0FFmpeg LogoThe Ultimate Codec Guide

How data is stored in a multimedia file

Files on a computer (or on an optical disk like DVD or BluRay) need to contain more than 1 type of data.  A typical movie will include at least 1 video “stream” and one audio “stream”.  Most movies include multiple languages and subtitles, each of which requires an additional stream.  Each of these streams is effectively a file in its own right, but they are all stored together within a “container” which also starts each of them at the right time (subtitles don’t start immediately at the start of a movie for example) and keeps them in sync as well as storing meta-data about each of them – i.e. what language they are, what framerate and resolution the video is, and what compression standards have been used.

Video streams

Video streams contain only the video part of the movie.  They are compressed using a particular system and there should be meta-data about their resolution, framerate, if they are interlaced or progressive and details of the encoding system used.

Audio streams

Audio streams contain only the audio part of the movie.  There are usually multiple audio streams included with the movie and each has its own stream.  They can be compressed in a range of ways or in some cases they are uncompressed raw audio.  There should be meta-data about their bitrate, resolution, language, number of channels and details of the encoding system used.

Other streams (e.g. Closed Captions)

Many movies contain other streams, most commonly closed captions (also known as subtitles).  These come in several formats, but are typically just text with time stamps.  They are so small relatively that no compression is used.  There should be meta-data about their language.

Containers which bundle all the streams together

All of the streams that comprise the movie are bundled together and kept in sync by a container.  The container should contain (and provide) all the meta-data about each stream.  The container also enables the collection of streams to be stored as a single file.  The most common container formats are VOB (used by DVD) and M2TS (used by BluRay).  Other common container formats on PC are TS, MPEG, AVI, MKV, WMV, WTV, DVR-MS and MP4.  The obsolete HD-DVD format used EVO containers.

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