Understanding AudioParams: Precision control of web audio nodes

Understanding AudioParams

This post is part of the series Make Noise with Web Audio API and originally appeared on Sonoport Tech Blog.


Today I would like to go into detail about AudioParams, which are a super neat and important part of web audio. We have seen them before, such as when we set the frequency value of a filter or the amplitude of a GainNode. All AudioParams have methods that we can use to manipulate their values. Using these methods, we can set envelopes and create more finite control structures of our audio signal using automation.

Let’s first look at an example.

Say I want to add an envelope to a square wave:

window.AudioContext = window.AudioContext || window.webkitAudioContext;

var audioContext = new AudioContext();
var osc = audioContext.createOscillator();
var currentTime = audioContext.currentTime;

osc.type = 'square';
osc.gain.value = 0.2;
osc.frequency.value = 0;

osc.connect(audioContext.destination)

osc.start(0);
osc.stop(4);

I do not advise you to run this any of this code as is, as your browser will run it upon page load. If you do, the sound will appear immediately, and if you didn’t set your gain properly, you’re gonna have a bad time.

I cannot stress this enough. When dealing with your audio signal ALWAYS use gain nodes and never set them above 1.0. ALWAYS. Forever. I blasted my ears the other day running this code and you know what?

I had a bad time.

Instead, use the buttons at the bottom of the page. There is also a link to a repository where you can download the js file and a jsfiddle so you can have fun tinkering inside of your own browser!


Above, we have set up our AudioContext, assigned it the the variable audioContext, as well as created an Oscillator and assigned it to the variable osc, and assigned our audioContext.currentTime to the variable currentTime. We then initialize our oscillator’s frequency value, osc.frequency.value at 0. The .value part of osc.frequency is our AudioParam!

Finally, we connect our oscillator to our audio output osc.connect(audioContext.destination)

Now, we will use the AudioParam methods setValueAtTime & exponentialRampToValueAtTime.

osc.frequency.setValueAtTime(440, currentTime)
osc.frequency.exponentialRampToValueAtTime(5000, currentTime + 1.0);

osc.frequency.exponentialRampToValueAtTime(200, currentTIme + 2.0)

So, let’s analyze what we have just written.

The setValueAtTime AudioParam method will assign a specific numerical value at a specific point in time relative to the AudioContext.currentTime. The exponentialRampToValueAtTime AudioParam method will move a value exponentially to a certain number at a specified time relative to our initial setValueAtTime value.

I know that was a mouthful! But stay with me. It will (hopefully) all become clear in the code examples below.

The setValueAtTime simply sets an AudioParam value (the first argument) to be assigned at a specific startTime (the second argument). The AudioParam will be set to the assigned value a the assigned startTime. exponentialRampToValueAtTime ramps to a value (the first argument) exponentially until the specified endTime argument (the second argument). We also can use linearRampToValueAtTime, which has the same functionality except it operates linearly and not exponentially.

We cannot use the exponentialRampToValueAtTime method without first using the setValueAtTime method. The latter assigns a starting point that the former must use as a reference for it’s own beginning.


Another AudioParam method is setValueCurveAtTime. This method gives us another form of AudioParam automation. It accepts in array of values, and automates these values based on a beginning time argument and a duration time argument.

An example :

var waveArray = new Float32Array(5);
waveArray[0] = 200;
waveArray[1] = 700;
waveArray[2] = 2000;
waveArray[3] = 100;
waveArray[4] = 440;

osc.frequency.setValueCurveAtTime(waveArray, currentTime + 4, 4);

The above code will automate the frequency between these set values evenly, beginning after 4 seconds and lasting for 4 seconds. Add this code to our existing AudioParam example, and we should have something like this.

window.AudioContext = window.AudioContext || window.webkitAudioContext;

var audioContext = new AudioContext();

var globalGain = audioContext.createGain();

var currentTime = audioContext.currentTime;
var osc = audioContext.createOscillator();
console.log(osc);

osc.type = 'square';
osc.frequency.value = 0;

// Attack
osc.frequency.setValueAtTime(440, currentTime);
osc.frequency.exponentialRampToValueAtTime(5000, currentTime + 2);

// Decay
// osc.frequency.setValueAtTime(5000, currentTime + 3)
osc.frequency.exponentialRampToValueAtTime(200, currentTime + 3);

var waveArray = new Float32Array(5);
waveArray[0] = 200;
waveArray[1] = 700;
waveArray[2] = 2000;
waveArray[3] = 100;
waveArray[4] = 440;

osc.frequency.setValueCurveAtTime(waveArray, currentTime + 4, 4);

osc.connect(globalGain);
globalGain.connect(audioContext.destination);

osc.start(currentTime);
osc.stop(10);

The final two AudioParam methods are setTargetAtTime and cancelScheduledValues.

setTargetAtTime takes three arguments. The first is the target, the value that will be transitioned to. The second is the startTime, the time relative to AudioContext.currentTime, the transition will begin. The third is the timeConstant, or the duration of the transition.

So, if we write our code like this:

target.setValueAtTime(440, currentTime);

target.frequency.setTargetAtTime(880, currentTime + 1, 0.2);

Here, we are targeting the frequency value of an oscillator. First, use the setValueAtTime method to a frequency of 440 at the currentTime. This will let the setTargetAtTime method know at what value to start from. If you do not use setValueAtTime before any other AudioParam method, you will get errors and bugs and you will have a bad time.

Try the buttons to hear how each method sounds different, and see if you can follow along with the code examples above.

JSFIDDLE


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Thomas Roberson

Thomas Roberson

Design Associate at Sonoport
Thomas is an American guitarist, electronic musician, sound designer, and audio engineer. Born in New Orleans, Louisiana, he has roots from a deep cultural and musical heritage. He has a solid understanding of sound synthesis and digital signal processing, as well as knowledge of several audio-programming languages including Max/MSP, Pure Data, and Supercollider. He has collaborated with choreographers, film-makers, and has assisted in the creation of sound art installations.
Thomas Roberson

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