What’s the difference between… MP3, MP4 and M4A?

QI want to sort my messy collection of digital movies and music. I’ve gathered files from all over the place and I’ve never given them names. Windows Media Player seems to play them all.

However, using Windows Explorer to tidy things up, I noticed that some of my audio files have an a .m4a extension while others are .mp3. Oddly, some of my audio files have a .mp4 extension, which I thought was for video. What’s the difference between all these file types? Charlie Bolton

AWe’ll start with MP4. MP is short for MPEG, which in turn is short for Moving Picture Experts Group — the name of the industry consortium that introduced the technology in 2001. MPEG-4 is its full name, and it’s just one of many standards defined by the consortium (at http://mpeg.chiariglione.org, see screenshot).

People often think of MP4 as a videofile format but this isn’t strictly true. In fact, MP4 is a ‘container’ that carries digital video or audio or, most commonly, a combination of the two. But note that we said video OR audio – meaning an MP4 file may contain only audio. It can also be a variety of other things, as we’ll explain.

In an effort to distinguish audio from video files, Apple came up with the .m4a file extension – so some of your digital-audio files probably originated from Apple’s itunes Store or an ipod. Other MP4 file-extension variants include .m4r for ringtones, .m4b for audiobooks and .m4p for audio files protected by digital-rightsmanagement ( DRM) encryption. However, not every company adopted this standard, so some audio-only MP4 files have an .mp4 file extension.

That leaves us with MP3 and the .mp3 file extension. This was also introduced by MPEG, in 1993, but a tad confusingly is a subset of the earlier MPEG-1/2 standards. Its full technical name is MPEG-1 or MPEG-2 Audio Layer III. In common use this was abbreviated to MP3, and .mp3 for the file extension – and it’s an audioonly format.