Operator: Shape FM Tone with Two Envelopes

Two operators, two different jobs

Operator has four oscillators, A–D, and eleven routing algorithms. The example uses the D>C>B>A chain, but C and D have zero level. That leaves B modulating A. A is the carrier: its signal reaches the output. B changes A's waveform through modulation, adding spectral components called sidebands. B is not separately mixed into the output in this routing.

The original MIDI bass pattern supplies pitch, timing and velocity; the instrument generates the sound. All four Wave controls are explicitly zero, the analytic sine setting. These are hand-authored teaching settings, not a factory preset or captured oscillator cycle. Keep those Wave values unchanged while following this lesson.

Select B and reduce B Level from 32% to zero. Listen for the sound becoming simpler, not just quieter. Restore it, then reduce A Level instead. That changes the audible carrier level. Knowing which job a control has is more useful than memorizing a bright preset: a modulator level can be a tone control, while a carrier level controls an audible contribution.

Make brightness decay faster than loudness

The A/D/S/R boxes belong to the selected operator. They mean attack time, decay time, sustain level and release time. A starts quickly, decays toward a 55% sustain, and releases after note-off. B has a shorter 180 ms decay and zero sustain. Consequently modulation is strongest near the beginning and fades while the carrier can continue.

Increase B's D toward 700 ms without changing A. Follow the bright portion of each bass note: it should occupy more of the note, rather than simply lengthen the entire note equally. Restore the shorter decay if the upper harmonics crowd the next onset. Then raise B's S a little to retain some edge throughout held notes. Zero sustain does not mean zero attack.

Ratios before exotic routing

A Mode and B Mode are Ratio. A Coarse at 1 follows the played fundamental; B Coarse at 2 runs at twice that frequency. Compare B Coarse at 1, 2 and 3 with a modest B Level. Integer ratios provide a useful starting point for harmonically organized spectra, though modulation depth still determines how dense they become.

Coarse zero is a special half-frequency ratio, not a mute. Fine is also not a conventional cents detune here: the implemented ratio multiplies Coarse by one plus Fine divided by 1000. Small nonzero values move away from exact integer relationships. Use that deliberately for clang or beating, and return Fine to zero when a clear bass center matters more.

Fixed mode substitutes the operator's Freq in hertz for its note-following ratio. A fixed modulator can give different harmonic relationships on different keys. It is useful for deliberately uneven color across a keyboard, not a guarantee that one setting will sound equally settled everywhere. Compare several notes before committing to it.

Read the algorithm as signal flow

Try A+B+C+D with only A and B enabled. Both are now audible carriers: B Level adds its own sine tone instead of modulating A. Return to the original chain afterward. Global Feedback feeds operator D back into itself; with D at zero it is not a useful way to brighten this two-operator example.

Finally use Cutoff to tame an already useful FM spectrum, not to conceal excessive modulation. Res emphasizes the filter region, and Vibrato adds pitch motion in semitones. Keep vibrato at zero while judging tuning. This implementation teaches four-operator synthesis; its familiar name does not establish sonic equivalence to another instrument.