Fully analog oscillator with an output switchable between sine & triangle and a square output. three selectable frequency ranges (low / mid / high) are available.
The AM input is used for modulating the XR-22's amplitude but it also acceps audio signals e.g. an instrurment and thus can be used as a ring modulator effect unit. The balance control will blend between the external audio signal to the "ring modulated) sine/triangle signal. Just like a classic dry/wet control at effect boxes. With the gain control you can mute the VCO and will listen to the ring modulation only.
So far everything is more or less "normal" but let's proceed to the second special feature: FSK alias Frequency Shift Keying.
The oscillator has two circuits controlling it's frequency: F1 and F2. Both have coarse and fine frequency controls, an adjustable CV input with inverter and a 1V/octave input. This way you can set two different base frequencies, feed the VCO with two different sequences and/or inject two different modulations.
With the FSK switch you can now select between F1 and F2 ... but much more interesting is the switch's lower position: in this one the VCO's square output will switch autonomously between F1 and F2. Even without any modulations this will create ramp waves at the sine/triangle output and pulses at the square output and in combination with the AM feedback and gain control (see above) even more nasty stuff. Alternatively you can use an external logic signal at the FSK input to do the switching!
The switch Σcom -> F2 couples all voltages at F1 and uses them both for F1 and F2 ("com." = common). This will keep frequency rations intact when doing inernal FSK switching. Nice side-effect: now the CV and 1V/oct input if F2 are free and usable for two more modualtions!
Confusing? Yes, it is in the beginning but give it a try and just play and experiment. Help an artist and get youself one!