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External flicker fixer. A flicker fixer or scan doubler is a piece of computer hardware that de-interlaces an output video signal. The flicker fixer accomplishes this by adjusting the timing of the natively interlaced video signal to suit the needs of a progressive display for example a CRT computer monitor.
A scan line (also scanline) is one line, or row, in a raster scanning pattern, such as a line of video on a cathode-ray tube (CRT) display of a television set or computer monitor. [1] On CRT screens the horizontal scan lines are visually discernible, even when viewed from a distance, as alternating colored lines and black lines, especially when ...
When the overall interlaced framerate is 60 frames per second, a pixel (or more critically for e.g. windowing systems or underlined text, a horizontal line) that spans only one scanline in height is visible for the 1/60 of a second that would be expected of a 60 Hz progressive display - but is then followed by 1/60 of a second of darkness ...
Initially intended for use by computer monitors and video cards, the standard made its way into consumer televisions. The parameters defined by standard include horizontal blanking and vertical blanking intervals, horizontal frequency and vertical frequency (collectively, pixel clock rate or video signal bandwidth), and horizontal/vertical sync ...
The higher bandwidth used by 80-column text mode results in random short horizontal lines appearing onscreen (known as "snow") if a program writes directly to video memory during screen drawing. The BIOS avoids the problem by only accessing the memory during horizontal retrace, or by temporarily turning off the output during scrolling.
This part of the line display process is the Horizontal Blank. [1] [2] In detail, the Horizontal blanking interval consists of: front porch – blank while still moving right, past the end of the scanline, sync pulse – blank while rapidly moving left; in terms of amplitude, "blacker than black".
In a CRT, the vertical scan rate is the number of times per second that the electron beam returns to the upper left corner of the screen to begin drawing a new frame. [3] It is controlled by the vertical blanking signal generated by the video controller, and is partially limited by the monitor's maximum horizontal scan rate.
The flicker of a CRT monitor can cause various symptoms in those sensitive to it such as eye strain, headaches [9] in migraine sufferers, and seizures in epileptics. [10]As the flicker is most clearly seen at the edge of our vision there is no obvious risk in using a CRT, but prolonged use can cause a sort of retinal shock where the flickering is seen even when looking away from the monitor.