How to Use FM Synthesis (fm6op)

DecentSampler includes a built-in 6-operator FM synthesizer that implements the 32 classic algorithm topologies from the Yamaha DX7. You can use it alongside traditional samples, as a pure synthesizer, or anywhere else an <oscillator> element is valid.

Quick Start

Set waveform="fm6op" on an <oscillator> element and configure the FM parameters on the parent <group>:

<group fmAlgorithm="5"
       fmOp1Ratio="1.0"  fmOp1Level="0.9"
       fmOp2Ratio="14.0" fmOp2Level="0.5"
       fmOp2Attack="0.001" fmOp2Decay="0.4" fmOp2Sustain="0.0" fmOp2Release="0.1"
       fmOp6Feedback="0.0"
       attack="0.001" decay="0.5" sustain="0.0" release="1.5">
  <oscillator waveform="fm6op"
              loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

Concepts

Operators

The fm6op oscillator has six operators numbered 1–6. Each operator is a sine-wave oscillator with:

A frequency ratio (fmOpNRatio) — multiplies the played note’s frequency.
1.0 = fundamental, 2.0 = one octave up, 0.5 = one octave down.
An output level (fmOpNLevel) — how much the operator contributes (0.0–1.0).
A feedback amount (fmOpNFeedback) — how much the operator feeds its own output back into its phase input (0.0–1.0). In the DX7 architecture only one operator per algorithm physically carries the feedback path; see the note on feedback below.
An independent per-operator ADSR envelope (fmOpNAttack, fmOpNDecay, fmOpNSustain, fmOpNRelease).
An optional DX7-compatible 4-stage rate/level envelope (fmOpNEgType="dx7" plus fmOpNEgRate1–fmOpNEgLevel4).

Carriers and Modulators

Each algorithm divides operators into two roles:

Carrier — its output is added to the final audio signal you hear.
Modulator — its output is fed into the phase input of another operator, adding harmonics and timbral complexity. Modulators are not heard directly.

Algorithms

The fmAlgorithm attribute (1–32) selects the routing topology. A higher algorithm number generally means more carriers (brighter, additive sound). A lower number generally means deeper modulation chains (more harmonically rich FM timbres).

Some useful algorithm categories:

Algorithms	Topology	Character
1–8	Deep chains (1 or 2 carriers)	Rich, complex FM timbres — basses, pads, strings
9–19	Mixed chains + parallel	Balanced, versatile — pianos, plucks, bells
20–28	3+ carriers	Bright, additive-leaning — choir, organs, leads
29–32	4–6 carriers	Additive/organ-like — drawbar organs, rich pads

Algorithm 32 is a special case: all six operators are carriers, each producing a sine wave at its own ratio. This is pure additive synthesis.

Feedback

Each algorithm designates exactly one operator as the feedback source. In the DX7 this is always Op 6 (the fmOp6Feedback attribute), so fmOp6Feedback is the feedback parameter that works in all 32 algorithms. Setting feedback on other operators has no audible effect unless the active algorithm routes them as the feedback source.

Small feedback values (0.0–0.15) add subtle harmonics and warmth. Larger values (0.3–0.6) create bright sawtooth-like waves. Very high values (0.7–1.0) produce noise and distortion.

Detune

Each operator can be pitch-detuned using DX7-compatible detune values (fmOpNDetune, range -7 to +7). Detune creates subtle pitch offsets that add richness and movement to the sound:

0 = no detune (default)
Positive values (1–7) = sharpen the pitch slightly
Negative values (-7 to -1) = flatten the pitch slightly

The DX7 detune algorithm produces larger pitch offsets at lower notes and smaller offsets at higher notes, matching the classic hardware behavior. Common use cases:

Chorus-like thickness: Set operators at the same ratio but slightly different detune values (e.g., Op1 detune=0, Op2 detune=2).
Detuned stacks: Create evolving textures by detuning multiple carriers in algorithm 32.
Bell-like inharmonicity: Slight detunes on modulators add metallic character.

Fixed Frequency Mode

By default, each operator’s frequency is a ratio of the played note (fmOpNMode="ratio"). You can instead set an operator to fixed frequency mode (fmOpNMode="fixed") and specify an absolute frequency in Hz using fmOpNFixedFreq:

<group fmAlgorithm="1"
       fmOp1Ratio="1.0" fmOp1Level="0.8"
       fmOp2Mode="fixed" fmOp2FixedFreq="987.5" fmOp2Level="0.4">

In this example, Op2 always oscillates at 987.5 Hz regardless of which note is played. This creates:

Metallic/bell timbres: Fixed-frequency modulators add inharmonic partials.
Detuned octaves: A fixed carrier at a specific frequency creates tension against the played note.
Special effects: Ring modulation and clangorous textures.

