Dictionary

Some terms used in the area of Audio/Video and their meaning.

Aspect Ratio

The ratio of width to height of a video.

Bitrate

The number of bits of data to be played per second.

Chroma

Greek for color, the part of a video file or signal that encodes the color portion.

Codec

A part of the program which understands a type of video or audio (short for Compression/Decompression). DivX and Theora are examples of video codecs; MP3 and Vorbis are audio codecs. The output stream produced when a codec to audio or video is generally "muxed" into a container format, such as AVI or Ogg. As certain codecs are often associated with certain container formats, the name of the container is often used to imply the codec, such as "Ogg", which usually refers to a Vorbis stream in an Ogg container.

Decode

To understand and play a file, VideoLAN needs to decode it. It does this with a decoder. See codec.

Deinterlacing

Deinterlacing is the process of converting interlaced video (a sequence of fields) into a non-interlaced form (a sequence of frames). This is a fundamentally impossible process that must always produce some image degradation, since it ideally requires "temporal interpolation" which involves guessing the movement of every object in the image and applying motion correction to every object.

ES

An Elementry Stream, a single channel of audio, video or subtitles (without a container).

Framerate

The number of frames of video displayed (or encoded to be displayed) per unit time, usually expressed in frames per second (fps) or Hertz (Hz). One Hertz is equivilant to one frame per second.

Keyframe

A frame of video which is stored as a complete image, not just as the changes from the previous image.

Motion Compensation

Part of the video compression process. New frames normally store changes in the image since the previous frame. If the scene is moving as a whole (such as panning), motion compensation moves the reference frame to line up with the new frame. This means that there are less changes to be stored since the previous frame, and so less data needs to be stored.

Overlay

Displaying an image on top of the video

PS

Abbreviation for Program Stream.

Sample rate

Usually used with audio, the frequency at which a signal is digitally sampled, usually expressed in Hertz (Hz) or kiloHertz (kHz). One Hertz is equivilant to one sample per second, One kiloHertz is a thousand (1000) Hertz.

Transcode

Transcoding is changing the format of a file. This can be for the purpose of changing the audio or video's bitrate, codec, or other attributes, to reduce disk usage or for compatibility with a certain program/device.

It is important to note that transcoding can be highly detrimental to quality when dealing with lossy codecs, particularly for video. This is because the second time a stream is encoded lossily the codec has less information to work with, causing it to produce a cruder approximation of the original. As with many other quality issues, this problem can be worked around by increasing the bitrate, though some quality loss (as well as possible re-encoding of the previous codec's artifacts) will inevitably occur.

Also note that, provided both container formats support the codec, transcoding is not necessary to switch container formats. For example, an XviD video stream in an AVI file can be losslessly remuxed into an Ogg file.

TS

Abbreviation for Transport Stream, as in MPEG TS.