Overscan crashes - the outer part of a television raster that is not displayed on most CRTs due to the presence of an empty area and possible distortions in this part of the image. In analogue television, they perform a function similar to printing sorties in the printing industry .
Purpose
Departures along the edges of the active part of the television raster are designed to mask the distortion of the video signal and the blanking margin at the edges of the frame. The latter has been used since the beginning of the existence of television , based on cathode ray tubes , and is formed due to the stock of the duration of the quenching pulses exceeding the duration of the reverse course of the scans [1] . Another reason for using sorties was the “swimming” effect, that is, fluctuations in the size of the raster depending on the brightness of the transmitted scene, making it even more difficult to standardize the size of the displayed frame.
Since then, the scan of household televisions has been adjusted so that part of it goes beyond the screen: this was especially true in televisions with very rounded picture tubes used during the development of standards. With the advent of video recording, crashes are used to mask the distortion of the video signal associated with the switching of video heads that occurs at the beginning of the active part of the half-frame . Inaccuracy of switching and recording defects of the clock signal inherent in most VCRs led to distortion of the upper edge of the image. The presence of sweeps forces the manufacturers of transmitting equipment and TVs to limit the useful part of the transmitted image. Domestic televisions with a kinescope do not display this area to provide clear edges, but the clipping of departures is not standardized, allowing for discrepancies between devices of different manufacturers, models and even individual copies [2] . At the same time, studio monitors and electronic viewfinders of professional transmitting cameras show the transmitted raster in full for precise control of the scene composition. Such a setting, as in video cards , is called English. Underscan . Departures restricting the television frame from four sides in foreign sources were called English. Overscan and should be taken into account when digitizing analog video, despite the fact that they contain an image. Most video cards display an image with raster crashes, allowing you to display the image from a computer to a TV screen in full. In some cases, this leads to the fact that defects in the edges of the image obtained during the scanning of film or the digitization of analog video become visible.
Modern Departure Use
Improving the design of the sweep and cathode ray tube power generators made it possible to precisely standardize the sizes of the displayed part of the raster. Modern video cameras and televisions are based on completely different technologies with fixed display elements and, in comparison with tube ones, generally do not allow image size fluctuations and its “inversion”. However, in modern analog television, sweep crashes are necessary for compatibility with televisions cutting them off. In addition, when playing a home analog video, moving the scan out of the screen helps to hide image defects.
Even in the absence of distortion, overhead information often falls into the departure area. The blanking area in analog television is used to transmit overhead information, such as subtitles , teletext and test signals used to configure the transmitting equipment [3] . A type of SMPTE / EBU time code called VITC (Standard Vertical Interval Time Code ) is standardized for recording by rotating heads of video recorders during a frame blanking pulse in that part of the lines adjacent to the active part of the frame. Therefore, VITC bits are displayed as “squares” in the upper part of the image when the raster size is too small in Underscan mode . On most home TVs that cut off flights, these artifacts are not observed.
Display in computers
CRT monitors for computers are set to Underscan compressed scan mode with adjustable black bars at the edges of the screen. On LCD monitors connected via DVI , there is no such setting, because all pixels have a fixed position. Thus, all modern computers can provide a full raster display, including crashes. However, analog video interfaces, such as VGA , are subject to temporal distortion, which is critical for LCD monitors. When a video or animation is created for viewing on a computer (for example, Flash movies), there is no need to leave additional content outside the boundaries of the plot-important area. If such content is created to be shown on television, crashes are necessary for compatibility with different types of TVs.
Video game consoles are designed so that the plot-important part of the game is in an even smaller area intended for captions. Older systems were made with protective framing of the image, for example, the Nintendo Entertainment System prefix gave an image with black stripes at the edges visible on some TVs. The new systems fit the content in a frame so that the plot-important part is visible, while filling the spare area with unnecessary details.
Due to the wide variety of home computers that were manufactured in the 1980s and early 1990s , many machines, such as Sinclair Research Ltd ZX Spectrum or Commodore 64 (C64), had a crash protection frame. Some other computers, such as Commodore International Amiga , allowed you to change the video signal to create a non-working area. In the case of C64 and Atari ST , it was possible to remove the no-fly zone using special coding. This effect was called full-screen mode on a 16-bit Atari demonstration computer, and it allowed us to improve the technique of resource-intensive vertical moving of the image, which was later called sync-scrolling .
Video Capture
There is no unanimous opinion on the need to trim departures during video capture [2] . There are supporters of digitizing the full frame, who consider cropping as a violation of the integrity of the composition. However, video recordings on household video cassettes, especially VHS , require trimming due to distortion along the edge of the scan, especially noticeable on archived recordings. In addition, without cutting off departures, in most cases black blanking strips become visible. In the case of departures, it is preferable to trim the black area, slightly increasing the amount of data.
Video capture cards operating as HD (high resolution) video do not take into account raster crashes and are designed to fully display a frame free from blanking margin. High definition video, originally designed for a square pixel, does not need to be cropped or scaled. Additional trimming is necessary if there are empty areas formed due to the mismatch of the aspect ratio of the original image and the standard used, mainly when using letterboxing .
A small subsequent cropping may be necessary to remove the black bars visible on the frame with a 14: 9 aspect ratio. In addition, the width of the cropping can unpredictably change depending on the presence of plot-important details at the edges of the video.
See also
- Blanking reserve
- Display borders
- Hidden Coughing
- Print Departures
Notes
- ↑ Television, 2002 , p. 49.
- ↑ 1 2 Analog source resolution . Guide to Understanding Video Sources . "Digital FAQ". Date of treatment December 10, 2012. Archived December 16, 2012.
- ↑ Television, 2002 , p. 547.
Literature
- V.E. Dzhakonia. Tv - M.,: “Hot line - Telecom”, 2002. - S. 41-56. - 640 s. - ISBN 5-93517-070-1 .