11

Arduino programming: Sound, envelopes, and interrupts

SCHOOL OF MAKING

if (trigger) {

playSound(261);

} else {

stopSound();

}}

If you now run all the code we’ve just written, you

should find that your Arduino generates a tone at

pitch equivalent to a middle ‘C’ on a piano keyboard.

But this is only part of the project, because a simple

tone isn’t all that exciting. To solve this, we’re going

to change the sound during playback using something

called an ‘envelope’ to modulate the playback pitch.

An audio envelope describes how much a sound

changes over time, from the moment it’s triggered

to when it’s released. Envelopes are typically used to

change the amplitude and pitch of a sound over the

duration of a note, and the most common envelope

type consists of four stages: attack, decay, sustain,

and release, also written as ADSR. Attack, decay, and

release are time durations that indicate how fast or

slowly the audio changes, whereas sustain is a level

that is held while the note is being triggered.

ENVELOPE GENERATOR

Before we start creating our own envelope, we need

to add a few global variables:

const int pitchEnv[] = {500, 250, 200};

const int pitchMax = 255;

The array is going to hold the attack, decay, and sustain

values, with the first two being durations and the final

element being level value. As we’re going to use this

envelope to vary the pitch of our sound, we’ve called

it pitchEnv, along with pitchMax to hold the maximum

value (amplitude) we want the envelope to reach on

the initial attack. Apart from its name, though, there’s

no reason why the envelope can’t be used to control

any other audio-related value to modulate the sound.

Before we write the envelope generator code itself,

we need to patch the envelope effect into our current

code. This is as simple as adding the following to the

beginning of the playSound function:

current to the pin connected to the speaker. It can all

be done with a single line, which we’ll place within its

own function:

void playSound(int pitch) {

tone (piezoPin, pitch);

}

We’ll pair the above function with another to turn the

sound off:

void stopSound(){

noTone(piezoPin);

}

All we now need to do is write the simple loop()

function to trigger either the playSound function or

the stopSound function depending on the state of the

trigger Boolean:

void loop()

{

Below

A piezo buzzer is a

handy component.

It works reasonably

well as a speaker and

can also be used as a

crude microphone

Envelopes are typically

used to change

the amplitude and

pitch of a sound over the

duration of a note

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