AN3823| Application Note
Maxim/Dallas > App Notes > VIDEO CIRCUITS Keywords: Analog Switching, Video Switching, Video Switch, Analog Switch, Video
May 08, 2006
APPLICATION NOTE 3823
Switching Video Using Analog Switches
Although digital video is increasing in popularity, analog video remains in use in many applications. This article describes standard analog video formats and ways to switch between analog video sources.
Introduction
Historically, RS-170 was the original black-and-white video standard in which video is divided into frames, with typical frame rates of 30 (U.S.) or 25 (Europe) frames per second. Broadcast television divides each frame into two interlaced fields. For North American television, a horizontal rate of 15750 lines was displayed in 1/30 of a second using two fields of interlaced video per frame. By convention, black is the most negative active video signal with sync tip going below black. The DC levels for various portions of the signal are shown in Figure 1.
Figure 1. Black-and-white signal DC levels. White level to black level was defined to be 100 IRE units for the active video portion, and white to sync tip was therefore 140 IRE units. If the signal was attenuated, the ratio of white to black and white to sync tip remained constant; whether the signal was attenuated or amplified, the value of the white to sync tip signal was 140 IRE units. Today's composite video baseband signal (CVBS) is a derivative of RS-170 with white to sync tip nominally set to 1.0V (white at +0.714, blank at 0V, and sync tip at -0.286). This could be considered as an RS-170 signal attenuated by 71%, as all IRE values still hold. The National Television Standards Committee (NTSC) system juggled some horizontal and vertical values and very cleverly fit bandwidth-reduced color information into essentially the same baseband signal. Because the black-and-white information was already being transmitted, which was necessary for early black-and-white compatibility, the black-and-white signal n
eeded to remain part of any color system. This information has become know as Y, or luminance information. Y consists of the sum of RGB signals1.
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