Fixed frequency is often combined with ratio mode operators in the same patch for hybrid harmonic/inharmonic sounds.

Velocity Sensitivity

Each operator can respond to MIDI velocity using fmOpNVelocitySensitivity (range 0–7, DX7 convention):

0 = no velocity response (default) — operator always plays at full level.
1–7 = increasing velocity scaling — level varies from near-zero at vel=1 to full at vel=127.

Velocity sensitivity is most useful on:

Carriers: Make the overall volume respond to playing dynamics.
Modulators: Make the brightness/harmonic complexity respond to velocity — lighter touches produce simpler tones, harder strikes add more harmonics.

Example: Velocity-sensitive electric piano (algorithm 5):

<group fmAlgorithm="5"
       fmOp1Ratio="1.0"  fmOp1Level="0.85" fmOp1VelocitySensitivity="3"
       fmOp2Ratio="14.0" fmOp2Level="0.45" fmOp2VelocitySensitivity="6"
       fmOp2Attack="0.001" fmOp2Decay="0.6" fmOp2Sustain="0.0" fmOp2Release="0.1">
  <oscillator waveform="fm6op" loNote="0" hiNote="127" rootNote="60"/>
</group>

Here, Op1 (carrier) has moderate velocity response (3), while Op2 (modulator) has high velocity response (6) — soft notes are mellow, loud notes are bright.

Per-Operator Envelopes

Each operator has its own ADSR envelope that shapes how that operator’s contribution evolves over time:

Carrier envelopes shape the timbre loudness over time (like a VCA).
Modulator envelopes shape the modulation depth: high modulation at attack = bright attack transient; low modulation after decay = simpler sustained tone.

Sentinel value (-1.0 for release): If you omit a per-operator release, or set it to -1.0, the operator has no independent release — it is fully controlled by the outer <group> ADSR’s release stage. This is the default, so you only need to specify per-operator release when you want the operator to decay independently while the note is held.

DX7-Compatible 4-Stage Operator Envelope

In addition to the standard ADSR envelope, each operator can use a 4-stage rate/level envelope that matches the parameter conventions of classic 6-operator DX7 synthesizers. This mode is activated per-operator by setting fmOpNEgType="dx7".

The DX7 envelope has four segments, each defined by a rate (how fast the level moves) and a target level (where it stops):

Stage	Attributes	Meaning
Attack	`fmOpNEgRate1`, `fmOpNEgLevel1`	Rise from silence to L1 at speed R1
Decay	`fmOpNEgRate2`, `fmOpNEgLevel2`	Move from L1 to L2 at speed R2
Sustain	`fmOpNEgRate3`, `fmOpNEgLevel3`	Move toward L3 at speed R3; held while key is down
Release	`fmOpNEgRate4`, `fmOpNEgLevel4`	Move from current level to L4 at speed R4 after key-off

All rates and levels use the range 0–99, matching standard DX7 patch banks:

Rate 99 = near-instant transition. Rate 0 = extremely slow.
Level 99 = full amplitude. Level 0 = silence.

Default flat envelope (instant attack, infinite sustain, instant release):

R1=99  L1=99   ← instant rise to full
R2=99  L2=99   ← instant move (stays at full)
R3=0   L3=99   ← rate 0: holds at L3 indefinitely while key is down
R4=99  L4=0    ← instant fall to silence after key-off

When fmOpNEgType="dx7" is set, the fmOpNAttack/fmOpNDecay/fmOpNSustain/fmOpNRelease ADSR attributes are ignored for that operator.

Example: Brass patch with DX7 envelope

Classic brass sounds have a fast attack to L1, a small dip to a slightly lower sustain level, and a moderate release. This example uses algorithm 1, with Op1 as the carrier and Op2 as a modulator with its own shape:

<group fmAlgorithm="1"
       attack="0.001" decay="0.0" sustain="1.0" release="0.3"
       fmOp1Ratio="1.0"  fmOp1Level="0.9"
       fmOp1EgType="dx7"
       fmOp1EgRate1="99" fmOp1EgLevel1="99"
       fmOp1EgRate2="86" fmOp1EgLevel2="94"
       fmOp1EgRate3="0"  fmOp1EgLevel3="94"
       fmOp1EgRate4="60" fmOp1EgLevel4="0"
       fmOp2Ratio="1.0"  fmOp2Level="0.55"
       fmOp2EgType="dx7"
       fmOp2EgRate1="99" fmOp2EgLevel1="99"
       fmOp2EgRate2="70" fmOp2EgLevel2="60"
       fmOp2EgRate3="0"  fmOp2EgLevel3="60"
       fmOp2EgRate4="55" fmOp2EgLevel4="0"
       fmOp6Feedback="0.08">
  <oscillator waveform="fm6op" loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

DX7 rate/level values from standard patch banks can be used directly without conversion — the numbers are the same.

