How to change audio channels without re-encoding?

[Boxcart]

I'm recording video with a Blackmagic card and the input has 5.1 audio. Apparently it's a longstanding flaw of Blackmagic cards that the center channel is #4 and the LFE is #3, when it is supposed to be the other way around. The result is my video has quiet dialogue since the center (dialogue heavy) channel is being interpreted as the LFE. How can I flip these two channels to their proper positions without re-encoding?

I've heard ffmpeg can do it with a specific command, but have not been able to find anything for my situation in particular.

Thanks.

[jack_666]

Have a read

https://trac.ffmpeg.org/wiki/AudioChannelManipulation

[Boxcart]

Have a read

https://trac.ffmpeg.org/wiki/AudioChannelManipulation

Thanks - I read through that but couldn't find an option for my specific situation. It doesn't seem like that one is described there.

[ProWo]

From the link above:

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

So your command is

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

[Boxcart]

From the link above:

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

I see. So if I have it correctly I would do "ffmpeg -i input.wav -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" output.wav" ?

[ProWo]

I see. So if I have it correctly I would do "ffmpeg -i input.wav -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" output.wav" ?

Yes

[Boxcart]

I see. So if I have it correctly I would do "ffmpeg -i input.wav -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" output.wav" ?

Yes

Great thanks I'll give this a try. And I'd assume this would be lossless given the lack of encoding following this process?

[ProWo]

Yes, it's lossless.

[Boxcart]

Yes, it's lossless.

Alright I was able to get it to work to fix the channels however it does not appear lossless. The input audio was 6,912 kb/s, 48 kHZ, 24 bits, 6 channel, and the output is 4,608 kb/s, 48 kHZ, 16bits, 6 channels. I used the exact command above. This is from MediaInfo.

[poisondeathray]

Is the original big or little endian?

-c:a pcm_s24be => 24bit big endian
-c:a pcm_s24le => 24bit little endian

[Boxcart]

Is the original big or little endian?

-c:a pcm_s24be => 24bit big endian
-c:a pcm_s24le => 24bit little endian

Little

[poisondeathray]

Add to the command line before the output file:

-c:a pcm_s24le

[Boxcart]

Add to the command line before the output file:

-c:a pcm_s24le

Thanks, it's now processing losslessly but failing with the error "Filesize 6045185208 invalid for wav, output file will be broken." The output file sounds great and MediaInfo confirms lossless/same but there's only 80 minutes out of a 2 hour input mkv.

[poisondeathray]

what ffmpeg version are you using ? Try git version or a very recent version

[Boxcart]

what ffmpeg version are you using ? Try git version or a very recent version

I tried with the latest 5.1.2 version from VideoHelp and a version from github, same problem.

[poisondeathray]

I don't know, might be a bug, you'd have to file a bug report , if it's not already reported

The search isn't great on their bug reporting site, but you'd check synonyms and related phrases
https://trac.ffmpeg.org/timeline

You'd need the full console report , you can add -report to the command line to write it to a text file

5.1.2 is considered "ancient" by developers, you have to check the git head, so today's version, or depending on where you are, that's "expired" already . They won't accept a bug report if your ffmpeg build is even a day old

[Cornucopia]

WAV file format is 32bit, so has a 4GB limit. You file is 6+GB. You must save as another format, such as W64 (the 64bit variant of wav) or something else, like FLAC.

Scott

[Boxcart]

WAV file format is 32bit, so has a 4GB limit. You file is 6+GB. You must save as another format, such as W64 (the 64bit variant of wav) or something else, like FLAC.

Scott

Thanks, I just tried FLAC using:

"ffmpeg -i C:\Users\Admin\Videos\AUDIOFIX.mkv -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le output.flac"

and it failed with the error: "[flac @ 000002515ff589c0] Invalid audio stream. Exactly one FLAC audio stream is required.
[out#0/flac @ 000002515ff588c0] Could not write header (incorrect codec parameters ?): Invalid argument
[vost#0:1/png @ 000002515fee4f00] Error initializing output stream:"

I tried W64 (by changing output extension from .wav to .w64 in the code) and it processed the output file which has the right size and correct properties in MediaInfo, but I can't get it to play in anything.

