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AudioChannelManipulation

Manipulating audio channels with ffmpeg

stereo → mono stream

stereo to mono diagram

Mix a single stereo stream down to a mono stream. Both channels of the stereo stream will be downmixed into the stream:

ffmpeg -i stereo.flac -ac 1 mono.flac

Note: Any out of phase stereo will cancel out.

stereo → 2 × mono files

stereo to 2 mono outputs diagram

Output each channel in stereo input to individual mono files:

ffmpeg -i stereo.wav -map_channel 0.0.0 left.wav -map_channel 0.0.1 right.wav

or with the pan audio filer:

ffmpeg -i stereo.wav -filter_complex "[0:0]pan=1c|c0=c0[left];[0:0]pan=1c|c0=c1[right]" -map "[left]" left.wav -map "[right]" right.wav

stereo → 2 × mono streams

stereo to 2 mono streams diagram

Output each channel in stereo input to individual mono streams in one output file with the channelsplit audio filter:

ffmpeg -i in.mp3 -filter_complex channelsplit=channel_layout=stereo out.mka

Note: Your player will likely play the first stream by default unless your player allows you to select the desired stream.

mono → stereo

mono to stereo diagram

Create a stereo output from one mono input:

ffmpeg -i input.mp3 -ac 2 output.m4a

or with the amerge audio filter:

ffmpeg -i input.mp3 -filter_complex "[0:a][0:a]amerge=inputs=2[aout]" -map "[aout]" output.m4a

Note: These examples will not magically create a "true" stereo output from the mono input, but simply place the same audio into both the left and right channels of the output (both channels will be identical).

2 × mono → stereo

2 mono files to stereo diagram

Create a stereo output from two mono inputs with the amerge audio filter:

ffmpeg -i left.mp3 -i right.mp3 -filter_complex "[0:a][1:a]amerge=inputs=2[aout]" -map "[aout]" output.mka

6 × mono → 5.1

6 mono inputs to 5.1 output

Combine 6 mono inputs into one 5.1 (6 channel) output with the amerge audio filter:

ffmpeg -i front_left.wav -i front_right.wav -i front_center.wav -i lfe.wav -i back_left.wav -i back_right.wav \
-filter_complex "[0:a][1:a][2:a][3:a][4:a][5:a]amerge=inputs=6[aout]" -map "[aout]" output.wav

All inputs must have the same sample rate and format. If inputs do not have the same duration the output will stop with the shortest.

5.1 → 6 × mono

5.1 to individual channels

Split a 5.1 channel input into individual per-channel files:

ffmpeg -i in.wav \
-filter_complex "channelsplit=channel_layout=5.1[FL][FR][FC][LFE][BL][BR]" \
-map "[FL]" front_left.wav \
-map "[FR]" front_right.wav \
-map "[FC]" front_center.wav \
-map "[LFE]" lfe.wav \
-map "[BL]" back_left.wav \
-map "[BR]" back_right.wav

5.1 → stereo

5.1 to stereo diagram

To downmix you can simply use -ac 2:

ffmpeg -i 6channels.wav -ac 2 stereo.wav

Notes:

  • By default when using -ac 2 the LFE channel is omitted. See "Digital Audio Compression Standard (Document A/52:2012)", sections 6.1.12 and 7.8 for more downmixing info.
  • ffmpeg integrates a default down-mix (and up-mix) system that should be preferred (the -ac option) over the pan filter unless you have very specific needs.

If you want to map specific channels and drop the rest you can use the pan audio filter. This will map the FL (Front Left) of the input to the FL of the output, and the FR (Front Right) of the input to the FR of the output:

ffmpeg -i 6channels.wav -af "pan=stereo|c0=FL|c1=FR" stereo.wav

You can also map specific channels by number. This example will map the first and third channels of the input to the first and second channels of the output.

ffmpeg -i 6channels.wav -af "pan=stereo|c0=c0|c1=c2" output.wav

If the = in a channel specification is replaced by <, then the gains for that specification will be renormalized so that the total is 1, thus avoiding clipping noise. See the pan audio filter documentation for additional information and examples.

2 × stereo → stereo

2 stereo inputs to 1 stereo output diagram

Combine two stereo inputs into one stereo output with the amerge and pan audio filters:

ffmpeg -i input1.wav -i input2.wav -filter_complex "[0:a][1:a]amerge=inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3[aout]" -map "[aout]" output.mp3

Or use -ac 2 instead of the pan audio filter:

ffmpeg -i input1.wav -i input2.wav -filter_complex "[0:a][1:a]amerge=inputs=2[aout]" -map "[aout]" -ac 2 output.mp3

Note: The output produced with the pan audio filter may not be identical to the output produced with -ac 2, so you'll have to listen to your outputs or view audio statistics to determine which output suits you.

2 stereo inputs to 1 stereo output diagram, alt

A similar situation as above, but instead use the left and right channels from the first input to make the left channel out the output, and use the left and right channels of the second input to make the right channel of the output.

Just change the channel specifications in the pan filter:

ffmpeg -i input1.wav -i input2.wav -filter_complex "[0:a][1:a]amerge=inputs=2,pan=stereo|c0<c0+c1|c1<c2+c3[aout]" -map "[aout]" output.mp3

The pan audio filter has to be used in this situation instead of -ac 2 unlike the previous example.

