NTSC Composite Video Signals, and the RS - 170A Standards

Part 3 - the Sync Signal

Once we have the right test equipment at hand, we must have at least a BASIC understanding of the function of a video signal. How exactly does the video signal put a picture on the screen? The answer is in " deflection ". The cathode ray tube puts a still dot on the screen, focussed toward the center. This dot is then " deflected " vertically downward while at the same time moving horizontally from left to right.

In this manner, 1 still picture is placed on the TV monitor. This is repeated 59 times per second, to give the illusion of movement. It was found years ago that by flipping at least 30 still pictures per second, an illusion of fluid motion is produced. This is the basic principle behind cartoons. The original thought was that by deflecting the signal 60 times per second, it would double the rate of 30, decreasing the "flicker" effect, while at the same time using the 60 Hz US power line rate for a "syncronizing" source. The problem came about when video took on color, and the 60 Hz signal generated unsavory results. So the " frame rate " was changed to 59 frames per second.

If the signal coming from the television station carried video information only, and the TV set at home generated its own deflection signal without a reference, the signal would look scrambled at worst, rolling from top to bottom at best. There had to be some way to " lock " the TV sets at home with the signal coming from the TV station, so that they would both start at the same place in the picture, at the same time. Here's where " Syncronizing " or Sync signals come into play.


A video signal would have a special " blip " added to it, that would cause it to know when to start a new line. This " blip " was called the Horizontal Sync Pulse. The Horizontal Sync Pulse is added to the beginning ( or end, depending on how you look at it ) of each horizontal line of video. It tells the TV set when to begin a new horizontal line.

Well thats a great way to find the beginning of a line, but what about the beginning of a frame or page? This stops the "scrambled" look, but doesn't do a thing for the video rolling from top to bottom at home.

The answer to this problem is Vertical Sync. Vertical Sync looks like a series of blips, which lasts for a pre-determined amount of time. When a TV set sees the vertical deflection pulses, it begins scanning a new set of lines from the top of the screen. It does this twice for each picture frame. Each frame is actually made up of 2 interlacing " fields ". The first field ( shown in red on the chart ) is started by the first set of ertical Sync Pulses. The second field ( shown in blue on the chart ) is started by a second set of Vertical Sync Pulses.

For the purposes of this course, it is not necessary that you know all the nuances of the Vertical Sync Pulse, but you should understand the difference between Horizontal and Vertical Sync, and their purpose in the video signal.

You should at least, by now, understand that they play an integral part in the alignment of the video picture on the screen. But keep in mind, that this is only the BASICS. You will need to go into greater detail on each Horizontal line in order to actually syncronize two or more video sources to each other for broadcast purposes.
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