Editing Lua modulation API (section)

=== Native-rate audio ===
Input audio is resampled to the Lua processing rate by default. In this mode, <code>ctx:audioN()</code> matches <code>ctx:n()</code>, <code>ctx:audioSampleRate()</code> matches <code>ctx:sampleRate()</code>, and the one-to-one helpers can be used normally.

A modulation may set <code>resample_audio = false</code> to receive audio at the source's native rate:

<syntaxhighlight lang="lua">
simtx.modulation {
    name = "Custom resampler",
    sample_rate = 240000,
    resample_audio = false,
    process = function(ctx)
        local outputCount = ctx:n()
        local outputRate = ctx:sampleRate()
        local inputCount = ctx:audioN()
        local inputRate = ctx:audioSampleRate()

        -- Read native input with ctx:audio(index).
        -- Produce outputCount samples with ctx:iq() or ctx:generate().
        -- Keep filter and FIFO state in Lua across calls.
    end,
}
</syntaxhighlight>

The input and output blocks cover the same elapsed duration, subject to sample-count rounding. Their sample counts can differ. During extreme downsampling, an output block can occasionally contain zero samples.

When the input and output rates or block sizes differ, <code>ctx:map()</code>, <code>ctx:passthrough()</code>, <code>ctx:gain()</code>, and <code>ctx:mixTone()</code> raise an error because they require one input sample per output sample. Use <code>ctx:audio()</code> to read the native block and write the resampled result with <code>ctx:iq()</code> or <code>ctx:generate()</code>. The extension is responsible for its resampling filter, FIFO, and state.

The registration fields are documented in [[Lua extension API#Modulation declaration|Lua extension API]].