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Wednesday, January 21, 2009

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Number of frames per second
Frame rate, the number of still pictures per unit of time of video, ranges from six or eight frames per second (frame/s) for old mechanical cameras to 120 or more frames per second for new professional cameras. PAL (Europe, Asia, Australia, etc.) and SECAM (France, Russia, parts of Africa etc.) standards specify 25 frame/s, while NTSC (USA, Canada, Japan, etc.) specifies 29.97 frame/s. Film is shot at the slower frame rate of 24frame/s, which complicates slightly the process of transferring a cinematic motion picture to video. The minimum frame rate to achieve the illusion of a moving image is about fifteen frames per second.

[edit] Interlacing
Video can be interlaced or progressive. Interlacing was invented as a way to achieve good visual quality within the limitations of a narrow bandwidth. The horizontal scan lines of each interlaced frame are numbered consecutively and partitioned into two fields: the odd field (upper field) consisting of the odd-numbered lines and the even field (lower field) consisting of the even-numbered lines. NTSC, PAL and SECAM are interlaced formats. Abbreviated video resolution specifications often include an i to indicate interlacing. For example, PAL video format is often specified as 576i50, where 576 indicates the vertical line resolution, i indicates interlacing, and 50 indicates 50 fields (half-frames) per second.
In progressive scan systems, each refresh period updates all of the scan lines. The result is a higher perceived resolution and a lack of various artifacts that can make parts of a stationary picture appear to be moving or flashing.
A procedure known as deinterlacing can be used for converting an interlaced stream, such as analog, DVD, or satellite, to be processed by progressive scan devices, such as TFT TV-sets, projectors, and plasma panels. Deinterlacing cannot, however, produce a video quality that is equivalent to true progressive scan source material.