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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11If a synthesizer calculates its LFO only once per audio block, the LFO’s actual update rate is the audio sample rate divided by the block size. In that implementation, changing the host buffer can change the modulation sequence—and a requested rate above the sequence’s Nyquist limit aliases to a different frequency. That is a bug in the implementation described here, not a rule that applies to every synth LFO.
What “running at the buffer size” means
Let fs be the audio sample rate and N the number of audio samples in a host block. If the synth calculates one LFO value for each block, it produces fs/N modulation values per second. The sampled control signal’s Nyquist frequency is half of that: fs/(2N).
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A signal above that control-rate Nyquist limit cannot be represented at its requested frequency in the block-by-block sequence. It folds, or aliases, to another frequency. The audio engine may still render audio at the full sample rate; the limitation is that this particular modulation value is updated only at the slower block rate.
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In the implementation Lluis Estape describes, phase advances by the requested LFO rate multiplied by the block duration. That is sensible phase bookkeeping: it accounts for the time elapsed during the block. But the code evaluates sin() only once per block. Advancing the phase by the right amount does not create the modulation values that would have been produced by evaluating the waveform continuously or more frequently.
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The distinction is between how far phase advances and how often the resulting waveform is sampled. If only one sine value reaches the modulation destination per block, the destination still receives a block-rate sequence.
What the buffer-size examples show
At a fixed audio sample rate of 44.1 kHz, a larger block means fewer LFO updates each second and a lower control-rate Nyquist limit:
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| Block size | Control updates per second | Control-rate Nyquist | Reported result at a requested 20 Hz |
|---|---|---|---|
| 2048 samples | About 21.5 Hz | About 10.8 Hz | Estape reports an FFT measurement of a 1.5 Hz folded output. |
| 1024 samples | About 43.1 Hz | About 21.5 Hz | Estape reports the setting near 20.03 Hz. |
These figures and the FFT observations are from Estape’s article and its described code, not an independent test. The comparison illustrates why an LFO in this implementation can appear to change speed when the host block size changes: the requested rate is unchanged, but the control signal is sampled at a different rate.
How to audit an LFO in a synth
- Find the waveform evaluation. Inspect the processing path and determine whether the LFO waveform is calculated once per host block, once per smaller chunk, or for every audio sample.
- Check what reaches the destination. Count how many distinct modulation values can be applied during one audio block. A phase accumulator may update correctly while the destination still receives only one value per block.
- Calculate the control-rate limit. For block-based evaluation, divide the audio sample rate by the block size to get updates per second, then divide that result by two for the Nyquist limit.
- Compare buffer sizes at the same sample rate. Keep the requested LFO rate fixed and inspect the modulation sequence or its spectrum. A result that changes with the block size is evidence that the update interval is tied to that block size.
For the 44.1 kHz, 2048-sample case, the calculation is 44,100 ÷ 2048 ≈ 21.5 updates per second, with a Nyquist limit of about 10.8 Hz. A requested 20 Hz modulation rate is above that limit. This arithmetic helps interpret an observed result; it is not a substitute for measuring a particular plugin or code path.
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Fix: render in smaller, fixed chunks
Estape’s remedy divides each host block into chunks of up to 32 samples, advances the LFO phase by each chunk’s duration, and renders the corresponding synth segment. At 44.1 kHz, a 32-sample interval yields 44,100 ÷ 32 = 1,378.125 control updates per second and a Nyquist limit of 689.0625 Hz. Estape rounds these to 1,378 updates per second and 689 Hz.
Because the inner update interval is fixed, it is no longer determined by the host’s outer block size in this implementation. The 32-sample choice is suitable for the article’s stated maximum requested LFO rate of 20 Hz; it is not a universal setting. An implementation that supports higher modulation rates needs an appropriately higher update rate or sample-rate processing.
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The code also changes phase wrapping from a single conditional subtraction to subtracting floor(phase). That handles increments spanning more than one cycle, rather than assuming phase advances by less than one cycle each update.
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Smoothing can soften abrupt changes between modulation values, but it cannot reconstruct the requested frequency after aliasing has occurred. Estape reports that smoothed cutoff modulation sounded smooth while running at the wrong frequency. Treat smoothing and the choice of LFO update rate as separate design decisions.
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Not every synthesizer LFO is block-rate
The behavior depends on the instrument and processing path. Official manuals document other arrangements: Ableton says Operator’s LFO can run at audio rates and act as an additional oscillator; Ableton’s Live 12 effects documentation describes LFO time modes, sync divisions, and stepped quantization; Native Instruments documents Free, Sync, and Osc modes for Massive X.
For any specific product, distinguish how often its modulation is updated from how its rate is set. A free-running rate, a tempo-synchronized rate, and an oscillator mode describe different aspects of behavior; the mode names alone do not establish the internal update rate. Consult the relevant product documentation or test the plugin at more than one host buffer size.
Quick Recap
Sources
- Lluis Estape, “Your LFO runs at the buffer size: auditing the control rate of my own synth,” DEV Community. The page text gives a September 23 posting date without a year; its arithmetic, code discussion, and FFT results are the author’s reported findings.
- Ableton Live 12 Instrument Reference
- Ableton Live 12 Audio Effect Reference
- Native Instruments Massive X Modulators manual
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