Manipulating audio channels

stereo → mono stream

Downmix both channels

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

Warning: Any out of phase stereo will cancel out.

The following filtergraph can be used to bring out of phase stereo in phase prior to downmixing:

-af "asplit[a],aphasemeter=video=0,ametadata=select:key=lavfi.aphasemeter.phase:value=-0.005:function=less,pan=1c|c0=c0,aresample=async=1:first_pts=0,[a]amix"

Choose a specific channel

This example uses the channelsplit audio filter to include only the right channel:

ffmpeg -i stereo.wav -filter_complex "[0:a]channelsplit=channel_layout=stereo:channels=FR[right]" -map "[right]" front_right.wav

• If you only want the left channel use FL instead of FR. See ffmpeg -layouts for a list of channel names. If you are working with a video file, you can use -map 0:0 -c:v copy to preserve the video stream.

Example using the pan audio filter:

ffmpeg -i stereo.wav -af "pan=mono|c0=c1" mono.m4a

• Alternatively, you could use c0=FR instead.
• If want the left channel instead use c0=c0 or c0=FL.

stereo → 2 × mono files

Output each channel in stereo input to individual mono files with the channelsplit audio filter:

ffmpeg -i stereo.wav -filter_complex "[0:a]channelsplit=channel_layout=stereo[left][right]" -map "[left]" left.wav -map "[right]" right.wav

or with the -map_channel option:

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

or with the pan audio filter:

ffmpeg -i stereo.wav -filter_complex "[0:a]pan=1c|c0=c0[left];[0:a]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 "[0:a]channelsplit=channel_layout=stereo" output.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[a]" -map "[a]" 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 join audio filter:

ffmpeg -i left.mp3 -i right.mp3 -filter_complex "[0:a][1:a]join=inputs=2:channel_layout=stereo[a]" -map "[a]" output.mp3

or the amerge audio filter:

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

6 × mono → 5.1

Combine 6 mono inputs into one 5.1 (6 channel) output with the join 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]join=inputs=6:channel_layout=5.1[a]" -map "[a]" output.wav

The join audio filter also allows you to manually choose the layout:

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]join=inputs=6:channel_layout=5.1:map=0.0-FL|1.0-FR|2.0-FC|3.0-LFE|4.0-BL|5.0-BR[a]" -map "[a]" output.wav

Another method using the amerge audio filter which is somewhat less flexible than the join filter shown above:

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[a]" -map "[a]" output.wav

With amerge 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 using the channelsplit audio filter:

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[a]" -map "[a]" 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[a]" -map "[a]" -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[a]" -map "[a]" 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 with the channelmap audio filter:

ffmpeg -i stereo.mp3 -filter_complex "channelmap=map=FL-FR|FR-FL:channel_layout=stereo" output.mp3

Or with the pan audio filer:

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

or with -map_channel:

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

Mute a channel

This example uses the pan audio filter to mute the first channel (front left) but keep the second channel (front right) as is:

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

Example using the -map_channel option:

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

Remap channels

The channelmap audio filter can re-arrange the channel layout. For example, to switch the FL and FR channels in a 5.1 input:

ffmpeg -i input.wav -filter_complex "channelmap=map=FL-FR|FR-FL|FC-FC|LFE-LFE|BL-BL|BR-BR:channel_layout=5.1" output.wav

Arbitrary channel reduction & mapping

The pan audio filter can be used to reduce the number of channels in a stream with an arbitrary mapping of input to output channels. For example, given a multi-channel WAV input file captured from an audio interface, to extract channels 3 and 4 into a stereo output any of the following commands would work:

ffmpeg -i in.wav -af 'pan=2c|c0=c2|c1=c3' out.wav
ffmpeg -i in.wav -filter 'pan=2c|c0=c2|c1=c3' out.wav
ffmpeg -i in.wav -filter_complex '[0:a] pan=2c|c0=c2|c1=c3' out.wav

Here, 2c specifies that the output should have two channels, and c0=c2 and c1=c3 define the mapping of input to output channels. The channel numbers start at zero, thus we define (zero-based) output channels 0 and 1 to come from input channels 2 and 3, respectively.

The -filter_complex version is only necessary when more than one filter is employed.

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Virtual Binaural Acoustics

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

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

SOFA files can be found at ​sofacoustics.org.

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Multiple channel layouts

This example combines two layouts (5.1+2.0) into a single arbitrary layout using the pan audio filter:

ffmpeg -i input -filter_complex "pan=5.1+downmix|FL=c0|FR=c1|FC=c2|LFE=c3|BL=c4|BR=c5|DL=c6|DR=c7[a]" -map "[a]" out.wav

The output in the example resulted in one audio stream with 8 channels.

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Merged and mapped audio channels

This is born from screencasting, where you might want to record an interview over teleconference and still be able to isolate the audio streams to better deal with background noise later in post-processing.

This example outputs a video with three audio options: the merged speaker+microphone streams, the speaker stream alone, and the microphone stream alone.

x11grab provides stream 0, and is video only.

stream 1 is the pulse "audio monitor", or what the speakers are playing.

stream 2 is the pulseaudio default audio input device, meant to be a microphone, and in this case, stereo.

"[a]" is the generated stream from merging stream 1 and stream 2, using amerge and then pan to retain stereo channels instead of 4.0 channels as seen above in "2 × stereo → stereo".

These streams are then mapped in the order desired.

#for "-f pulse -i 0", 0 is the pulseaudio id 
#of the audio that is going to the speakers 
#as determined from "pacmd list-sources"
ffmpeg -video_size 1920x1080 -framerate 60 \
  -f x11grab -i :0.0 \
  -f pulse -i 0 \
  -f pulse -i default \
  -filter_complex "[1:a][2:a] amerge=inputs=2,pan=stereo|c0<c0+c2|c1<c1+c3[a]" \
  -map 0 \
  -map "[a]" \
  -map 1 \
  -map 2 \
  -c:v libx264 -crf 17 -preset ultrafast  \
  output_filename.mkv

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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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List channel names and standard channel 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
TSL            top side left
TSR            top side right
BFC            bottom front center
BFL            bottom front left
BFR            bottom front right

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(back)      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
7.1(top)       FL+FR+FC+LFE+BL+BR+TFL+TFR
octagonal      FL+FR+FC+BL+BR+BC+SL+SR
cube           FL+FR+BL+BR+TFL+TFR+TBL+TBR
hexadecagonal  FL+FR+FC+BL+BR+BC+SL+SR+TFL+TFC+TFR+TBL+TBC+TBR+WL+WR
downmix        DL+DR
22.2           FL+FR+FC+LFE+BL+BR+FLC+FRC+BC+SL+SR+TC+TFL+TFC+TFR+TBL+TBC+TBR+LFE2+TSL+TSR+BFC+BFL+BFR