The MP4 format – Parts, containers and extensions
When we talk about an .mp4 file or the MP4 format in general, what we are talking about is what is technically known as MPEG-4 Part 14. It is a standard format (ISO / IEC 14496-14) and it is a container format for multimedia tracks. In other words, this format defines how audio and video tracks (called data streams) can be contained in the file in various formats, and can even contain subtitles as well.
Within this container format, within the .mp4 file, the audio and video tracks can be encoded in various formats, as appropriate for the application to be given. Although in theory it supports many different formats (almost any) for these audio and video tracks, in practice the players of this format support only a few specific types, the most frequent being:
Audio: AAC (Advanced Audio Codec, which when they are loose are files with extensions .m4a or .3gp), or the MP3 format.
Video: the different variants of the MPEG format.
MPEG or Moving Picture Experts Group is a group of “authorities” and manufacturers in the field of audio and video that came together at the request of ISO in the late 1980s to create file encoding standards for this type of multimedia information, and thus guaranteeing compatibility between the media to be played and the playback devices. The first version of the standard, MPEG-1 came out in 1993, and since then there have been many new versions, and within these what they call “Parts”, which are concrete aspects of the standard and also extensions to the specification for concrete things, or improvements to the base format that they modify.
The most widespread version of this MPEG format is 4, or MPEG-4, which appeared in late 1998, and is what we know as MP4, due to the extension of its files. This version is divided into several sub-standards or “Parts” that describe certain issues of the format (for example, 14 the container, as I said at the beginning) and certain extensions.
Within the parts of the standard, part 10 describes an advanced encoding format that is what we also know as H.264, but that is actually also called MPEG-4 Part 10 and that is what Blue-Ray disks used , for example. That is why on many Internet sites they talk about H.264 and MPEG-4 being the same. And it is true, but not entirely, since in MPEG-4 parts 2, 12 or 14 also describe other compression formats that are MPEG-4, and the container format can also contain MPEG in previous versions of lower quality as MPEG -1 or MPEG-2.MPEG-4 Part 10 or H.264 or AVC format
Let’s recap what we know so far: when we talk about MP4 or .mp4 files nowadays, or about the “MP4 format” we are really talking about the container format. Within this one there can be audio and video in various formats, so speaking only of MP4 is not correct (or it is not precise at least) and it is necessary to specify more. Given that the format used to compress the video stream that contains the MP4 is of vital importance, the “surname” of said format is usually put to indicate well what we are referring to.
Thus, MP4 / H.264 is referred to for videos that use MPEG-4 part 10 and that are highly compatible and high-quality videos. It is also common to talk about the MP4 / MPEG-4 format, which in this case usually refers to Part 2 of MPEG-4, which is the original format that was released, and is of lower quality, so it is usually use to broadcast IP television (video surveillance) or distribute certain multimedia content (some “pirate” movies, multimedia CDs …) as it generally weighs less.
The MP4 / H.264 format is the one that must be used to play directly in browsers, since it is the one that all of them support today in all operating systems. It is much more efficient than MPEG-4 Part 2 (the original) when encoding, and offers much better quality even if we use a small bit rate or a reduced bandwidth when reproducing it. It resists transmission errors very well even if some packages are lost, so it works very well for streaming, video conferencing and similar applications. This format is also known as AVC (Advanced Video Coding), so sometimes you can read MP4 / AVC, but it is the same.
As a counterpart, the H.264 format is more complex and more complicated to encode (it may require up to 3 times more computing power than the original MPEG-4 format) and to decode (twice the power), but today with the devices that the average user has (both computers and mobiles, etc …) this is of little importance.
