Why won't iTunes play demux / split audio that VLC will?

[sapphire]

I've succeeded in demuxing the audio from an MP4 video and producing an audio file that plays in VLC. My problem is that iTunes 10.7 won't play the same audio file. I gave the file an .mp3 extension and used Get Info to ensure iTunes is the default application for opening it. But when I double-click the file, nothing happens.

Here's the demux procedure I used in VLC:

1. File --> Convert/Stream

2. Selected MP4 input video file

3. Choose Profile --> Customize: Encapsulation = MP4/MOV; Video codec: No selection; Audio codec: Keep original audio track

4. Choose destination: xxxx.mp3

As I said, VLC plays the resulting audio file just fine, but iTunes won't.

I'm wondering if my iTunes problem has to do with my encapsulation choice. I would think MP3 (or maybe AAC) would be best, but those aren't options, and I'm unsure that the native audio format is in an MP4 anyway.

[kdean]

Native audio format for an mp4 is usually an AAC so naming it .m4a instead of .mp3 may make a difference.

You can also try dropping the original file on iMediaHud to see what it's exact specs are. (https://www.macupdate.com/app/mac/40255/imediahud)

You can get a small bit of that information while playing the video by viewing the Media Information Window (Command I as in Information). Switch to Codec Details and click the triangles for the different streams.

[sapphire]

Native audio format for an mp4 is usually an AAC so naming it .m4a instead of .mp3 may make a difference.

That did the trick -- iTunes plays the file fine now. Thanks!

You can get a small bit of that information while playing the video by viewing the Media Information Window (Command I as in Information). Switch to Codec Details and click the triangles for the different streams.

Right, and it shows the codec is AAC. Thanks again.

Here's a side-note: I discovered that QuickTime Player can also demux the audio, using File --> Export. It offers no options (codec or bit rate) and gives me a 10.4-MB 256-kbps AAC with an .mp4 extension. The file VLC produces using "Keep original audio track" is a 3.8-MB 96-kbps AAC. The source video file is 25.8 MB.

If I use VLC's Media Information --> Statistics to monitor the bitrate while the video plays, I see approx. 600 - 800 kbps typically (that's video + audio combined). So I suspect the 96-kbps file I get from VLC with the "Keep original" etc. option is the "true" audio quality, while QuickTime Player is up-sampling.

Do you suppose anything is gained by up-sampling (which I could do with VLC, as well)? I don't see how that could add audio info that isn't present in the original, and both versions sound equally mediocre to me, but perhaps there's a benefit I'm unaware of.

[kdean]

96 kbps is low to begin with but up-sampling will gain you nothing.

As you can see Quicktime is not actually demuxing since that would normally give you the original audio data. It's simply encoding it to a new format. Best to not re-encode an already low quality track since it could make it sound worse.