Interlaced video

Interlaced video is a technique for doubling the perceived frame rate of a video display without consuming extra bandwidth. The interlaced signal contains two fields of a video frame captured at two different times. This enhances motion perception to the viewer, and reduces flicker by taking advantage of the phi phenomenon.

This effectively doubles the time resolution (also called temporal resolution) as compared to non-interlaced footage (for frame rates equal to field rates). Interlaced signals require a display that is natively capable of showing the individual fields in a sequential order. CRT displays and ALiS plasma displays are made for displaying interlaced signals.

Interlaced scan refers to one of two common methods for "painting" a video image on an electronic display screen (the other being progressive scan) by scanning or displaying each line or row of pixels. This technique uses two fields to create a frame. One field contains all odd-numbered lines in the image; the other contains all even-numbered lines.

A Phase Alternating Line (PAL)-based television set display, for example, scans 50 fields every second (25 odd and 25 even). The two sets of 25 fields work together to create a full frame every 1/25 of a second (or 25 frames per second), but with interlacing create a new half frame every 1/50 of a second (or 50 fields per second).[1] To display interlaced video on progressive scan displays, playback applies deinterlacing to the video signal (which adds input lag).

The European Broadcasting Union has argued against interlaced video in production and broadcasting. They recommend 720p 50 fps (frames per second) for the current production format—and are working with the industry to introduce 1080p 50 as a future-proof production standard. 1080p 50 offers higher vertical resolution, better quality at lower bitrates, and easier conversion to other formats, such as 720p 50 and 1080i 50.[2][3] The main argument is that no matter how complex the deinterlacing algorithm may be, the artifacts in the interlaced signal cannot be completely eliminated because some information is lost between frames.

Despite arguments against it,[4][5] television standards organizations continue to support interlacing. It is still included in digital video transmission formats such as DV, DVB, and ATSC. New video compression standards in development, like High Efficiency Video Coding, do not support interlaced coding tools and target high-definition progressive video such as ultra high definition television.

Progressive scan captures, transmits, and displays an image in a path similar to text on a page—line by line, top to bottom. The interlaced scan pattern in a CRT display also completes such a scan, but in two passes (two fields). The first pass displays the first and all odd numbered lines, from the top left corner to the bottom right corner. The second pass displays the second and all even numbered lines, filling in the gaps in the first scan.

This scan of alternate lines is called interlacing. A field is an image that contains only half of the lines needed to make a complete picture. Persistence of vision makes the eye perceive the two fields as a continuous image. In the days of CRT displays, the afterglow of the display's phosphor aided this effect.

This page was last edited on 1 July 2018, at 17:56 (UTC).
Reference: under CC BY-SA license.

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