AudioNode.channelInterpretation

The channelInterpretation property of the AudioNode interface represents an enumerated value describing the meaning of the channels. This interpretation will define how audio up-mixing and down-mixing will happen.

When the amount of channels doesn't match between an input and an output, up- or down-mixing happens according the following rules. This can be somewhat controlled by setting the AudioNode.channelInterpretation property to speakers or discrete.

Interpretation Input channels Output channels Mixing rules
speakers 1 (Mono) 2 (Stereo) Up-mix from mono to stereo.
The M input channel is used for both output channels (L and R).
output.L = input.M
output.R = input.M
1 (Mono) 4 (Quad) Up-mix from mono to quad.
The M input channel is used for non-surround output channels (L and R). Surround output channels (SL and SR) are silent.
output.L = input.M
output.R = input.M
output.SL = 0
output.SR = 0
1 (Mono) 6 (5.1) Up-mix from mono to 5.1.
The M input channel is used for the center output channel (C). All the others (L, R, LFE, SL, and SR) are silent.
output.L = 0
output.R = 0

output.C = input.M
output.LFE = 0
output.SL = 0
output.SR = 0
2 (Stereo) 1 (Mono) Down-mix from stereo to mono.
Both input channels (L and R) are equally combined to produce the unique output channel (M).
output.M = 0.5 * (input.L + input.R)
2 (Stereo) 4 (Quad) Up-mix from stereo to quad.
The L and R input channels are used for their non-surround respective output channels (L and R). Surround output channels (SL and SR) are silent.
output.L = input.L
output.R = input.R
output.SL = 0
output.SR = 0
2 (Stereo) 6 (5.1) Up-mix from stereo to 5.1.
The L and R input channels are used for their non-surround respective output channels (L and R). Surround output channels (SL and SR), as well as the center (C) and subwoofer (LFE) channels, are left silent.
output.L = input.L
output.R = input.R
output.C = 0
output.LFE = 0
output.SL = 0
output.SR = 0
4 (Quad) 1 (Mono) Down-mix from quad to mono.
All four input channels (L, R, SL, and SR) are equally combined to produce the unique output channel (M).
output.M = 0.25 * (input.L + input.R + input.SL + input.SR)
4 (Quad) 2 (Stereo) Down-mix from quad to stereo.
Both left input channels (L and SL) are equally combined to produce the unique left output channel (L). And similarly, both right input channels (R and SR) are equally combined to produce the unique right output channel (R).
output.L = 0.5 * (input.L + input.SL)
output.R = 0.5 * (input.R + input.SR)
4 (Quad) 6 (5.1) Up-mix from quad to 5.1.
The L, R, SL, and SR input channels are used for their respective output channels (L and R). Center (C) and subwoofer (LFE) channels are left silent.
output.L = input.L
output.R = input.R
output.C = 0
output.LFE = 0
output.SL = input.SL
output.SR = input.SR
6 (5.1) 1 (Mono) Down-mix from 5.1 to mono.
The left (L and SL), right (R and SR) and central channels are all mixed together. The surround channels are slightly attenuated and the regular lateral channels are power-compensated to make them count as a single channel by multiplying by √2/2. The subwoofer (LFE) channel is lost.
output.M = 0.7071 * (input.L + input.R) + input.C + 0.5 * (input.SL + input.SR)
6 (5.1) 2 (Stereo) Down-mix from 5.1 to stereo.
The central channel (C) is summed with each lateral surround channel (SL or SR) and mixed to each lateral channel. As it is mixed down to two channels, it is mixed at a lower power: in each case it is multiplied by √2/2. The subwoofer (LFE) channel is lost.
output.L = input.L + 0.7071 * (input.C + input.SL)
output.R = input.R
+ 0.7071 * (input.C + input.SR)
6 (5.1) 4 (Quad) Down-mix from 5.1 to quad.
The central (C) is mixed with the lateral non-surround channels (L and R). As it is mixed down to two channels, it is mixed at a lower power: in each case it is multiplied by √2/2. The surround channels are passed unchanged. The subwoofer (LFE) channel is lost.
output.L = input.L + 0.7071 * input.C
output.R = input.R + 0.7071 * input.C
output.SL = input.SL
output.SR = input.SR
Other, non-standard layouts Non-standard channel layouts are handled as if channelInterpretation is set to discrete.
The specification explicitly allows the future definition of new speaker layouts. This fallback is therefore not future proof as the behavior of the browsers for a specific amount of channels may change in the future.
discrete any (x) any (y) where x<y Up-mix discrete channels.
Fill each output channel with its input counterpart, that is the input channel with the same index. Channels with no corresponding input channels are left silent.
any (x) any (y) where x>y Down-mix discrete channels.
Fill each output channel with its input counterpart, that is the input channel with the same index. Input channels with no corresponding output channels are dropped.

 

Syntax

var oscillator = audioCtx.createOscillator();
oscillator.channelInterpretation = 'discrete';

Value

An enumerated value representing a channelInterpretation.

Example

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

var audioCtx = new AudioContext();

var oscillator = audioCtx.createOscillator();
var gainNode = audioCtx.createGain();

oscillator.connect(gainNode);
gainNode.connect(audioCtx.destination);

oscillator.channelInterpretation = 'discrete';

Specifications

Specification Status Comment
Web Audio API
The definition of 'channelInterpretation' in that specification.
Working Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 10.0webkit 25.0 (25.0) No support 15.0webkit
22 (unprefixed)
(Yes)
Feature Android Firefox Mobile (Gecko) Firefox OS (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support ? 26.0 1.2 ? ? ?

See also

Document Tags and Contributors

 Contributors to this page: LiuTong, fscholz, chrisdavidmills
 Last updated by: LiuTong,