SuperCollider audio installation series: volume 1

SuperCollider audio installation series: volume 1
using an envelope to control audio bus output volume

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I am currently working on an interactive audio installation using SuperCollider that will use sensors to track people in the space. When they stand in certain areas audio will fade up or down. I wanted this process to be controlled by an envelope triggered by the presence of a person. I also wanted the ability to define a delay time before the audio was faded up or down. Here's how I did it.

If you just can't wait: Envelope to Audio Bus.scd

First we'll create an audio bus to send the output of our instrument to. This is a single channel audio bus on the default server 's'.

~envelope_bus = Bus.audio(s, 1);

Next, we'll create a basic SynthDef to play an infinite sequence of notes and PBind to use the SynthDef.

(
SynthDef(\tones,{|out=0, gate=1, freq=400|
    var sig, env;
     sig = SinOsc.ar(freq, 0, 1);
     env = EnvGen.kr(Env.asr, gate, doneAction:2);
     sig = sig*env; Out.ar(out, sig);
}).add;
)
(
Pbind(
    \instrument, \tones,
    \freq, Pseq([400, 600, 800, 740, 920, 360], inf),
    \out, ~envelope_bus
).play;
)

And here's the interesting bit. We create a SynthDef to control the output of the private audio bus to the public audio bus using a controllable envelope.

(
SynthDef(\audio_output_envelope, {|out=0, atk=4, rel=4, wait=0|
    var env;
    env = EnvGen.kr(Env.asr(atk,releaseTime: rel, curve: \sin),
        Latch.kr(\gate.kr, TDelay.kr(\trig.tr, wait)));
    Out.ar(out, \sig.ar*env);
}).add;
)

Let's break down the envelope generator itself. The generic envelope has this structure:

EnvGen.kr(envelope type, gate)

If you remember, I was looking for an option to delay the onset of the envelope change once I received a gate signal. I could not find a way to directly delay the changing of an incoming argument to a SynthDef. Instead I'm using the delay feature of the Trigger, TDelay. A trigger, however, is momentary, so if we just make the gate signal a trigger

TDelay.kr(\gate.tr, wait)

the envelope immediately closes again.  I need it to stay open or closed depending but I still want to use the delay. We can use the Latch UGen to accomplish this we just need to add an additional trigger to start the process.

Latch.kr(\gate.kr, TDelay(\trig.tr, wait)

Then we can activate the gate and let the audio through like so.

x.set(\gate, 1, \trig, 1);
x.set(\gate, 0, \trig, 1);

And set the wait times based on what we need.

x.set(\wait, 1, \gate, 0, \trig, 1);
x.set(\wait, 2, \gate, 1, \trig, 1);

A link to the full working demo SuperCollider file is available above.
I'll be adding more examples of using SuperCollider for interactive audio installations in the future.