ALM Busy Circuits - Akemie's Castle

Dual voltage controlled oscillator with authentic digital 4-operator FM using new-old-stock Yamaha ICs.
Each operator features individual control of level, frequency multiplier and waveform selection via potentiometers and CVs (!). Six selectable algorithms determine the interaction between operators and oscillators. Both oscillators have a feedback parameter and oscillator A offers a chord mode with voltage controllable inversion.

The two operators (OPs) access the two oscillators equally or in a different way, depending on the selected algorithms, six of which are available.

The upper two algorithms, these with orange LEDs, allow for using the module as two individual oscillators because only two OPs influence one oscillator:

1. OP1 modulates OP2's frequency (two-operator FM) and affects oscillator A. OP3 is mixed with OP4 and affects VCO B
2. OP1 is mixed with OP2 and the sum influences VCO A. OP3 modulates OP4, result affects Oscillator B

The other four algorithms affect both oscillators in exactly the same way and they offer more complex four-operator structures:
3. OP1 modulates OP2, result modulates OP3 and it's result modulates OP4.
4. OP1 modulates OP2 and OP3 modulates OP4. These two FM chains are mixed then.
5. OP2 modulates OP3, the result modulates OP4. This is mixed with operator 1.
6. OP2 modulates OP3. The result is mixed with OP1 and OP4.

Each operator has potentiometers and CV inputs for three parameters:

• "Wave" selects one of 8 waveforms; most are derivates of sine but there's a square, too.
• "Multiplier" sets the frequency multiplication factor from 1-15, derived from the oscillator's initial frequency.
• "Level" determines the operator's level. Turning the control at high levels clicks may be audible. Very low values may not mute the operator entirely - there might be some bleed.

Feedback, a type of auto-modulation, can be set for each oscillator separately but by default A setting is normalized to B as long as nothing is patched to the Fb input of VCO B. the feedback sounds coarse and can affect the tuning a little bit.

In oscillator A a chord function is integrated, generating one of 16 possible chords. Those are selectable by CV or manually and depending on the chord they have two to five voices. It is also possible to invert the chords via CV - the higher the voltage the higher the amount of chord inversion.