Combine left and right channel audio in MP4

[stevet]

Hi. I just recorded a song for a church service tomorrow. I hear it fine, but some folks say they only hear one side. As it turns out, I'm feeding audio into the computer with a Focusrite 2i2 Scarlett interface, and it has two channels, so I feed guitar into one and mike into the other. BUT, then I have guitar in right channel and mic in left, and I guess some listeners are not listening in stereo. Is there a way to combine left and right into mono? I don't have any expertise with a video editor, just using Windows Camera and Windows Photo on my PC. I tried exporting the audio to Audacity and that works GREAT on the audio, I just can't put it back into the MP4.

[johns0]

Use MP4Box with gui to remux the audio into the mp4.

[stevet]

Thanks for the tip. So I downloaded MP4Box gui, and started playing with it. Here's what I did:

I demuxed the original MP4 that I recorded into an AAC audio file and an Advanced H264 video track. Here's what that produced. Interesting that the .aac file 2 is 3:45 now when in the container it was 3:30


[Attachment 52506 - Click to enlarge]

Then I remuxed the H264 video track with the .wav file (that was originally extracted from the same file I got the video from, then sweetened and converted to mono). , the resulting muxed file had only video, no audio and is only 2:44 in length.

So, then I took the video file and muxed it with an mp3 of the same audio file. The original MP4, the extracted audio mp3, and the extracted audio wav are all 3:30.
The muxed files with the mp3 has 3:30 of audio, so the audio is right, but it only has 2:44, of video, and the video freezes at that point while the audio plays on till 3:30.

So if I could figure out how to get the length of the video to be 3:30 and fine tune it to get it in sync this would fix my problem.

Any ideas?

[davexnet]

Not sure what is going on with MP4Box, here's another method.
Install Avidemux and open the original file.
On the left keep video as "Copy" and set "Audio Output" to AAC(FDK), filters Remix/mono

lower left - choose your output file type (defaults to mkv) if you want mp4
hit the button and choose mp4 muxer
File/save

[hello_hello]

I demuxed the original MP4 that I recorded into an AAC audio file and an Advanced H264 video track. Here's what that produced. Interesting that the .aac file 2 is 3:45 now when in the container it was 3:30

There's no duration info for raw AAC, so programs usually look at the bitrate and take a guess. The reported duration should go back to what it was originally if you put the AAC back in a container.So if I could figure out how to get the length of the video to be 3:30 and fine tune it to get it in sync this would fix my problem. You may need to specify the type of AAC to keep Mp4Box happy. I'm not sure but it'd be something like, LC or SBR/HE.

Any ideas?

I can't speak with much MP4 authority as most MP4 muxers suck and blow at the same time, so I'm an MKVToolNixGUI and MKV guy. If you're not married to MP4 maybe give it a try. It'll open MP4s and convert the container misery to MKV.
Keeping in mind I know almost nothing about MP4, I tend not to extract streams unless I need to. Obviously to convert the audio to mono it has to be extracted, but for out-putting MKVs I extract only the streams I have to extract, open the source file with MKVToolNixGUI, deselect the streams in the source I don't want to keep, add the new stream and start muxing. The reason for doing it that way, is it should keep any stream delays and not mess with the AV sync, and it won't double the length of the video stream as it appears to have done going by your screenshot, and you probably won't need to worry about the AAC audio type. Of course that might be impossible for MP4 muxers. I don't know.

Keeping in mind I know nothing about determining MP4 stream delays, how did you extract them? Most extraction programs write any delay info to the file name when extracting, so the delay can be re-applied when muxing. There may not be delays, but there often is.

This is how the file name of an audio stream is written after extracting it from an MKV with gMKVExtractGUI.

Episode 1_track2_[eng]_DELAY 0ms.ac3

[VenusAndMars]

Not sure what is going on with MP4Box, here's another method.
Install Avidemux and open the original file.
On the left keep video as "Copy" and set "Audio Output" to AAC(FDK), filters Remix/mono

lower left - choose your output file type (defaults to mkv) if you want mp4
hit the button and choose mp4 muxer
File/save

Now, this is the way to do it. Excellent, easy-to-follow instrux; beautiful .