Television scanning

(communications) The process of scrutinizing the brightness of each element of detail contained in the image of a scene to be transmitted by television.

The process used to convert a three-dimensional image intensity into a one-dimensional television signal waveform. The image information captured by a television camera conveys color intensity (in terms of red, green, and blue primary colors) at each spatial location, with horizontal and vertical coordinates, and at each time instance. Thus, the image intensity is multidimensional, since it involves two spatial dimensions and time. It needs to be converted to a unidimensional signal so that processing, storage, communications, and display can take place.

The television scene is sampled many times per second in order to create a sequence of images (called frames). Then, within each frame, sampling is done vertically to create scan lines. Scanning proceeds sequentially, left to right and top to bottom. In a television camera, an electron beam scans across an electrically photosensitive target upon which the image is focused. At the other end of the television chain, with raster scanned displays, an electronic beam scans and lights up the picture elements in proportion to the light intensity. While it is convenient to think of all the samples of a single frame occurring at a single time (similar to the simultaneous exposure of a single frame for film), the scanning in a camera and in a display results in every sample corresponding to a different instance in time, and successive lines occur later in time. See also Television camera tube; Television receiver.

There are two types of scanning approaches: progressive (also called sequential) and interlaced. In progressive scanning, the television scene is first sampled in time to create frames, and within each frame all the raster lines are scanned from top to bottom. Therefore, all the vertically adjacent scan lines are also temporally adjacent and are highly correlated even in the presence of rapid motion in the scene. Film can be thought of as naturally progressively scanned, since all the lines were originally exposed simultaneously, so the correlation between adjacent lines is guaranteed. Almost all computer displays (except some low-end computers) are sequentially scanned. See also Electronic display.

In interlaced scanning, all the odd-numbered lines in the entire frame are scanned first, and then the even numbered lines. This process produces two distinct images per frame, representing two distinct samples of the image sequence at different points in time. The set of odd-numbered lines constitute the odd field, and the even-numbered lines make up the even field. All current television systems use interlaced scanning. One principal benefit of interlaced scanning is to reduce the scan rate (or the bandwidth). This is done with a relatively high field rate (a lower field rate would cause flicker), while maintaining a high total number of scan lines in a frame (lower number of lines per frame would reduce resolution on static images). Interlace cleverly preserves the high-detail visual information and, at the same time, avoids visible large-area flicker at the display due to temporal postfiltering by the human eye.

An agreement (not a standard) on high-definition formats has been reached and is in use, enabling the transition from analog to digital television. In early 2001, there were more than 150 stations broadcasting high-definition television. The format has 1920 active pixels per line and 1080 active (out of a total of 1125) lines in a frame. The frames may be interlaced or progressive and the frame rate is 29.97 Hz. Progressive frames at a rate of 23.976 Hz are also permitted to accommodate film material. An alternative progressive-only format that provides additional temporal resolution at the expense of some spatial resolution has also been approved and is currently in use. This format has 1280 active pixels per line and 720 active (out of a total of 750) lines per frame, and the frame rates permitted are 23.976 Hz, 29.97 Hz, or 59.94 Hz. Both high-definition formats have square pixels and a 16 × 9 aspect ratio. See also Data compression; Image processing; Television.