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class:: DetectSilence
summary:: Detect when input falls below an amplitude threshold
categories:: UGens>Synth control, UGens>Analysis>Amplitude
Description::
When the absolute value of the input signal remains below the threshold for a given window of time, output 1. Otherwise, output 0. If the output transitions from 0 to 1, doneAction is also evaluated.
code::
// this frees after the Decay has become quiet enough for a long enough time
{ var signal = Decay.ar(Impulse.ar(0), 2, PinkNoise.ar(0.2)); DetectSilence.ar(signal, doneAction: Done.freeSelf); signal }.play;
::
If the input signal starts with silence at the beginning of the synth's duration, then DetectSilence will wait indefinitely until the first sound before starting to monitor for silence. To avoid a hanging silent sound where the input may remain zero, you can add an code::Impulse.ar(0):: to its input.
DetectSilence does not distinguish a DC-biased signal from a loud signal. If your signal has DC bias, you should wrap it in link::Classes/LeakDC::.
warning:: DetectSilence can be tricky with multi-channel input! See below. ::
classmethods::
method::ar, kr
argument::in
The input signal.
argument::amp
When input falls below this for long enough, evaluate
code::doneAction:: .
argument::time
The minimum duration for which input must fall below
code::amp:: before this triggers.
argument::doneAction
An integer representing the doneAction. See
link::Classes/Done:: for more detail.
returns::
This UGen outputs 1 if silence is detected, otherwise 0.
Examples::
code::
(
SynthDef("detectSilence-help", { arg out;
var z;
z = SinOsc.ar(Rand(400, 700), 0, LFDNoise3.kr(8).max(0)).softclip * 0.3;
DetectSilence.ar(z, doneAction: Done.freeSelf);
Out.ar(out, z);
}).add;
)
Synth("detectSilence-help");
Synth("detectSilence-help");
Synth("detectSilence-help");
(
Task({
loop({
Synth("detectSilence-help");
[0.5, 1].choose.wait;
})
}).play;
)
::
section::DetectSilence and multiple inputs
Be careful feeding multiple inputs into DetectSilence, since multichannel expansion can lead to confusing behavior. This code
code::DetectSilence.ar([left, right] + Impulse.ar(0), doneAction: 2)::
spawns strong::two:: DetectSilences, and the enclosing Synth frees itself when left OR right becomes silent. (The code::Impulse.ar(0):: is there to ensure that DetectSilence starts detecting silence right away rather than waiting for non-silence.)
Usually you want to detect silence for both inputs, so what's a good way to change that OR into an AND? The solution is strong::not:: to sum code::(left + right) / 2.sqrt::, because there may be phase cancellation in that sum. Instead, use DetectSilence's output instead of its doneAction, and use FreeSelf to do the freeing. Recall that DetectSilence outputs 1 if silence is detected, and 0 otherwise. Taking the product of multiple DetectSilences will return 1 only if all the DetectSilences are 1. So here is the correct solution to freeing a Synth when both code::left:: and code::right:: first fall silent:
code::FreeSelf.kr(DetectSilence.ar([left, right] + Impulse.ar(0)).product)::
It's also possible to mix the signals with full-wave rectification to avert phase cancellation issues (code::[left, right].abs.sum / 2.sqrt::).
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