthesiz