How to Explore Filter FM Artifacts Using a Held Dual-Oscillator Tone

You need two oscillators feeding a filter, a way to sustain their output, and a documented route that applies an audio-frequency signal to filter cutoff. Check the modulation source and destination before starting. Sequential’s Prophet Rev2, for example, uses Oscillator 1 as the source for its dedicated Audio Mod control. Its user guide describes this as modulation of filter cutoff. Prophet Rev2 User’s Guide, version 1.2.2

For a modular setup, distinguish the filter’s audio input from its cutoff control input. Doepfer’s A-121-2 documentation identifies separate audio and frequency-control inputs, including an attenuated FCV2 input for setting modulation depth. That distinction matters when tracing the patch: the oscillator mix enters the audio path, while the modulation signal enters the cutoff control path. Doepfer A-121-2 12dB Multimode Filter

Use a filter modulation route that your instrument documents as suitable for audio-rate operation. A generic modulation assignment should not be assumed to have the same behavior as a dedicated filter FM control. If the available documentation does not establish the necessary routing, choose an instrument or patch configuration that does.

Start with an initialized patch. Hold one key manually, use the instrument’s hold function, or send one sustained MIDI note. Set the amplifier envelope so the tone remains steady after its attack. Disable the arpeggiator and sequencer so the note does not retrigger during comparisons.

Use these proposed starting conditions:

• Select triangle waves for both oscillators if available; otherwise choose the same simple waveform on each.
• Tune both oscillators to the same nominal pitch, then add only a small amount of detuning if desired.
• Make both oscillators audible at moderate mixer levels.
• Turn off oscillator sync, oscillator pitch modulation, waveform modulation, noise and effects.
• Set filter envelope amount and other cutoff modulation amounts to zero.
• Select a low-pass response, start with low resonance, and set filter FM depth to zero.

Hold a comfortable middle-register note and slowly move cutoff through its range. Choose a position where the tone is clearly audible and a small cutoff adjustment changes its character. Keep that position for the first comparison.

Record several seconds of this baseline if recording is available. Describe what you already hear: a stable tone, slow pulsing, an uneven texture or a slight pitch difference. Any feature present with filter FM at zero belongs in your reference description; do not attribute it to the modulation you add later.

Keep the note, oscillator tuning, mixer balance and cutoff fixed. Raise filter FM depth from zero in small steps, pausing at each setting. Choose three reproducible positions: zero, a subtle change and a pronounced change. Exact knob percentages are less useful than settings you can return to reliably.

At each position, listen for a specific difference: added upper texture, a more prominent tonal component, a change in the apparent pitch center, or a change in how the sustained sound moves. These are listening categories, not predictions that every filter will produce each result.

Return to zero between comparisons. If possible, record equal-length excerpts with a short pause between them. Keep the recording path unchanged and compare at similar playback loudness, so a louder take does not automatically become your preferred one.

Sequential describes the Rev2’s Audio Mod as capable of adding harmonic movement and high-frequency sizzle, particularly with some resonance in four-pole mode. Treat that as guidance for that instrument; use your own controlled comparisons to establish what your filter does. Prophet Rev2 User’s Guide

Once you find an audible modulation depth, explore cutoff, resonance and oscillator tuning separately. Restore the same starting patch before each pass. This makes the experiment interpretable even when the resulting sound is complex.

Pass	Change only	Keep fixed	Question to answer
Cutoff	Base cutoff position	FM depth, tuning, resonance	Where is the added texture most distinct?
Depth	Filter FM amount	Cutoff, tuning, resonance	Where does the texture first become useful?
Resonance	Resonance amount	Cutoff, depth, tuning	Does emphasis around cutoff reveal another character?
Tuning	Modulating oscillator pitch	Other oscillator pitch, cutoff, depth	Which tuning relationships are worth saving?
Waveform	Modulating oscillator waveform	Tuning, cutoff, depth	Which source shape gives the preferred result?

For the tuning pass, try unison, an octave above, and a deliberately offset interval. These are test conditions rather than promised tonal outcomes. If the modulation source is also audible in the oscillator mix, changing its pitch changes both the input sound and the modulation. Write down both roles instead of describing the comparison as an isolated change to modulation frequency.

For waveform comparisons, first return to the original tuning. Compare only two waveforms at a time and restore the baseline between them. Avoid simultaneously increasing resonance or drive to make the difference more dramatic; those settings can be explored in their own passes.

Use the following diagnostic sequence whenever a new texture appears. It is a practical method derived from the documented separation of audio, cutoff control and resonance controls; it does not establish a circuit-level explanation from listening alone.

1. Set filter FM depth to zero. Note whether the texture disappears, weakens or remains.
2. Restore FM and remove one oscillator from the audio mix. Use its mixer level, preserving the modulation route where the instrument allows this.
3. Restore the mix and remove the other oscillator. Compare the two isolated audio contributions with the combined tone.
4. Lower resonance. Check whether the feature depends on the resonant setting.
5. Lower the signal entering the filter. Keep modulation depth fixed where possible, then compare recordings at similar playback loudness.

The distinction between muting an audio contribution and disabling an oscillator is essential. Turning off the oscillator’s waveform or engine may also remove the modulation source. Before interpreting a mute test, confirm that the oscillator continues to feed the filter FM route.

Input level deserves its own comparison. Doepfer explicitly documents distortion around the midpoint and above on the A-106-5’s input level control with typical A-100 oscillator signals. Its A-121-2 documentation also identifies input-level-dependent distortion. Those thresholds belong to those modules, but they demonstrate why a rough sound should be checked at lower filter input levels. Doepfer A-106-5, Doepfer A-121-2

Treat self-oscillation as a separate experiment. The filter may contribute a tone of its own, so the patch no longer consists only of two oscillator signals passing through a resonant filter.

This behavior depends on the filter. Doepfer states that the A-121-2 can produce a sine-like oscillation without audio input at sufficient resonance, while the A-106-5 does not self-oscillate. Maximum resonance therefore does not define a comparable test across these two designs. Doepfer A-121-2, Doepfer A-106-5

For a filter that supports it, first set FM depth to zero and remove the oscillator audio feed. Raise resonance until you establish whether the filter produces a sustained tone. Then introduce the documented filter FM source. Label this recording separately from your held dual-oscillator experiment, and restore the original audio mix and resonance afterward.

If the filter does not self-oscillate, continue exploring the sustained input tone. Self-oscillation is an optional branch of the procedure, not a prerequisite for finding useful cutoff-modulation textures.

Choose one setting from the comparisons and make a compact patch record. Include the held note, oscillator waveforms and interval, mixer levels, modulation source, filter response, cutoff, resonance and FM depth. Record the state of keyboard tracking and any effects you later add.

Use observations that say what changed. For example: “With this held note and oscillator interval, the upper texture becomes prominent at depth B, disappears at zero depth, and remains when the modulating oscillator is removed from the audio mix.” This is an illustrative reporting format, not a claimed test result. It preserves more useful information than a label such as “metallic.”

Finally, play the patch above and below the original note and give it the envelope shape you intend to use. Keep the sustained reference recording alongside the musical version. You will then have both a usable sound and a clear account of the settings that produced its character.

• Prophet Rev2 User’s Guide, version 1.2.2 — Sequential
• A-121-2 12dB Multimode Filter — Doepfer
• A-106-5 12dB SEM Filter — Doepfer