A container format (informally, sometimes called a wrapper) or metafile is a file format that allows multiple data streams to be embedded into a single computer file, file, usually along with metadata for identifying and further detailing those streams. Notable examples of container formats include archive files (such as the ZIP (file format), ZIP format) and formats used for multimedia playback (such as Matroska, MPEG-4 Part 14, MP4, and Audio Video Interleave, AVI). Among the earliest cross-platform container formats were Distinguished Encoding Rules and the 1985 Interchange File Format.

Design

Although containers may identify how data or metadata is encoded, they do not actually provide instructions about how to decode that data. A Computer program, program that can open a container must also use an appropriate codec to decode its contents. If the program doesn't have the required algorithm, it can't use the contained data. In these cases, programs usually emit an error message that complains of a missing codec, which users may be able to acquire. Container formats can be made to wrap any kind of data. Though there are some examples of such file formats (e.g. Microsoft Windows's Dynamic-link library, DLL files), most container formats are specialized for specific data requirements. For example, since Audio coding format, audio and Video coding format, video streams can be coded and decoded with many different algorithms, a container format may be used to provide the appearance of a single file format to users of multimedia playback software.

Considerations

The differences between various container formats arise from five main issues: # Popularity; how widely supported a container is. # Overhead. This is the difference in file-size between two files with the same content in a different container. # Support for advanced codec functionality. Older formats such as AVI do not support new codec features like B-frames, Variable bitrate, VBR audio or Variable frame rate, VFR video natively. The format may be "hacked" to add support, but this creates compatibility problems. # Support for advanced content, such as chapters, subtitles, meta-tags, user-data. # Support of streaming media.

Single coding formats

In addition to pure container formats, which specify ''only'' the wrapper but not the coding, a number of file formats specify ''both'' a storage layer and the coding, as part of modular design and forward compatibility. Examples include JPEG File Interchange Format (JFIF) for containing JPEG data, and Portable Network Graphics (PNG). In principle, coding can be changed while the storage layer is retained; for example, Multiple-image Network Graphics (MNG) uses the PNG container format but provides animation, while JPEG Network Graphics (JNG) puts JPEG encoded data in a PNG container; in both cases however, the different formats have different Magic number (programming), magic numbers – the format specifies the coding, though a MNG can contain both PNG-encoded images and JPEG-encoded images.

Multimedia container formats

The container file is used to identify and interleave different data types. Simpler container formats can contain different types of audio formats, while more advanced container formats can support multiple audio and video streams, Subtitle (captioning), subtitles, chapter-information, and meta-data (Tag (metadata), tags) — along with the synchronization information needed to play back the various streams together. In most cases, the file header, most of the metadata and the synchro chunks are specified by the container format. For example, container formats exist for optimized, low-quality, internet video streaming which differs from high-quality Blu-ray streaming requirements. Container format parts have various names: "chunks" as in RIFF and PNG, "atoms" in QuickTime/MP4, "packets" in MPEG-TS (from the communications term), and "segments" in JPEG. The main content of a chunk is called the "data" or "payload". Most container formats have chunks in sequence, each with a header, while TIFF instead stores offsets. Modular chunks make it easy to recover other chunks in case of file corruption or dropped frames or bit slip, while offsets result in Serial communications, framing errors in cases of bit slip. Some containers are exclusive to audio: * Audio Interchange File Format, AIFF (Interchange File Format, IFF file format, widely used on Mac OS platform) * WAV (Resource Interchange File Format, RIFF file format, widely used on Microsoft Windows, Windows platform) * Extensible Music Format (XMF), XMF (Extensible Music Format) Other containers are exclusive to still images: * FITS (Flexible Image Transport System) still images, raw data, and associated metadata. * Tagged Image File Format, TIFF (Tagged Image File Format) still images and associated metadata. * Macintosh PICT resource (PICT), superseded by Portable Document Format, PDF in Mac OS X * Windows Metafile (WMF) = (EMF) Enhanced Metafile * Encapsulated PostScript (EPS) * Computer Graphics Metafile (CGM) * Portable Document Format (PDF) * Corel Draw File (CDR) * Scalable Vector Graphics (SVG) * Rich Text Format file (RTF) Other flexible containers can hold many types of audio and video, as well as other media. The most popular multi-media containers are: * 3GP (used by many mobile phones; based on the ISO base media file format) * Advanced Systems Format, ASF (container for Microsoft Windows Media Audio, WMA and Windows Media Video, WMV, which today usually do not use a container) * Audio Video Interleave, AVI (the standard Microsoft Windows container, also based on Resource Interchange File Format, RIFF) * DVR-MS ("Microsoft Digital Video Recording", Proprietary software, proprietary video container format developed by Microsoft based on Advanced Systems Format, ASF) * Flash Video (FLV, F4V) (container for video and audio from Adobe Systems) * Interchange File Format, IFF (first platform-independent container format) * Matroska (MKV) (not limited to any coding format, as it can hold virtually anything; it is an open standard container format) * MJ2 - Motion JPEG 2000 file format, based on the ISO base media file format which is defined in MPEG-4 Part 12 and JPEG 2000 Part 12 * QuickTime File Format (standard QuickTime video container from Apple Inc.) * MPEG program stream (standard container for MPEG-1 and MPEG-2 elementary streams on reasonably reliable media such as disks; used also on DVD-Video discs) * MPEG transport stream, MPEG-2 transport stream (a.k.a. MPEG-TS) (standard container for digital broadcasting and for transportation over unreliable media; used also on Blu-ray Disc video; typically contains multiple video and audio streams, and an electronic program guide) * MPEG-4 Part 14, MP4 (standard audio and video container for the MPEG-4 multimedia portfolio, based on the ISO base media file format defined in MPEG-4 Part 12 and JPEG 2000 Part 12) which in turn was based on the QuickTime file format. * Ogg (standard container for Xiph.Org Foundation, Xiph.org audio formats Vorbis and Opus (audio format), Opus and video format Theora) * RealMedia, RM (RealMedia; standard container for RealVideo and RealAudio) * WebM (subset of Matroska, used for web-based media distribution on online platforms; container for royalty-free audio formats Vorbis/Opus (audio format), Opus and video formats VP8/VP9/AV1) There are many other container formats, such as FFmpeg#NUT, NUT, Material Exchange Format, MXF, General Exchange Format, GXF, ratDVD, SVI, VOB and DivX Media Format

References

External links