AudioChannelManipulation

Manipulating audio channels with ffmpeg

Contents

1. stereo → mono stream
2. stereo → 2 × mono files
3. stereo → 2 × mono streams
4. mono → stereo
5. 2 × mono → stereo
6. 6 × mono → 5.1
7. 5.1 → 6 × mono
8. 5.1 → stereo
9. 2 × stereo → stereo
10. Mix both stereo channels to stereo
11. Switch stereo channels
12. Virtual Binaural Acoustics
13. Mute a channel
14. Statistics
15. Layouts

stereo → mono stream

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

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

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

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

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

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

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

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

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.

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

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 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/

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Mute a channel

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

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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

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Layouts

Output from ffmpeg -layouts:

Individual channels:NAME        DESCRIPTIONFL          front leftFR          front rightFC          front centerLFE         low frequencyBL          back leftBR          back rightFLC         front left-of-centerFRC         front right-of-centerBC          back centerSL          side leftSR          side rightTC          top centerTFL         top front leftTFC         top front centerTFR         top front rightTBL         top back leftTBC         top back centerTBR         top back rightDL          downmix leftDR          downmix rightWL          wide leftWR          wide rightSDL         surround direct leftSDR         surround direct rightLFE2        low frequency 2Standard channel layouts:NAME           DECOMPOSITIONmono           FCstereo         FL+FR2.1            FL+FR+LFE3.0            FL+FR+FC3.0(back)      FL+FR+BC4.0            FL+FR+FC+BCquad           FL+FR+BL+BRquad(side)     FL+FR+SL+SR3.1            FL+FR+FC+LFE5.0            FL+FR+FC+BL+BR5.0(side)      FL+FR+FC+SL+SR4.1            FL+FR+FC+LFE+BC5.1            FL+FR+FC+LFE+BL+BR5.1(side)      FL+FR+FC+LFE+SL+SR6.0            FL+FR+FC+BC+SL+SR6.0(front)     FL+FR+FLC+FRC+SL+SRhexagonal      FL+FR+FC+BL+BR+BC6.1            FL+FR+FC+LFE+BC+SL+SR6.1(back)      FL+FR+FC+LFE+BL+BR+BC6.1(front)     FL+FR+LFE+FLC+FRC+SL+SR7.0            FL+FR+FC+BL+BR+SL+SR7.0(front)     FL+FR+FC+FLC+FRC+SL+SR7.1            FL+FR+FC+LFE+BL+BR+SL+SR7.1(wide)      FL+FR+FC+LFE+BL+BR+FLC+FRC7.1(wide-side) FL+FR+FC+LFE+FLC+FRC+SL+SRoctagonal      FL+FR+FC+BL+BR+BC+SL+SRhexadecagonal  FL+FR+FC+BL+BR+BC+SL+SR+TFL+TFC+TFR+TBL+TBC+TBR+WL+WRdownmix        DL+DR