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The Flight > The Flight - Pg. 226

226 Chapter 12 Frame Line 1 Line 2 Line N Figure 12.1: Video image scanning Table 12.1: International video standards examples USA Standard Frames per second Number of lines NTSC 29.97* 525 Europe, Asia PAL 25 625 France and Others SECAM 25 625 Note: NTSC used to be 30 frames per second, but the introduction of the new color standard changed it to 29.97, to accommodate a specific frequency used by the color sub-carrier crystal oscillator. refreshed fast enough for our eyes to be tricked into believing that the entire image is present at the same time and, if there is motion, it is fluid and continuous. In different parts of the world, slightly different and therefore incompatible systems have been developed over the years, but the basic mechanism remains the same. What changes eventually is the number of lines composing the image, the refreshing frequency, and the way the color information is encoded. Table 12.1 illustrates three of the most commonly used video standards adopted in the USA, Europe and Asia. All those standards encode the "luminance" information (that is, the underlying black and white image) together with synchronization information in a similarly defined composite signal. The name composite is used to describe the fact that this video signal combines and transmits three different pieces of information in one: the actual luminance information and both horizontal and vertical synchronization information (Figure 12.2). The horizontal line signal is composed of: l l The horizontal synchronization pulse, used by the display to identify the beginning of each line The so-called "back porch", which creates a dark frame around the image