921b osc

moog921b · sources · schema v1

921B Oscillator (moogafakkin System 55/35 clone — categorized under Ports -> moogafakkin). The slaved VCO driven by a 921A bus: it reads freq_bus (V/oct pitch) + width_bus (pulse width) CONTROL INPUTS from a 921A rather than carrying its own 1V/oct jack, and presents FOUR fixed-level simultaneous waveform outs off one common core — sine, triangle, saw, rectangular — across 1 Hz–40 kHz. The FREQUENCY pot is a 2-octave fine trim; the RANGE switch sets the octave footage; DC MODULATE is DC-coupled LINEAR FM (non-1V/oct, ±Hz); AC MODULATE is cap-coupled LINEAR FM (a DC-blocking high-pass runs first so a DC offset on the modulator doesn't bend the pitch); the SYNC input + 3-position SYNC switch (off / lo=soft / hi=hard) drive oscillator sync. With nothing patched the bus normals to C4 @ 50% duty so the 921B still sounds standalone, but it is designed to be driven by a 921A driver. DSP forks the shared own-code moogafakkin VCO core (clean-room polyBLEP/polyBLAMP band-limited oscillator + hard/soft sync, the same core the 921 VCO uses) - not a port of any moogafakkin schematic or copyleft source, permissive only. Beige moogafakkin faceplate (the intrinsic always-on look shared by the moogafakkin module family).

A clean-room recreation of the Moog 921B Oscillator — the slaved (sound-making) half of the System 55/35 two-part oscillator. It has NO 1V/oct jack of its own: its pitch comes from a 921A driver's FREQ BUS, so several 921Bs sharing one 921A play in unison and you tune the whole bank from the driver. Off one core it presents four fixed-level simultaneous waveform outputs — sine, triangle, saw, rectangular — across ~1 Hz to 40 kHz, with two linear-FM inputs (a DC-coupled and an AC/cap-coupled one) and a hard/soft sync input. The FREQ (fine) and RANGE (octave footage) knobs offset its pitch relative to the bus, so each 921B in a bank can be detuned or octave-shifted off the shared pitch. Mental model: a 921 VCO whose pitch is fed by the bus instead of a knob, built for stacked unison/detune voices.

the faceplate

5 inputs · 4 outputs · 5 params

about this module

Hover a control, or open the patch panel and hover a jack.

inputs

id	cable	what it does
`freq_bus`	cv	1V/oct pitch CV from a 921A driver's freq bus (0 = C4) — this is how the 921B gets its pitch. Patch the driver's FREQ BUS output here; the FREQ + RANGE knobs offset on top of it. control voltage (CV)
`width_bus`	cv	Pulse-width CV from a 921A driver's width bus; sets the rectangular output's duty cycle. Unpatched it normals to 0.5 (a 50% square). control voltage (CV)
`dc_mod`	audio	DC-coupled linear FM input: added to the frequency in Hz and scaled by the FM knob. Because it is DC-coupled, a steady offset here shifts the pitch (good for envelope-to-pitch), and audio here gives linear FM. audio signal
`ac_mod`	audio	AC-coupled (cap-coupled) linear FM input: a DC-blocking high-pass runs first, so a DC offset on the modulator does NOT bend the pitch — only its alternating content modulates. Scaled by the FM knob; use it for vibrato/FM that won't drift the tuning. audio signal
`sync`	audio	External sync source: each rising edge resets (hard) or nudges (soft) the oscillator's phase per the SYNC switch, locking it to the incoming signal for hard-sync sweeps. audio signal

outputs

id	cable	what it does
`sine`	audio	The pure sine tap (fixed level), the fundamental with no harmonics. audio signal
`triangle`	audio	The triangle tap (fixed level) — soft and hollow, gentler than the saw. audio signal
`saw`	audio	The band-limited sawtooth tap (fixed level) — the brightest, all-harmonics waveform. audio signal
`rect`	audio	The rectangular / pulse tap (fixed level); its duty cycle follows the width bus (50% when unpatched). audio signal

params

id	label	range	default	curve
`fine`	Freq	-12..12st	0	linear
`range`	Range	-5..5oct	0	discrete
`modAmount`	FM	-1..1	0	linear
`syncMode`	Sync	-1..1	0	linear
`level`	Level	0..2	1	linear

controls

control	what it does
Freq	FREQ — a ±12-semitone (2-octave) fine tuning offset applied on top of the bus pitch, for detuning this 921B against others in the bank.
Level	Output gain on every waveform tap, 0 to 2 (1 = unity) — the 921B's built-in VCA.
FM	FM — linear-FM depth, ±1, shared by both the DC and AC modulate inputs; at 0 both FM jacks are silent.
Range	RANGE — octave footage, a discrete ±5-octave coarse offset on top of the bus pitch, to stack a 921B an octave (or several) above/below its siblings.
Sync	SYNC switch: -1 = soft sync (a gentle phase nudge), 0 = off, +1 = hard sync (a full phase reset on each edge of the sync input).

source

moog921b.ts on GitHub.