JX-8P cross modulation

I've visited this before, but I got a bit confused when looking at the circuit, so here is some details of what is going on.

JX-8P cross modulation is in fact amplitude modulation of DCO-1 by DCO-2. 

Here is the circuit:

 

The strange four-triangles are switches. The three NPN transistors work as VCOs. Two of them are controlled by the CV/sample-and-hold circuits and control the output of each DCO to the mixer. The third, AM VCA, is controlled by the output of DCO-2.

As a side note, noise is injected through the 4.7k resistor in the top center, instead of DCO-1 (presumably, they could be mixing though but that makes the whole setup a bit strange.

Standard mode

Then switch D and C are closed (they receive the same control signal), the output of DCO-1 is routed to the DCO-1 VCA through D. C grounds the base of the AM VCA so it shuts off. In practice it works out something like this:

Cross modulation

When D and C are open, the DCO signal is routed through the AM VCA, like this:

 

Note that in both cases, the output of DCO-2 can still be added to the mix - but I'm not sure if the synth actually allows this or if it would have any use in practice.

DCOs

I was confused about the DCO saw generator too, for a little while - I couldn't see how it could have a variable charge current when the negative input was connected through a 100k resistor to the negative supply. That's of course wrong - the downwards arrow points to IC53 (and IC48 on the output) which are the "DCO Self Adj" (IC48) and the "Analog DMUX" (IC53, e.g. CV), so the charging current is CV controllable.

Further reading 

After studying the circuit I found a great explanation over at electric druid that has a more detailed explanation of what is going on - most interestingly, the saw wave is rounded after the VCAs. It is also stated that the output of the AM VCA is turned off when the input is negative, and speculated that this means that the saw wave is in fact never below gnd. This is highly likely, as the control voltages from IC48 and IC53 probably never go below 0.

https://electricdruid.net/roland-cross-mod-metal-sync/