Amplitude modulation is multiplication
Amplitude modulation sounds like it should be one thing, but it is a family of at least four, and the only property they share is that they all multiply one signal by another. The differences follow directly from what the second signal is: a slow envelope, a fast envelope, a gate, or an oscillator.
That multiplication has a well-known consequence in the frequency domain. If you multiply a sine at frequency f by another at frequency m, the result contains frequencies at f plus m and f minus m. If the second signal is not a sine but a full-scale sine wave that only ever takes plus and minus one, the original frequency disappears entirely and you are left with only the two sidebands.
| Effect | Modulator is | Creates new frequencies | Main use |
|---|---|---|---|
| Tremolo | Slow envelope, 0.5 to 8 Hz | No | Amplitude movement |
| Vibrato | Pitch, 4 to 7 Hz | Yes, via pitch | Pitch movement |
| Chopper | Hard square gate | No | Rhythmic chops |
| Auto-pan | Slow panning value | No | Stereo movement |
| Ring modulation | Full oscillator | Yes, sum and difference | Metallic timbre |
Tremolo and its neighbour vibrato
Tremolo is an LFO writing to a gain parameter. The waveform of the LFO decides the character: sine gives a smooth swell, triangle gives a rise and fall with an audible turning point, square gives abrupt on and off, and a random or S&H shape gives something less predictable. Depth is the amount of gain change, so full depth means complete silence at the trough.
- Rate below about 4 Hz reads as pulsing and is usually a rhythmic or vintage effect.
- Rate between 4 and 7 Hz overlaps the vibrato range, which is where tremolo and vibrato get confused.
- Depth below about 15 percent is usually inaudible on a dense mix and obvious on a single sustained note.
- Tremolo on a stereo source applied in mid-side avoids collapsing the image that a linked gain control causes.
Vibrato is the same LFO rate routed to pitch instead of gain, which makes it a frequency modulation effect and does create new content through the sideband mechanism above. The two get separated mostly by ear: at 5 Hz you cannot reliably tell which parameter is moving without watching the pitch. Common synthesis practice links them for a small amount and separates them for a large amount, since full vibrato plus full tremolo sounds seasick rather than expressive.
Auto-pan is a position control, not an amplitude control
Auto-pan writes to a pan parameter over time, and the signal is never attenuated in the process. What changes is where it appears, and that is why a hard-panned repeat sounds like a different object rather than a fainter one. Pan law is the hidden variable: a -3 dB centre pan sums to unity in mono, while a -6 dB law at hard left loses 6 dB when folded down.
This gives auto-pan a specific strength that stereo widening does not: it creates the impression of a source moving through a real room. The trade-off is that it does nothing to the mono fold beyond whatever pan law costs, and a listener on headphones will hear the pan sweep while a listener on a mono system hears only level change from the law.
Ping-pong delay is auto-pan applied to delay repeats rather than to a single voice, and it inherits exactly the same caveats. Sync it to a dotted eighth and the sweep lands with the beat, which reads as designed; leave it unsynced and it reads as a fault.
Chopper and ring modulation, the two that differ
A chopper is tremolo pushed to a hard gate. The LFO is a square, the depth is at or near full, and the rate is usually tempo-synced, so the output is a rhythmic pattern rather than a texture. That makes it a drum and arrangement tool rather than a mixing one, and it is the reason a gated guitar sound exists at all. Its cost is harmonics: a hard gate is a non-smooth waveform, so it generates intermodulation products that a gentle tremolo does not.
Ring modulation multiplies by an oscillator running at audio rate, and it is the one on this list that genuinely rewrites the spectrum. Ring a 220 Hz sine with a 440 Hz oscillator and the output contains 660 Hz and 220 Hz. Ring a full harmonic-rich tone with a low modulator and you get a metallic, bell-like result with the fundamental largely removed. Partial-based timbre makes the difference obvious, which is a useful way to hear it.
Two practical warnings. Ring modulation at audio-rate frequencies is a strong, unusual sound and sits forward in the mix, so it usually needs level and space rather than more brightness. And a true ring mod has no memory of the original pitch, which means it cannot be used to reinforce a root note. If you need the root to stay, blend a dry path with the ring-modulated one rather than trying to correct the result afterwards.
The bottom line
Tremolo multiplies a signal by a moving level and leaves its spectrum alone, auto-pan moves it between two speakers, a chopper interrupts it on a grid, and only ring modulation creates sum and difference frequencies. Choosing wrongly costs you either tone colour or spectrum, so decide which one you are after before reaching for the control.
Hear the difference without a meter
432Hz MASTER shifts pitch by one fixed ratio, so any change in colour you hear between a tremolo and a ring mod is the modulation itself.
Frequently asked questions
What is the difference between tremolo and ring modulation?
Tremolo multiplies the signal by a level envelope, so the spectrum stays where it was and only the amplitude moves. Ring modulation multiplies by an oscillator, which produces the sum and difference of the two and shifts the timbre completely.
What does a chopper actually do?
It gates the signal hard on and off using a square LFO, producing a rhythmic gate pattern from the tremolo depth. It stays on the grid if the rate is tempo-synced, which makes it a rhythmic effect rather than a timbral one.
Why does ring modulation change the pitch?
Because multiplication in the time domain is convolution in the frequency domain. Every partial in the carrier appears alongside a copy offset by the modulator, so a 440 Hz tone ring-modulated at 100 Hz contains 540 Hz and 340 Hz as well as the original.
Is auto-pan the same as stereo widening?
No. Auto-pan moves an existing signal between two speakers in time. Widening uses decorrelation, phase manipulation or a different signal, and produces something that still sounds wide when the movement stops.