so, I've been reading a lot about downmixing 5.1 to stereo, but as documentation provides, it creates L and R channels, so is the LFE channel being included down the mix? Is it completely lost? I've tried using -ac 2.1, but there's no option for that. It actually returns Expected int64 for ac but found 2.1. Sorry if this sounds like a newb question, but I can't seem to find a dead end answer to this.

  • I would expect the LFE channel to be mixed in to both the left and right channels, but either at a -3dB level or -10dB level. FYI -3dB is half the power, so the sum of two halves equals the original signal. -10 dB is half as loud.
    – sawdust
    Apr 11, 2018 at 20:38
  • @sawdust Nope, according to ATSC guidelines, it's skipped.
    – slhck
    Apr 12, 2018 at 12:45

1 Answer 1


Yes. See the FFmpeg Wiki. By default the LFE channel is omitted when using -ac 2.

You can of course create your own mixes if you want to keep the LFE channel.

  • Ah, so that's what it meant, it was kind of confusing to me, thanks! But what if I want to keep LFE since I have a 2.1 sound system? I tried typing -ac 2.1 but it's of no use.
    – José
    Apr 13, 2018 at 14:02
  • Then you have to create separate channel mapping. The ac option cannot be used for that. I don't know from the top of my head how that works though, have to get back to you.
    – slhck
    Apr 13, 2018 at 14:16

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