[poisondeathray]

If source is in a mkv container, did you try putting it in a mkv container ? If there is video as well, you can use -c:v copy

ffmpeg -i C:\Users\Tom\Videos\AUDIOFIX.mkv -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le output.mkv"

[Boxcart]

If source is in a mkv container, did you try putting it in a mkv container ? If there is video as well, you can use -c:v copy

ffmpeg -i C:\Users\Admin\Videos\AUDIOFIX.mkv -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le output.mkv"

I attempted it but cancelled it since it's running much, much slower that way. Assuming it works I could let it do it and then swap the audio in the mkv's?

[poisondeathray]

Why swap the audio later in a 2nd step? It would take even longer. You should be able to swap it in the same command at the same time. Or what are the details of the "video" or your situation ?

Use a different source and destination drives

[Boxcart]

Why swap the audio later in a 2nd step? It would take even longer. You should be able to swap it in the same command at the same time. Or what are the details of the "video" or your situation ?

Use a different source and destination drives

I'm not in front of the PC right now but I will give that a try. Will this code provided above accomplish that?

ffmpeg -i C:\Users\Admin\Videos\AUDIOFIX.mkv -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le output.mkv"

Thanks!

[poisondeathray]

I'm not in front of the PC right now but I will give that a try. Will this code provided above accomplish that?

If source is in a mkv container, did you try putting it in a mkv container ? If there is video as well, you can use -c:v copy

If "AUDIOFIX.mkv" has the video stream as well, add -c:v copy to copy the video stream at the same time . Otherwise you need to use 2 inputs and -map to specify which streams

To specify different destination drive, you can set the path of output.mkv

ffmpeg -i C:\Users\Tom\Videos\AUDIOFIX.mkv -c:v copy -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le "D:\someotherpath\output.mkv"

[Boxcart]

I'm not in front of the PC right now but I will give that a try. Will this code provided above accomplish that?

If source is in a mkv container, did you try putting it in a mkv container ? If there is video as well, you can use -c:v copy

If "AUDIOFIX.mkv" has the video stream as well, add -c:v copy to copy the video stream at the same time . Otherwise you need to use 2 inputs and -map to specify which streams

To specify different destination drive, you can set the path of output.mkv

ffmpeg -i C:\Users\Admin\Videos\AUDIOFIX.mkv -c:v copy -filter_complex "channelmap=map=FL-FL|FR-FR|FC-LFE|LFE-FC|BL-BL|BR-BR:channel_layout=5.1" -c:a pcm_s24le "D:\someotherpath\output.mkv"

The video stream are identical. I'll give it a shot, thanks again

[ProWo]

Yes, it's lossless.

Alright I was able to get it to work to fix the channels however it does not appear lossless. The input audio was 6,912 kb/s, 48 kHZ, 24 bits, 6 channel, and the output is 4,608 kb/s, 48 kHZ, 16bits, 6 channels. I used the exact command above. This is from MediaInfo.

Do you really need 24 bit audio?

[poisondeathray]

I'm guessing the "much slower" was because you had video, and it was re-encoding the video because you didn't specify a video codec for -c:v (it defaults to libx264)

-c:v copy means copy the video stream. Stream copy should be very fast
-vn means discard the video stream - so it would be audio only in the mkv container if you wanted to do something in another step

[Boxcart]

Yes, it's lossless.

Alright I was able to get it to work to fix the channels however it does not appear lossless. The input audio was 6,912 kb/s, 48 kHZ, 24 bits, 6 channel, and the output is 4,608 kb/s, 48 kHZ, 16bits, 6 channels. I used the exact command above. This is from MediaInfo.

Do you really need 24 bit audio?

Yes, since that's the output I'm capturing and I'm going for lossless.

[Boxcart]

All set, that latest idea of doing the whole mkv worked. Thanks!