Per-Operator ADSR Example: DX7 E. Piano Transient

The famous DX7 Electric Piano transient comes from the modulator decaying quickly on attack while the carrier sustains:

<group fmAlgorithm="5"
       attack="0.001" decay="0.5" sustain="0.0" release="1.5"
       fmOp1Ratio="1.0"  fmOp1Level="0.90"
       fmOp2Ratio="14.0" fmOp2Level="0.55"
       fmOp2Attack="0.001" fmOp2Decay="0.40" fmOp2Sustain="0.0" fmOp2Release="0.15"
       fmOp3Ratio="1.0"  fmOp3Level="0.70"
       fmOp4Ratio="14.0" fmOp4Level="0.40"
       fmOp4Attack="0.001" fmOp4Decay="0.30" fmOp4Sustain="0.0" fmOp4Release="0.12"
       fmOp5Ratio="1.0"  fmOp5Level="0.55"
       fmOp6Ratio="14.0" fmOp6Level="0.30" fmOp6Feedback="0.0"
       fmOp6Attack="0.001" fmOp6Decay="0.25" fmOp6Sustain="0.0" fmOp6Release="0.10">
  <oscillator waveform="fm6op"
              loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

Here Op6/Op5/Op4/Op3/Op2 are modulators. The modulator envelopes are short (decay=0.25–0.40, sustain=0) so they decay to zero quickly — creating the bright “tine” transient at the note start. The outer <group> ADSR shapes the overall volume fade.

Real-Time Modulation with Bindings

All FM operator parameters support real-time modulation. This lets you connect LFOs, envelopes, MIDI CCs, or UI knobs to FM parameters for expressive control.

Example: Knob controls modulation depth (morph timbre)

<ui width="812" height="375">
  <tab>
    <labeled-knob x="350" y="30" width="100" height="100"
                  label="FM Depth" parameterName="FMDepth"
                  minValue="0" maxValue="1" value="0.5">
      <binding type="general" level="group" position="0"
               parameter="OSCILLATOR_FM_OP2_LEVEL"
               translation="linear"
               translationOutputMin="0.0" translationOutputMax="1.0"/>
    </labeled-knob>
  </tab>
</ui>

Supported binding parameters

Parameter	Range	What it controls
`OSCILLATOR_FM_ALGORITHM`	1–32	Routing topology (usually set once, not modulated)
`OSCILLATOR_FM_OP1_LEVEL` … `OSCILLATOR_FM_OP6_LEVEL`	0.0–1.0	Per-operator modulation depth / carrier level
`OSCILLATOR_FM_OP1_RATIO` … `OSCILLATOR_FM_OP6_RATIO`	Any positive value	Per-operator frequency ratio (detune, octave shifts)
`OSCILLATOR_FM_OP1_FEEDBACK` … `OSCILLATOR_FM_OP6_FEEDBACK`	0.0–1.0	Per-operator self-feedback

See Appendix B for a full listing.

Classic Sound Recipes

Drawbar Organ (Algorithm 32 — all carriers)

Algorithm 32 turns the FM engine into a pure additive synthesizer. Map each operator to a harmonic of the fundamental:

<group fmAlgorithm="32"
       attack="0.001" decay="0" sustain="1.0" release="0.05"
       fmOp1Ratio="1.0" fmOp1Level="0.85"  fmOp1Release="0.05"
       fmOp2Ratio="2.0" fmOp2Level="0.65"  fmOp2Release="0.05"
       fmOp3Ratio="3.0" fmOp3Level="0.45"  fmOp3Release="0.05"
       fmOp4Ratio="4.0" fmOp4Level="0.30"  fmOp4Release="0.05"
       fmOp5Ratio="6.0" fmOp5Level="0.18"  fmOp5Release="0.05"
       fmOp6Ratio="8.0" fmOp6Level="0.10"  fmOp6Feedback="0.08"  fmOp6Release="0.05">
  <oscillator waveform="fm6op" loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

Short per-operator releases (0.05 s) produce the characteristic fast key-off click of a tonewheel organ.

