Add AAC track to MP4 file

[whoisthebaron]

Hi all

I am considering a purchase of an iPad, however I'm concerned that my library of MP4 files are all created with only the AC3 codec (which I believe the iPad does not natively support).

Is there therefore a simple way to add a an AAC audio track (by converting/copying the AC3 track that already exists) without touching the video?

I've searched high and low for a Handbrake-oriented solution but my understanding is that there is no video passthrough support in Handbrake and other solutions I've tried (MP4box, FFmpeg) don't appear to compile correctly on my Windows 7 64-bit OS.

I am somewhat of a noob when it comes to this so any help much appreciated!

Cheers

[transporterfan]

XMedia Recode perhaps. Video to copy, audio to AAC, full batch processing. Should do the job.
Portable version available in the tools.

[whoisthebaron]

Thanks for the quick reply, I'll take a look at that application

As it happens I've just got FFmpeg to work using an experimental aac encoder and the output seems ok, however it seems far too easy! I've not tried it on an iPad yet but VLC Player will open and play the file ok and recognises the audio as an AAC codec. Is what I have done viable??

[transporterfan]

Try it and see. We don't know the flavor of the original encode. Hard to say off the bat.
XMedia has presets if you need to recode. VLC will play anything so it's not a good test program; other than to show the file itself is viable.

[whoisthebaron]

Thanks for the feedback - I'll report back once I've had a chance to test

[whoisthebaron]

Hi

Thought I'd drop some feedback in case anyone stumbles across this with a similar query.

Didn't end up using xMedia - I managed to get this working satisfactorily using ffmpeg.

The way to do it is to use ffmpeg to create a new mp4 file which copies the video, copies the AC3 track and then creates a new AAC track. Increases the file size slightly but I only wanted a 2ch stereo AAC track for playing on the iPad so not a massive impact (50-100Mb)

It's also important to make sure that you set the AAC track as the primary audio track, otherwise iTunes won't import it (as it seems to fail as soon as it picks up the AC3 track in its analysis). I was concerned this would impact my playback on other devices (I use a R-Pi driven XBMC distribution to play on my HDTV) but this was unfounded as the XBMC player can be configured to default to an AC3 codec which will give you the best of both worlds - surround sound in your living room, and stereo through earphones for your iPad.

The CLI command text required to "convert" your file (ie make a new version with the tracks you need) is as follows:

ffmpeg32 
-i "Filename.mp4" /
-map 0:0 -map 0:1 -map 0:1 /
-c:v copy / 
-c:a:0 aac -b:a 128k -ac 2 -strict -2 -cutoff 15000 / 
-c:a:1 copy /	
"Filename 2.mp4"

I understand there are other aac codec libraries available with ffmpeg but this was the most straightforward fix for me.