Description
With progressive scan, an image is captured, transmitted, and displayed in a path similar to text on a page: line by line, from top to bottom. The interlaced scan pattern in a CRT display also completes such a scan, but only for every second line. This is carried out from the top left corner to the bottom right corner of a CRT display. This process is repeated again, only this time starting at the second row, in order to fill in those particular gaps left behind while performing the first progressive scan on alternate rows only.
Such scan of every second line is called interlacing. A field is an image that contains only half of the lines needed to make a complete picture. The afterglow of the phosphor of CRT displays, in combination with the persistence of vision results in two fields being perceived as a continuous image, which allows the viewing of full horizontal detail with the same bandwidth that would be required for a full progressive scan but with twice the perceived frame rate and with the necessary CRT display refresh rate to prevent flicker. Interlacing is used by all the analog broadcast television systems in current use.
In common shorthand format identifiers like 576i50 and 720p50, the frame is specified for progressive scan formats, but for interlaced formats, the field rate is typically specified, which is twice the frame rate. This can lead to confusion because industry-standard SMPTE timecode formats always deal with frame rate, not field rate. To avoid confusion, SMPTE and EBU always use frame rate when specifying interlaced formats, i.e. 480i60, 576i50, 1080i50, and 1080i60 become 480i/30, 576i/25, 1080i/25, and 1080i/30 and it is asserted that each frame in an interlaced signal always contains two sub-fields in sequence.
Read more about this topic: Interlaced Video
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