Bell / FM Marimba (Algorithm 5 — three 2-op pairs)

Inharmonic ratios + all-decay envelopes = percussive bell tones:

<group fmAlgorithm="5"
       attack="0.001" decay="0" sustain="1.0" release="20"
       fmOp1Ratio="1.0"  fmOp1Level="1.0"
       fmOp1Attack="0.001" fmOp1Decay="4.0" fmOp1Sustain="0.0" fmOp1Release="2.0"
       fmOp2Ratio="3.5"  fmOp2Level="0.35"
       fmOp2Attack="0.001" fmOp2Decay="0.15" fmOp2Sustain="0.0" fmOp2Release="0.1"
       fmOp3Ratio="1.0"  fmOp3Level="0.70"
       fmOp3Attack="0.001" fmOp3Decay="3.0"  fmOp3Sustain="0.0" fmOp3Release="1.5"
       fmOp4Ratio="3.5"  fmOp4Level="0.28"
       fmOp4Attack="0.001" fmOp4Decay="0.12" fmOp4Sustain="0.0" fmOp4Release="0.09"
       fmOp5Ratio="1.0"  fmOp5Level="0.50"
       fmOp5Attack="0.001" fmOp5Decay="2.0"  fmOp5Sustain="0.0" fmOp5Release="1.0"
       fmOp6Ratio="3.5"  fmOp6Level="0.22"  fmOp6Feedback="0.0"
       fmOp6Attack="0.001" fmOp6Decay="0.10" fmOp6Sustain="0.0" fmOp6Release="0.08">
  <oscillator waveform="fm6op" loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

Note the long outer release (release="20") — this keeps the voice alive while the per-operator envelopes decay naturally to silence on their own.

Choir Pad (Algorithm 19 — shared modulator with detuned carriers)

Algorithm 19 routes a single Op6 modulator into both Op5 and Op4. Setting their ratios slightly apart creates a beating, chorus-like quality:

<group fmAlgorithm="19"
       attack="1.0" decay="0" sustain="1.0" release="20"
       fmOp4Ratio="1.003" fmOp4Level="0.75"
       fmOp4Attack="0.90" fmOp4Decay="0.0" fmOp4Sustain="1.0" fmOp4Release="1.5"
       fmOp5Ratio="1.0"   fmOp5Level="0.85"
       fmOp5Attack="0.90" fmOp5Decay="0.0" fmOp5Sustain="1.0" fmOp5Release="1.5"
       fmOp6Ratio="1.0"   fmOp6Level="0.30" fmOp6Feedback="0.08"
       fmOp6Attack="0.50" fmOp6Decay="0.30" fmOp6Sustain="0.20" fmOp6Release="0.5"
       fmOp1Ratio="1.0" fmOp1Level="0.60"
       fmOp1Attack="0.80" fmOp1Decay="0.0" fmOp1Sustain="1.0" fmOp1Release="1.5"
       fmOp2Ratio="1.0" fmOp2Level="0.50"
       fmOp2Attack="0.50" fmOp2Decay="1.0" fmOp2Sustain="0.50" fmOp2Release="1.0"
       fmOp3Ratio="3.0" fmOp3Level="0.35"
       fmOp3Attack="0.30" fmOp3Decay="0.50" fmOp3Sustain="0.30" fmOp3Release="0.5">
  <oscillator waveform="fm6op" loNote="0" hiNote="127" rootNote="60"
              loVel="0" hiVel="127" volume="1.0"/>
</group>

Tips

Start simple. Pick algorithm 5 and set only Op1 (carrier) + Op2 (modulator) to non-default values. Gradually raise fmOp2Level from 0 to 1 to hear FM modulation build from a sine wave to a complex timbre.
Keep loudness consistent. When many operators have high levels the output can clip. Lower volume on the <group> or reduce carrier levels accordingly.
Per-operator release tuning. Set per-operator releases independently to make different parts of the spectrum fade at different rates — carriers fade slowly (long release), modulators fade faster (short release) for a natural brightness decay.
Inharmonic ratios for metallic textures. Try ratios like 3.5, 7.07, 1.41 (sqrt(2)), or other non-integers on modulators for bells, gongs, and metallic sounds.
Operator 6 first. fmOp6Feedback is nearly always the feedback operator. Start by adjusting this first when you want to add harmonic richness.
Combine with samples. A group containing <sample> elements can share a track with an fm6op group. Use the groups’ output* routing to send them to different buses if needed.