Splitting, speeding up M4A audio file on a Mac

June 13, 2015 8:35 PM

Two part question: I have an M4A audio file that’s about three hours and 90 MB, and would like to split it into three tracks at specific time stamps. What’s the best or simplest way to do this on a Mac without re-encoding? There’s a bunch of stuff that Google returns, but I can’t tell what’s good and what’s not. After that, I’d like to increase the tempo of one file. Any recommendations for tools? Command line or GUI are fine. Thanks.
posted by jroybal to Computers & Internet (5 answers total)

Best answer: ffmpeg is the swiss army knife of (command line) media file tools. See this blog post for an example of how to use it to split a file.
posted by axiom at 9:10 PM on June 13, 2015

Best answer: Oh, and for the second part, you want the atempo command line argument to ffmpeg. I'm not terribly familiar with it but you can see a discussion here. Basically you want something like (to increase speed by 10%):

ffmpeg -i input.m4a -filter:a "atempo=1.1" -c:a libfaac -q:a 100 output.m4a

posted by axiom at 9:15 PM on June 13, 2015

I've relied on Audio Splitter for the splitting part, at least, for years. Its website doesn't say whether it supports M4A, and I can't remember whether I've tried it with that format, but other sites suggest that it does. There's a freeware version you can test it with.
posted by limeonaire at 9:34 PM on June 13, 2015

I use Fission from Rogue Amoeba to split and do quick tasks with audio. I'm pretty sure it does M4A files.

https://www.rogueamoeba.com/fission/
posted by pzarquon at 1:09 AM on June 14, 2015

To be 100% clear -- ffmpeg can definitely split without re-encoding but it seems likely to me that the "atempo" filter will decode and then re-encode. Changing tempo without changing pitch is complicated enough that I would doubt there is any software that can do it directly on an encoded audio file -- and if it did, I don't know if you'd get the best results.
posted by goingonit at 12:53 PM on June 14, 2015