Sunday, April 24, 2011

Interlaced Video

Interlaced video is a technique of doubling the perceived frame rate of a video signal without consuming extra bandwidth. Since the interlaced signal contains the two fields of a video frame shot at two different times, it enhances motion perception to the viewer and reduces flicker by taking advantage of the persistence of vision effect. This results in an effective doubling of time resolution as compared with non-interlaced footage. However, interlaced signals requires a display that is natively capable of showing the individual fields in a sequential order, and only traditional CRT-based TV sets are capable of displaying interlaced signals, due to the electronic scanning and lack of apparent fixed-resolution. Interlaced scan refers to one of two common methods for "painting" a video image on an electronic display screen by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image. A PAL-based television display, for example, scans 50 fields every second i.e. 25 odd and 25 even. The two sets of 25 fields work together to create a full frame every 1/25th of a second, resulting in a display of 25 frames per second, but with a new half frame every 1/50th of a second.
To display interlaced video on progressive scan displays, de-interlacing is applied to the video signal.
One of the most important factors in analog television is signal bandwidth which is measured in megahertz. The greater the bandwidth, the more expensive and complex is the entire production and broadcasting chain which consist of cameras, storage systems such as tape recorders or hard disks, broadcast and reception systems such as terrestrial, cable, and satellite transmitters and receivers, or the Internet, and end-user displays such as televisions or computer monitors. Given a fixed bandwidth instead, interlace can provide a video signal with twice the display refresh rate for a given line count versus progressive scan video at similar frame rate, for instance 1080i at 60 half-frames per second, vs. 1080p at 30 full frames per second. The higher refresh rate improves the portrayal of motion, because objects in motion are captured and their position is updated on the display more often, and when objects are more stationary the human vision combines information from multiple similar half-frames resulting in the same perceived resolution as progressive full frames. This technique is only useful though, if the source material is available in higher refresh rates. Cinema movies are typically recorded at 24fps, and gets no real benefit from common interlacing techniques.

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