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Interlace

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(Redirected from Interlacing)

Interlacing is a method of displaying images on a raster-scanned display device, such as a cathode ray tube (CRT). The method causes less visible flickering than non-interlaced methods. The display alternates between drawing the even-numbered lines and the odd-numbered lines of each picture. In the PAL and NTSC standards, the lower (even) field is always drawn first.

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Description

A non-interlaced raster display draws every line of a picture, or frame, in sequence from top to bottom. This takes a finite length of time, during which the image on the CRT begins to decay, resulting in flicker. Interlacing is used when bandwidth limitations make it impractical to transmit full frames at a fast enough rate to make flicker invisible. Instead, only half the lines from each frame are transmitted in what is known as a field. One field contains only the odd-numbered lines (forming the odd field), and the next contains only even-numbered lines (forming the even field). Because of persistence of vision, pairs of fields are perceived at the same time, giving the appearance of a full frame.

The alternatives to interlacing (assuming a directly-driven CRT display) are:

  • Doubling the bandwidth and transmitting full frames instead of each field. This produces little improvement in picture quality, providing the same effective resolution and flicker rate.
  • Using the same bandwidth, but transmitting progressive frames with half the amount of detail. The flicker rate remains the same.
  • Using the same bandwidth, but transmitting a full progressive frame instead of every two fields. The eye suffers more fatigue (eye-strain) than when viewing the interlaced display, because the flicker rate halves.
  • As above, but using a digital frame buffer to display each frame twice. This provides the same flicker rate as the interlaced signal, but with less smooth motion.

One common misconception is that an odd and even pair of fields represents a single frame. In reality, the camera scans in the same way as an interlaced display, reading even lines from its sensor only after it has finished reading the odd lines. Therefore, any moving part of the image will be in a different position. As an example, lines 122 and 124 in a 50 fields per second system is read approximately one fiftieth of a second after lines 123 and 125 were read. If the odd and even fields were combined into a single progressive frame, any parts with horizontal motion would display visible combing on their edges.

In an interlaced system, lines are drawn diagonally such that the right end of each line is two lines lower than the left end. The offset between the two fields is then produced by having both an odd number of overall lines and vertical flyback between the odd and even fields occur halfway through one line. For example, in PAL, the blanking period starts after 292.5 lines of the odd field have been transmitted, and lasts for 20 lines. When scanning begins again at the top of the screen, the scanning beam is still halfway across the picture. Because of the slant, the centre top of the picture is one line above the line begun at the top left corner.

In modern monitors and television sets, interlacing is being slowly superseded as the refresh rate of non-interlaced displays increases beyond the level at which flicker can be detected.

Broadcast television

The early televisions in the 1920s were unable to refresh at high speeds. Instead they were limited to 30 or 25 frames per second. This could potentially cause flickering on the screen. Interlacing solved this. Each frame is split into an odd and even field. When played back on a television, the tv would project the odd field followed by the even field. When an image was moving, then one of the fields would be offset from the other. The image, however, would look smoother than if it had merely shown a single frame.

For example, the PAL television system is based on displaying 50 fields per second, and as two fields contain the full resolution, it corresponds to 25 full video frames per second. Interlaced video is often used as a tradeoff between bandwidth requirements and smooth reproduction of movement.

Interlacing is used by all the analogue TV broadcast systems in current use (mainly NTSC, PAL and SECAM).

Disadvantages

The major disadvantage of interlacing is the reduction in vertical display resolution. Consider a TV image made of alternating white and black horizontal lines, each line being only one TV line high. Instead of a fine black and white striped image, an interlaced display would show a black and white flashing image at the frame rate of the video. This is the most dramatic example of an interference effect called twitter, caused when an interlaced image attempts to display vertical information at too high a spatial frequency.

Another disadvantage is that an interlaced signal cannot be displayed without artifacts on a non-scanning display device such as an LCD or plasma screen. To compensate for the problems, a computationally expensive deinterlacing process is needed.

Computer Images

Interlacing is done on some GIF and PNG images (that have been specifically designed to be viewed remotely) so that the viewer on a slow communications line can see what the image looks like before it is finished loading. GIF is interlaced by sending the lines in the order 0, 8, 16, ..., 4, 12, ..., 2, 6, 10, 14, ..., 1, 3, 5, 7, 9, .... PNG is interlaced using the Adam7 algorithm.

See Also

  • Progressive scan: the opposite of interlacing; the image is displayed line by line.
  • Deinterlacing: converting an interlaced video signal into a non-interlaced one
  • Telecine: a method for converting film frame rates to television frame rates using interlacing
  • Federal Standard 1037C: defines Interlaced scanning

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