2/10/2011
Common video Codecs
A video codec is software that enables video compression and or decompression for digital video. The compression usually employs lossy data compression. Historically, video was stored as an analog signal on magnetic tape. Around the time when the compact disc entered the market as a digital-format replacement for analog audio, it became feasible to also begin storing and using video in digital form and a variety of such technologies began to emerge.
Most video files have at least two different types: the container, and the codec.
* A container format is a computer file format that can contain various types of data, compressed in a manner of standardized codecs. The container describes the structure of the file: where the various pieces are stored, how they are interleaved, and which codecs are used by which pieces. It may also specify an audio codec as well as video.
* Codec, short for compressor/decompressor, allows you to store the digital music, photos, and movies in the form of compact, manageable files, which can then be opened or decompressed to deliver the original contents. The common video codecs include DivX, XviD, 3ivx, MPEG(MPEG-1,2,3,4), ReadVideo.
A video can be thought of as being made up of numerous snapshots, called frames. Frame rate is the number of still pictures per unit of time of video, and most often expressed in frames per second (fps). Motion pictures shot on film do so at 24 FPS. Both video and audio file resolution is measured in kilobytes per second (kbps) — that is, how many thousands of bytes it takes to store one second of sound or videos. This figure is known as the bitrate. A higher bitrate allows better video quality.
Detail information of common video codecs:
MPEG ("Moving Pictures Expert Group"): three video formats, MPEG 1, 2, and 4.
MPEG-1: Old, supported by everything (at least up to 352x240), reasonably efficient. A good format for the web.
MPEG-2: A souped-up version of MPEG-1, with better compression. 720x480. Used in HDTV, DVD, and SVCD.
MPEG-4: A family of codecs, some of which are open, others Microsoft proprietary.
MPEG spinoffs: mp3 (for music) and VideoCD.
MJPEG ("Motion JPEG"): A codec consisting of a stream of JPEG images. Common in video from digital cameras, and a reasonable format for editing videos, but it doesn't compress well, so it's not good for web distribution.
DV ("Digital Video"): Usually used for video grabbed via firewire off a video camera. Fixed at 720x480 @ 29.97FPS, or 720x576 @ 25 FPS. Not very highly compressed.
WMV ("Windows Media Video"): A collection of Microsoft proprietary video codecs. Since version 7, it has used a special version of MPEG4.
RM ("Real Media"): a closed codec developed by Real Networks for streaming video and audio. Maybe also a container?
DivX: in early versions, essentially an ASF (incomplete early MPEG-4) codec inside an AVI container; DivX 4 and later are a more full MPEG-4 codec.. No resolution limit. Requires more horsepower to play than mpeg1, but less than mpeg2. Hard to find mac and windows players.
Sorenson 3: Apple's proprietary codec, commonly used for distributing movie trailers (inside a quicktime container).
Quicktime 6: Apple's implementation of an MPEG4 codec.
RP9: a very efficient streaming proprietary codec from Real (not MPEG4).
WMV9: a proprietary, non-MPEG4 codec from Microsoft.
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