Container format

A container format is a type of file format that stores video, audio, metadata, and synchronization and error correction information following a pre-established format in its technical specification.

Some multimedia containers are: AVI, MPG, MOV (QuickTime Container), ASF (WMV and WMA Container), Ogg, OGM, RMVB and Matroska.

The video and audio tracks are usually compressed, being different codecs, used within each one of the containers, those in charge of decompressing the information for the sake of its reproduction.

When a container is created, the tracks are first encoded and then "joined" (multiplexed) following a typical pattern for each format.

When a file needs to be played, it first acts as a splitter, which knows the pattern of the container, and "separates" (demultiplexes) the audio and video tracks. Once separated, each one of them is interpreted by the decoder and reproduced.

In those containers with more than one track, it is the player (that is, the user) who selects the one to play.

It is therefore essential that the player has the necessary decoders to reproduce both the video and the audio, since otherwise the information cannot be interpreted correctly.

In short, not only do you need to know the container format to be able to separate the tracks, but you also need to be able to decode them.

In theory, a container format could wrap any kind of data. Although there are few examples of these types of file formats (one example being Windows Dynamic Link Libraries), most containers exist for certain groups of data. This is due to the specific needs of the desired information. Most relevant is the family of wrappers, found in multimedia file formats, where audio and/or video can be encoded with hundreds of algorithms from different alternatives that are stored in a smaller number of file formats. In this case, the algorithm or algorithms used to store the data is actually called a codec.

The main differences between some container formats and others are:

• Popularity. How much format is supported.
• Overhead. The size difference of two files with the same content when they are in different formats.
• Support for advanced codec functions. If they are old formats they do not support more recent features.
• Support for advanced content. For example, subtitles in videos.
• Support for medium streaming.