Demystifying the MP4 container format

7 June, 2021

Blog

By Alexander Nordstr m

In previous installments of this blog series, we've told you all about how our solutions help make sense of video quality and work collaboratively to improve it. This time around, we shift focus away from ourselves. Instead, we'll share some general knowledge about one of the technologies we help make more observable: the MP4 container format.

What is MP4? MP4 is a container format that allows video, audio and other related information to be bundled together in a single file. Among other things, it's commonly used in streaming video services.

Is it the same as MPEG-4? Yes and no (mostly no). The name MP4 comes from the MPEG-4 standard, but the container format is just one part of the MPEG-4 standard. (Part 14 to be precise.)

The MPEG-4 standard also contains many other parts, including specifications for how to encode and decode the video part of the video - the moving pictures - before combining it with audio using MP4 or another container. This codec (specified in Part 10) is called AVC or H.264 but is often just referred to as MPEG-4 for optimal confusion.

Just as the MPEG-4 AVC encoded video can be encapsulated in a different container than MP4 (such as MPEG-2 Transport Stream, Matroska, or Ogg), the MP4 container can encapsulate video content encoded with other codecs - perhaps most notably HEVC, also known as H.265.

The rest of this text is about MP4, the container format specified in MPEG-4 Part 14, not the video codec defined in MPEG-4 Part 10. Confused yet? No? Good!

Where is it used and why? MP4 files can be used for storing or streaming complete audiovisual content in a single file. Because it can easily be divided up into smaller pieces, it is widely used in streaming video services based on adaptive bitrate technologies such as Smooth Streaming, MPEG-DASH and HLS.

Apple's HLS technology entered the game early and initially used MPEG-2 Transport Streams as the container format. This is the format used in IPTV, cable, terrestrial and satellite distribution. It works well but has some unnecessary overhead to make room for channel lineup information and other things that just aren't applicable to streaming videos. Over time, HLS has evolved to also embrace MP4, but Transport Streams are still widely used.

Microsoft's Smooth Streaming technology was based around MP4 from the start. It can use Transport Streams, but this is rarely if ever seen. MPEG-DASH, in an attempt to create one standard to encompass them all, also supports both MP4 and MPEG-2 Transport Streams, but MP4 has always been the de facto favorite.

History - from QuickTime to an extension of ISO BMFF The MP4 container format is largely based on the QuickTime file format. If that name sounds familiar, you may be as old as I am. If you're not, kids, let me tell you a story about what Internet video was like in the mid-nineties.

A short, potato-quality clip would take hours or days to download (provided nobody picked up the phone in the meantime - don't ask), and then you had to download a video player supporting the file format used - none of this magical built-in browser support. Often, that player was QuickTime, another Apple technology. And it's this player's native file format that was used as the basis for the standardized MP4 container.

Subsequently, the core parts of the MP4 container format were broken out into the ISO base media file format (ISO BMFF), with MP4 being redefined as an extension of the ISO BMFF specification.

How is it structured? Conceptually, the format is organized into a series of boxes, which may, in turn, contain other boxes. Which types of boxes exist, what information they can contain, and where in the box hierarchy they can appear is clearly defined. Each box type has a four-character identifier.

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