FM pair: noise or a sample as a modulator, the SY77's operator input and RCM #153

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opened 2026-10-06 16:24:13 +00:00 by bcox · 0 comments

bcox commented

2026-10-06 16:24:13 +00:00

Owner

bcox asked on 2026-10-06 for a ticket for each gap docs/feature-survey.md names without one, whether or not it is ever done. Engine claims are from reading the tree at 6a3fede9 on main. Instrument claims are from that document's research, read from the manuals by research agents and not re-checked; the sources are listed there. Nothing here is built, measured or listened to.

Summary

The FM pair's two modulators are spectral tables (FmPairLayer::Patch::modulator, modulator2), so a modulator is periodic by construction. The SY77 lets an operator's input be white noise, or the voice's AWM sample (RCM: "By using an AWM waveform to modulate one or more AFM operators, new harmonics can be added to the AWM sample"); the Virus TI2's FM modes include noise and the inputs. The nearest route here is the VA layer: an oscillator on the sample shape or the input tap as another oscillator's cross-mod source, which is frequency modulation and not the phase modulation the SY77 runs.

What the tree has

FmPairLayer::Patch::IndexLaw with kPhase and kFrequency; Routing parallel or series; dsp/noise.h as a per-voice source the VA layer owns and the FM pair does not.
VaOscillator shapes kSample and the bus input, and cross_source naming an oscillator.

The work, as a sketch

A modulator kind on the pair: table, noise, or an input tap, read where the modulator table's sample is read and scaled by the same depth lane.
Noise through the modulator's formant band (kMod1FormantHz) gives the SY77's filtered-noise input.

Open

What RequiredBandwidthRatio says about an aperiodic modulator, which it worst-cases over a table's partials today (#68).

bcox asked on 2026-10-06 for a ticket for each gap `docs/feature-survey.md` names without one, whether or not it is ever done. Engine claims are from reading the tree at `6a3fede9` on `main`. Instrument claims are from that document's research, read from the manuals by research agents and not re-checked; the sources are listed there. **Nothing here is built, measured or listened to.** ## Summary The FM pair's two modulators are spectral tables (`FmPairLayer::Patch::modulator`, `modulator2`), so a modulator is periodic by construction. The SY77 lets an operator's input be white noise, or the voice's AWM sample (RCM: "By using an AWM waveform to modulate one or more AFM operators, new harmonics can be added to the AWM sample"); the Virus TI2's FM modes include noise and the inputs. The nearest route here is the VA layer: an oscillator on the sample shape or the input tap as another oscillator's cross-mod source, which is frequency modulation and not the phase modulation the SY77 runs. ## What the tree has - `FmPairLayer::Patch::IndexLaw` with `kPhase` and `kFrequency`; `Routing` parallel or series; `dsp/noise.h` as a per-voice source the VA layer owns and the FM pair does not. - `VaOscillator` shapes `kSample` and the bus input, and `cross_source` naming an oscillator. ## The work, as a sketch - A modulator kind on the pair: table, noise, or an input tap, read where the modulator table's sample is read and scaled by the same depth lane. - Noise through the modulator's formant band (`kMod1FormantHz`) gives the SY77's filtered-noise input. ## Open What `RequiredBandwidthRatio` says about an aperiodic modulator, which it worst-cases over a table's partials today (#68).