Digital Media Glossary

Editor's Note: The following is meant to be a helpful guide to common digital media terms, including many common encoding and container formats.  It is intended for the novice to intermediate user, and so many of the definitions are simplified and do not cover the full technical details involved.  While the information is believed to be accurate, OIT makes no guarantee as to the current accuracy of the information contained on this page.

If you do find any factual errors on this page, we would greatly appreciate it if you would let us know.


General Terminology

codec

A 'co'mpression / 'dec'compression algorithm, used to take a raw stream of audio and/or video data and make it smaller by removing elements that are deemed unnecessary, and later to take the compressed stream and restore the original version so that it can be replayed on a display and/or sound system.  Some codecs attempt to only remove elements that the average person would never miss, while others will notably reduce the image or sound quality, usually in order to make the content as small as possible for transmission over slow or low bandwidth connections.  Codec selection is usually based on what is more important: quality or size/speed of the transmission.

container format

The outer shell of a media file that organizes the stream(s) that it carries.  Most video files have one video data stream and one audio data stream, but can contain multiple audio streams (possibly in different languages, or to support special surround-sound systems), or even additional video streams (to support watching the same program from multiple angles).  The container format of a file is usually directly connected to the file extension or MIME type (e.g. Quicktime MOV, RealMedia RM, MPEG, MP4, Windows AVI, Windows WMV).

Note that a particular container format may support several different encoding formats, and no container format can handle every possible encoding format.  Thus, for example, you can have two different MOV files, one of which plays just fine on a computer, while the other fails to play, due to that computer having a codec for the encoding format of the first file, but no matching codec for the encoding format found in the second file.

encoding

The process of taking an analog audio or video wave and turning it into a stream of binary data (i.e. 1's and 0's).  Encoding is done by measuring the wave periodically and storing each measurement as a binary value, a process known as sampling.  The more frequent the measurements, the better the stored values will represent the original waveform, but this also results in more data to store and transfer.  To reduce the amount of data without losing an equal amount of quality, codecs are used to algorithmically analyze and intelligently compress the raw binary data.

encoding format

The output of the algorithm used by a codec to compress a video or audio stream.  A computer must have an appropriate codec installed to be able to create or playback an encoded stream.  The encoding format is specific to each stream and cannot be determined just by looking at the name of a file.  Instead, a media file must be examined using a player or preview tool capable of understanding that particular container format in order to determine which codec was used on each stream in the file.  Some examples of common encoding formats are H.264, WMV, Sorenson AVC, RealVideo, and DivX.

intermediate codec

A specialized, often proprietary codec used by many video editing suites to encode video and audio streams while a user is working on editing the content into a final product.  Intermediate codecs are usually not recognized by most mainstream video players and transcoders, and must be avoided when exporting your final video or audio project.  An example of an intermediate codec is the Apple ProRes codec used by Final Cut Pro.

sampling

Measuring an analog audio or video wave periodically and storing each measurement as a binary value.  See encoding for more information.

stream

A sequence of data that represents analog audio or video content.  A stream usually holds audio or video, but not both at the same time.

streaming video

To send only the portion of a stored video file that the viewer currently wants to see, rather than the viewer having to download an entire video file before being able to view it.  Streaming allows a viewer to quickly watch a small portion of a very large recording, and even jump around in that recording with minimal delays.  Streaming is also very useful for broadcasting live events, where the source is a live video camera instead of a stored video file.

Streaming can be used to help reduce unauthorized copying of video content, as it is more difficult (though not impossible) for a viewer to obtain a complete and usable copy of a streamed video file.

transcode

To convert the encoding format and/or container format of a media file, usually from a more proprietary format to a more universal format.


Common Container and Encoding Formats

AAC (Advanced Audio Coding) [Encoding, Audio]

Produced by the MPEG working group, this is the standard audio format for use in MP4 files in conjunction with H.264 video content.

DivX (Advanced Audio Coding) [Container]

Produced by the DivX, Inc. company, this container format is commonly used by programs that can rip (i.e. extract and copy) content from a DVD disc.   It usually has a H.264 or MPEG-4 video stream.

H.264 / MPEG-4 AVC [Encoding, Video]

Produced by the MPEG working group, this is currently one of the most common encoding formats, making it a good choice for universal compatibility.   However, it is covered by various patents that limit the ability to create software or hardware that can read or generate H.264 streams without permission and/or royalty payments to the MPEG-LA licensing group.

Flash Video [Container, Video]

A container format developed by Macromedia for their Flash embeddable web player (now known as Adobe Flash). FLV files traditionally contained Sorenson or VP6 encoded video, but today can contain H.264 video.

MP4 [Container]

MP4 files most commonly carry H.264 video streams with AAC audio streams.

MTS / M2TS [Container]

The Blu-ray Disc Audio-Video [BDAV] MPEG-2 Transport Stream container format. Many camcorders will produce video files in this format, which often contain H.264 encoded video.

OGG (ogg/ogv/ogx) [Container]

A free and open container format designed to support video and audio streams encoded in several free and open encoding formats, including Theora for video streams and Vorbis for audio streams.

RealMedia [Container & Encoding, Video]

A proprietary codec created by Real Networks that is primarily compatible only with their own player product, though the format can be converted to other more universal encoding standards.

Sorenson [Encoding, Video]

A proprietary codec used primarily in early QuickTime (MOV) video container files from before the mid-2000s.

WebM [Container]

A free open container format developed by Google and designed to use VP8 video encoding and Vorbis audio encoding.  VP8 was purported by Google to be free and open, but there are currently claims by Nokia and other groups that aspects of VP8 are covered by existing patents.

WMV (Windows Media Video) [Container & Encoding, Video]

A proprietary video format and file container type created by Microsoft for use primarily on Windows computers.  WMV files may be subject to Digital Rights Management controls, which usually render the files unplayable on any media player except for specific Microsoft authored players (e.g. Windows Media Player).  DRM controlled WMV files cannot be transcoded by the OIT Media Management system.