Writing Audio Plugins in MATLAB

When To Use

• Creating audio effect or generator plugins (classdef inheriting from audioPlugin or audioPluginSource)
• Troubleshooting validateAudioPlugin or generateAudioPlugin failures
• Converting a MATLAB audio algorithm into a deployable VST/AU plugin
• Integrating deep learning inference into a real-time audio plugin

When NOT To Use

• General MATLAB class authoring unrelated to audio plugins
• Audio file I/O, feature extraction, or analysis (no plugin involved)
• Simulink audio processing blocks
• Writing Audio Toolbox functions that are not plugins (e.g., audioDatastore, audioFeatureExtractor)

Structure

Every audio plugin is a classdef with %#codegen, public tunable properties, a Constant PluginInterface, and methods process + reset.

classdef MyPlugin < audioPlugin
%#codegen

    properties
        Gain = 0.5
        Cutoff = 1000
    end

    properties (Access = private)
        pSR = 44100
        pB = [1 0 0]
        pA = [1 0 0]
        pState = zeros(2, 2)  % (filterOrder, numChannels)
    end

    properties (Constant)
        PluginInterface = audioPluginInterface( ...
            audioPluginParameter('Gain', ...
                DisplayName='Gain', Label='dB', Mapping={'lin', 0, 1}), ...
            audioPluginParameter('Cutoff', ...
                DisplayName='Cutoff', Label='Hz', Mapping={'log', 20, 20000}), ...
            InputChannels=2, OutputChannels=2)
    end

    methods
        function y = process(plugin, x)
            [y, plugin.pState] = filter(plugin.pB, plugin.pA, x, plugin.pState);
            y = y * plugin.Gain;
        end

        function reset(plugin)
            plugin.pSR = getSampleRate(plugin);
            plugin.pState = zeros(2, 2);
            designFilter(plugin);
        end

        function set.Cutoff(plugin, val)
            plugin.Cutoff = val;
            designFilter(plugin); %#ok<MCSUP>
        end
    end

    methods (Access = private)
        function designFilter(plugin)
            wn = plugin.Cutoff / (plugin.pSR / 2);
            wn = max(eps, min(wn, 1 - eps));
            [plugin.pB, plugin.pA] = butter(2, wn);
        end
    end
end

Source plugins inherit audioPluginSource, omit InputChannels, and process takes no audio input — use getSamplesPerFrame(plugin) for output frame size.

System Object hybrids (matlab.System & audioPlugin) require (StrictDefaults), isInputSizeMutableImpl returning true, and stepImpl/resetImpl instead of process/reset. Use only when user requests Simulink compatibility.

Plugin Lifecycle

1. Constructor — Construct all sub-objects with literal arguments. getSampleRate returns 44100 here; use for initial buffer sizing only.
2. `reset` — Called when sample rate or frame size changes. Cache getSampleRate(plugin) in pSR. Recompute all SR-dependent values. Call reset() on every sub-object (after setting their sample rate).
3. `process` — Called per audio frame. Return double output sized [N, numOutputChannels]. Never assign to properties registered in audioPluginParameter — for output meters, keep properties public but unregistered.
4. Set methods — Recompute derived values (coefficients, buffer sizes) when parameters change. Add %#ok<MCSUP> only when the set method actually accesses another property through obj.PropertyName or calls a method that does (e.g., designFilter(plugin) from set.Cutoff). Do not add it preemptively — only suppress warnings that checkcode actually raises on that line.
5. Save/load (optional, MATLAB-only) — Only needed when the plugin has private state that reset cannot reconstruct from parameters and sample rate (e.g., loaded data, handle sub-objects with internal buffers). Not compiled into generated VSTs/standalones — DAW hosts persist parameters via the plugin interface and call reset on restore. See references/advanced-patterns.md for the pattern.

Codegen Requirements

All code must be codegen-compatible. validateAudioPlugin sweeps 5 sample rates (8000–192000), frame sizes 2.^(1:13)+1 (max 8193), and all parameter extremes.

Always use built-in functions over hand-implementations. Prefer toolbox functions in this order: Audio Toolbox → DSP System Toolbox → Signal Processing Toolbox → base MATLAB. If unsure whether a codegen-compatible built-in exists for an operation, consult references/available-functions.md before implementing manually.

Buffers and State

Pre-allocate all state to maximum needed size. Codegen locks property size from the constructor's last assignment — allocate at the maximum the parameter can reach, then index into the active region at runtime.

