====== ffmpeg - Audio - Audio Channels - Change Audio Channels - 5.1 to Stereo ======

{{:ffmpeg:audio:audio_channels:audio_channels_-_5.1_to_stereo.png?200|}}

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<WRAP important>
**WARNING:**  The resulting stereo mix will often have a much weaker volume than playing the 5.1 track natively.

  * This is because of how audio mixing works.
  * You cannot expect six sources of audio to sound as loud as two sources because there is simply not enough dynamic headroom. 
  * Just turn up the volume. 

</WRAP>

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===== Use ffmpeg built-in downmix =====

<code bash>
ffmpeg -i input.mp4 -c:v copy -ac 2 stereo_output.mp4
</code>

<WRAP info>
**NOTE:**  The **-ac 2** switch works by mixing proportions of the first 5 channels from the sources 6-channel stream - Back Left, Back Right, Front Left, Front Right and Front Center - into the Front Left and Front Right channels of the output stereo stream:

  * **-ac 2** tells ffmpeg to downmix the audio to stereo.
    *  It will then choose a reasonable default audio codec for a .mp4 file.


<WRAP important>
**WARNING:**  When using this option, audio from the LFE channel (the .1 in 5.1, reserved for the subwoofer and used for deep, low-frequency effects) is discarded completely when using this option.
</WRAP>


</WRAP>

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===== Using a custom mix algorithm for the stereo mix =====

This example takes a combination of various channels for the stereo mix.

<code bash>
ffmpeg -i "input.mkv" -c:v copy -af '-af "pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR"' -ac 2 -c:s: copy stereo_output2.mp4

# Downmix a 5.1 track to stereo and increase its volume level to 425 (where 256 is 100% of the original source volume level).
ffmpeg -i "input.mkv" -c dca -vol 425 -af "pan=stereo|c0=0.5*c2+0.707*c0+0.707*c4+0.5*c3|c1=0.5*c2+0.707*c1+0.707*c5+0.5*c3" "outputstereo.dts"
</code>

<WRAP info>
**NOTE:**  Maybe mix left/center/right into stereo, and throw out the back end subwoofer channels.
</WRAP>
 

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===== Map specific channels for the stereo mix, and drop the rest =====

This example takes only the front left and front right channel for the stereo mix.

<code bash>
ffmpeg -i "input.mkv" -c:v copy -af 'pan=stereo|c0=FL|c1=FR' -ac 2 -c:s: copy stereo_output2.mp4
</code>

<WRAP info>
**NOTE:**  This is probably a very bad way to downmix multi channel audio.

  * It completely discards all channels except for the first 2.
  * For most 5.1 audio source, most dialogue will be on the 3rd channel, FC. 
  * This discards that.

</WRAP>

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===== Map specific channels by number for the stereo mix, and drop the rest =====

This example will map the first and third channels of the input to the first and second channels of the output. 

<code bash>
ffmpeg -i "input.mkv" -af "pan=stereo|c0=c0|c1=c2" output.mkv
</code>

<WRAP info>
**NOTE:**  This is probably a very bad way to downmix multi channel audio.

  * It completely discards all channels except for the first 2.
  * For most 5.1 audio source, most dialogue will be on the 3rd channel, FC. 
  * This discards that.

</WRAP>



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===== References =====

https://trac.ffmpeg.org/wiki/AudioChannelManipulation#a5.1stereo