Mix both stereo channels to stereo

stereo to stereo mix diagram

The left and right channels of the output will each contain both the left and right channels of the input:

ffmpeg -i input.mp3 -af "pan=stereo|c0<c0+c1|c1<c0+c1" output.ogg

Switch stereo channels

switch stereo channels diagram

Switch left channel to right and right channel to left:

ffmpeg -i stereo.ogg -map_channel 0.0.1 -map_channel 0.0.0 output.wav

or with the pan audio filer:

ffmpeg -i stereo.ogg -af pan=stereo|c0=c1|c1=c0 output.wav

Virtual Binaural Acoustics

FFmpeg can produce virtual binaural acoustics files using sofalizer filter, most known channel layouts are supported for input, output is always stereo.

ffmpeg -i input.wav -af sofalizer=/path/to/sofa/file output.flac

SOFA files can be found on http://sofacoustics.org/data/database/ari/


Mute a channel

mute a stereo channel diagram

This example will mute the first channel (the left channel) but keep the second channel as is:

ffmpeg -i stereo.wav -map_channel -1 -map_channel 0.0.1 output.wav

Statistics

The astats audio filter can display information including length, DC offset, min/max levels, peak/RMS level dB:

$ ffmpeg -i input.wav -af astats -f null -
…
[Parsed_astats_0 @ 0x168a260] Channel: 1
[Parsed_astats_0 @ 0x168a260] DC offset: -0.001829
[Parsed_astats_0 @ 0x168a260] Min level: -0.605072
[Parsed_astats_0 @ 0x168a260] Max level: 0.607056
[Parsed_astats_0 @ 0x168a260] Peak level dB: -4.335430
[Parsed_astats_0 @ 0x168a260] RMS level dB: -20.298984
[Parsed_astats_0 @ 0x168a260] RMS peak dB: -12.303891
[Parsed_astats_0 @ 0x168a260] RMS trough dB: -35.352893
[Parsed_astats_0 @ 0x168a260] Crest factor: 6.283154
[Parsed_astats_0 @ 0x168a260] Flat factor: 0.000000
[Parsed_astats_0 @ 0x168a260] Peak count: 2
[Parsed_astats_0 @ 0x168a260] Channel: 2
[Parsed_astats_0 @ 0x168a260] DC offset: -0.001826
[Parsed_astats_0 @ 0x168a260] Min level: -0.585999
[Parsed_astats_0 @ 0x168a260] Max level: 0.608490
[Parsed_astats_0 @ 0x168a260] Peak level dB: -4.314931
[Parsed_astats_0 @ 0x168a260] RMS level dB: -20.519969
[Parsed_astats_0 @ 0x168a260] RMS peak dB: -12.056472
[Parsed_astats_0 @ 0x168a260] RMS trough dB: -36.784681
[Parsed_astats_0 @ 0x168a260] Crest factor: 6.460288
[Parsed_astats_0 @ 0x168a260] Flat factor: 0.000000
[Parsed_astats_0 @ 0x168a260] Peak count: 2
[Parsed_astats_0 @ 0x168a260] Overall
[Parsed_astats_0 @ 0x168a260] DC offset: -0.001829
[Parsed_astats_0 @ 0x168a260] Min level: -0.605072
[Parsed_astats_0 @ 0x168a260] Max level: 0.608490
[Parsed_astats_0 @ 0x168a260] Peak level dB: -4.314931
[Parsed_astats_0 @ 0x168a260] RMS level dB: -20.408071
[Parsed_astats_0 @ 0x168a260] RMS peak dB: -12.056472
[Parsed_astats_0 @ 0x168a260] RMS trough dB: -36.784681
[Parsed_astats_0 @ 0x168a260] Flat factor: 0.000000
[Parsed_astats_0 @ 0x168a260] Peak count: 2.000000
[Parsed_astats_0 @ 0x168a260] Number of samples: 1440706

Layouts

Output from ffmpeg -layouts:

Individual channels:
NAME        DESCRIPTION
FL          front left
FR          front right
FC          front center
LFE         low frequency
BL          back left
BR          back right
FLC         front left-of-center
FRC         front right-of-center
BC          back center
SL          side left
SR          side right
TC          top center
TFL         top front left
TFC         top front center
TFR         top front right
TBL         top back left
TBC         top back center
TBR         top back right
DL          downmix left
DR          downmix right
WL          wide left
WR          wide right
SDL         surround direct left
SDR         surround direct right
LFE2        low frequency 2

Standard channel layouts:
NAME        DECOMPOSITION
mono        FC
stereo      FL+FR
2.1         FL+FR+LFE
3.0         FL+FR+FC
3.0(back)   FL+FR+BC
4.0         FL+FR+FC+BC
quad        FL+FR+BL+BR
quad(side)  FL+FR+SL+SR
3.1         FL+FR+FC+LFE
5.0         FL+FR+FC+BL+BR
5.0(side)   FL+FR+FC+SL+SR
4.1         FL+FR+FC+LFE+BC
5.1         FL+FR+FC+LFE+BL+BR
5.1(side)   FL+FR+FC+LFE+SL+SR
6.0         FL+FR+FC+BC+SL+SR
6.0(front)  FL+FR+FLC+FRC+SL+SR
hexagonal   FL+FR+FC+BL+BR+BC
6.1         FL+FR+FC+LFE+BC+SL+SR
6.1         FL+FR+FC+LFE+BL+BR+BC
6.1(front)  FL+FR+LFE+FLC+FRC+SL+SR
7.0         FL+FR+FC+BL+BR+SL+SR
7.0(front)  FL+FR+FC+FLC+FRC+SL+SR
7.1         FL+FR+FC+LFE+BL+BR+SL+SR
7.1(wide)   FL+FR+FC+LFE+BL+BR+FLC+FRC
7.1(wide-side)FL+FR+FC+LFE+FLC+FRC+SL+SR
octagonal   FL+FR+FC+BL+BR+BC+SL+SR
downmix     DL+DR
Last modified 10 months ago Last modified on Dec 19, 2015, 12:09:06 PM

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