What exactly is the "cross modulation" on Roland Jupiter and JX analog synthesizers?

Question

Roland's later analog synthesizers have a setting for cross-modulation between the two oscillators. I'm trying to find out what the exact technical nature of this effect is. Is it phase modulation, linear frequency modulation, exponential frequency modulation, amplitude/ring modulation, or something else entirely? And does it go one way, or does it affect both oscillators, as is suggested by the term "cross-modulation"?

On the Jupiter-6, there are two faders in the oscillator-1 section to set the cross-modulation level manually and to have it influenced by envelope-1.

On the Jupiter-8, there is one fader in the oscillator-1 section to set the cross-modulation level manually, and it is indicated on the panel as a signal coming from oscillator-2.

On the JX-8P and JX-10, there is a switch in the oscillator-2 section to set the cross-modulation level from 0 to 3.

(On the JX-3P, there is a switch in the oscillator-2 section with settings "off", "sync" and "metal", but nothing to set the cross-modulation level. It looks as if the cross-modulation on the JX-3P is very different from the implementation on other models.)

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UPDATE

A few (vague) sources on internet forums suggest that the cross-modulation is exponential frequency modulation of oscillator-1 by oscillator-2.

I'd be interested to have this confirmed by someone who has access to a Jupiter, and ideally I'd like to know by what interval the pitch of oscillator-1 goes up and down when oscillator-2 is set to a square wave and low-fequency mode, and the cross-modulation level is set to maximum.

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Further Googling suggests that on the JX-8P and the JX-10, the four settings are 0: off, 1: oscillator sync (oscillator 2 synced to oscillator 1), 2: cross modulation (apparently oscillator 1 modulating the frequency of oscillator 2) and 3: oscillator sync combined with cross modulation.

So the cross modulation on the JX synthesizers seems to go in the same direction as the oscillator sync (oscillator 1 controls oscillator 2), unlike the Jupiter-8. The Jupiter-6 could do oscillator sync in both directions, so there you had the choice of both options.

Answer 1

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).

I do have a JP-8 and a JX-8P btw, do you still want something confirmed two years later? :-D

Answer 2

One would think that the 'metal' choice on the JX3P refers to Ring Modulation (RM). The reason for my thinking so is that the option was often used for bell like sounds, which is also typically associated with RM. I don't know this for a fact, of course, but it seems to reason.

Just the name brings back memories of making some of the coolest bells and S&H sounds, and I am very much partial to metallic timbres. Also, I recall it had Frequency Modulation (FM) which sort of infers a separation between FM and CM (in terms of Roland's use). Sadly I do not have the synthesizer anymore.

This does not answer your question anywhere near 'exactly', but I think you would need someone from Roland in order to find that kind of answer.

Answer 3

The characteristic feature of cross modulation is that the two signals have similar frequencies, both in the audio range. That creates quite a different effect from amplitude or frequency modulation driven by a low frequency oscillator (i.e. with a frequency between say 5 Hz and maybe as low as 0.01 Hz) where you can hear the effect of each individual cycles of the LFO, or slow changes in the sound of a drone.)

Probably the nearest equivalent to cross modulation would be ring modulation as in early analog synths, or "FM synthesis" as marketed by Yamaha in the 1980s, for example in their DX7 synth.

The frequency and amplitude content of the cross-modulated signal doesn't necessarily have any simple relation to the two inputs, and what you hear doesn't necessarily have a simple relation to what you would measure with a frequency spectrum analyser either, since your ears and brain are a highly nonlinear signal processing system! You could describe it as a "there are no rules - just play with it till you get something you like" type of effect!