Shift with indexed assignment:

buf(1:end-N) = buf(N+1:end);   % shift left by N
buf(end-N+1:end) = newData;     % fill tail

Never concatenate to shift — [buf(N+1:end); zeros(N,ch)] produces a variable-size result that codegen rejects on assignment to a fixed-size property.

Never use : on the column dimension when assigning to a fixed-size property — codegen treats x(i,:) as variable-size. Index columns explicitly: [x(i,1), x(i,2)].

Initialize arrays that will hold complex values with complex(zeros(...)) — not bare zeros(...). Codegen locks the real/complex attribute from the first assignment. If the array starts real, assigning FFT output into it later fails with "left-hand side constrained to be non-complex."

dsp.AsyncBuffer capacity must exceed the largest single write the plugin will receive. validateAudioPlugin delivers frames up to 8193 samples; account for that plus any overlap when sizing the buffer.

When filter() input comes from a sub-object (e.g., crossoverFilter), codegen cannot propagate the column count — the returned state becomes variable-size. Process channels explicitly:

[low(:,1), plugin.pState(:,1)] = filter(b, a, low(:,1), plugin.pState(:,1));
[low(:,2), plugin.pState(:,2)] = filter(b, a, low(:,2), plugin.pState(:,2));

Enum Parameters (Different-Length Values)

Use Style='dropdown' for 3+ values, 'vrocker'/'vtoggle' for exactly 2.

When enum values have different character lengths ('On'/'Off', 'Short'/'VeryLong'), prefer a separate int32 enum class file. The framework can handle char padding internally, but an explicit enum class produces cleaner generated code with typed dispatch instead of string comparisons:

% MyMode.m
classdef MyMode < int32
    enumeration
        Normal     (0)
        Aggressive (1)
        Subtle     (2)
    end
end

Plugin property: Mode = MyMode.Normal with Mapping={'enum','Normal','Aggressive','Subtle'}.

Derived Values from Parameters

When computing normalized frequency or delay indices from parameters, clamp the result:

wn = plugin.Cutoff / (plugin.pSR / 2);
wn = max(eps, min(wn, 1 - eps));   % keep in valid (0,1) range for butter/cheby

Clamp delay-line read indices to [1, bufferLength]. This prevents out-of-bounds at extreme sample rates or parameter settings that validateAudioPlugin will exercise.

Switch Completeness

Every switch that assigns a variable must include otherwise with a safe default — codegen requires all branches to define the same outputs.

Sub-Objects

Construct with literal arguments in the constructor. In reset, propagate sample rate then reset:

function reset(plugin)
    fs = getSampleRate(plugin);
    setSampleRate(plugin.pEcho, fs);
    reset(plugin.pEcho);
    plugin.pCompressor.SampleRate = fs;
    reset(plugin.pCompressor);
end

Call sub-plugins as process(plugin.pSub, x). Call System Objects as plugin.pObj(x). Forward parameter changes in set methods.

External Data and Runtime-Only Calls

• Load data files: coder.load('data.mat') in the constructor — baked into the binary
• Guard non-codegen calls: wrap fprintf/disp/plot in if isempty(coder.target)

Validation

validateAudioPlugin -nomex ClassName   % structural + testbench
validateAudioPlugin ClassName          % full MEX codegen
generateAudioPlugin ClassName          % produce VST/AU binary

After validation passes:

1. Run checkcode on all produced .m files — resolve every warning by renaming or restructuring, not by adding %#ok suppressions (except %#ok<MCSUP> in set methods). Re-run checkcode to confirm zero warnings remain.
2. Verify functional behavior: instantiate, setSampleRate, reset, process a test signal, confirm output matches intent.

Generation

Default output is VST 2. Use flags to select other formats:

When the user requests an output format unavailable on the current platform (e.g., AU on Windows, or any compiled binary on Linux), explain the constraint and suggest the closest alternative (-juceproject on Linux, -vst/-vst3 on Windows instead of AU).

References

• references/available-functions.md — Built-in streaming DSP objects (prefer over manual implementations)
• references/advanced-patterns.md — AsyncBuffer, modulated delay, save/load, codegen edge cases
• references/parameters.md — Mapping laws, multi-bus I/O, grid layout constraints
• references/grid-layout.md — Grid layout syntax, row allocation with DisplayNameLocation
• references/deep-learning.md — Neural network inference with coder.loadDeepLearningNetwork

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