Web Audio API split mono to stereo

Question

Lets say I have a source node that's connected to the destination node. Even if the audio is mono, I want to be able to control each ear's volume independently, like I can do when I have stereo audio with splitter and merger node.

Already tried to use splitter and merger nodes on the mono source node, but right channel comes out empty.

example for stereo:

var audioCtx = new AudioContext();
var source = audioCtx.createMediaElementSource(myAudio);
var gainNodeL = audioCtx.createGain();
var gainNodeR = audioCtx.createGain();
var splitter = audioCtx.createChannelSplitter(2);
var merger = audioCtx.createChannelMerger(2);

source.connect(splitter);
splitter.connect(gainNodeL, 0);
splitter.connect(gainNodeR, 1);

gainNodeL.connect(merger, 0, 0);
gainNodeR.connect(merger, 0, 1);

merger.connect(audioCtx.createMediaStreamDestination());

When I do this with mono audio, the right channel comes out empty.

Please add a minimal reproducible example.

user247702
– user247702

2019-03-04 15:10:22 +00:00
Commented Mar 4, 2019 at 15:10 — user247702
– user247702, Commented Mar 4, 2019 at 15:10
Added an example

Alon Dayan
– Alon Dayan

2019-03-04 18:07:38 +00:00
Commented Mar 4, 2019 at 18:07 — Alon Dayan
– Alon Dayan, Commented Mar 4, 2019 at 18:07

chrisguttandin · Accepted Answer · 2019-03-04 19:15:14Z

4

If a signal is only mono (or in other words its channelCount is 1) the ChannelSplitterNode is not necessary. I modified the example a bit. It does now split the mono signal of an Oscillator.

var audioCtx = new AudioContext();
var oscillator = audioCtx.createOscillator();
var gainNodeL = audioCtx.createGain();
var gainNodeR = audioCtx.createGain();
var merger = audioCtx.createChannelMerger(2);

oscillator.connect(gainNodeL);
oscillator.connect(gainNodeR);

gainNodeL.connect(merger, 0, 0);
gainNodeR.connect(merger, 0, 1);

merger.connect(audioCtx.destination);

oscillator.start();

function left () {
    gainNodeL.gain.value = 1;
    gainNodeR.gain.value = 0;
}

function right () {
    gainNodeL.gain.value = 0;
    gainNodeR.gain.value = 1;
}

function center () {
    gainNodeL.gain.value = 1;
    gainNodeR.gain.value = 1;
}

answered Mar 4, 2019 at 19:15

chrisguttandin

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2 Comments

Alon Dayan Over a year ago

It does work with oscillator node, but source node cant connect to 2 nodes, it throws an exception

chrisguttandin Over a year ago

Hi @AlonDayan, which error do you get? Replacing the oscillator with a MediaElementSourceNode works for me in Firefox and Chrome. In theory it should be okay to connect a node to as many other nodes as you want.

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