The Early Years of Television
One of the most confusing terms in home theater jargon is "video". What exactly is video and how does it differ from everything else? The term video is loosely defined as images originally produced by a video camera, as opposed to a film camera. Though this definition is simple and easy to digest, its simplicity may be cause for confusion. Why? High definition television and NTSC television are both often referred to as video.
So, for the sake of clarity, we will use the term "video" only in reference to television standards used in the United States prior to high definition television.
NTSC is the acronym for the National Television Standards Committee, which was originally responsible for the development of U.S. television and video standards. Today NTSC is still used to describe video standards.
Let's take a step back in time to learn more about televisions and standards. In the 1950's, the now familiar electronic color television hit the U.S. market. Since then, we've used the term "television" to describe both the broadcasted signal and the television set. The two are inseparable since it was the limitations of that era's technology that shaped video standards in the U.S.
Until the late 1970's, the only technology available to display a television signal (AKA NTSC video signal) was the CRT, or Cathode Ray Tube television. We all know what a CRT looks like (Figure 1) but might not be as familiar with the name.
Figure 1. Cathode Ray Tube
How does a CRT work? For starters, when you watch a CRT television you are, in fact, seeing the large end of the Cathode Ray Tube, which we all know as "the TV screen" (Figure 2). The back side of the TV screen is coated with dots of red, green and blue phosphors — materials that emit visible light when exposed to radiation. On a standard television set, phosphor dots are aligned in approximately 480 horizontal lines.
To create an image, an incoming color signal is split into red, green and blue components and fed to the CRT itself. (Why red, green and blue? Because red, green and blue light are the primary colors of light and can be used to create all of the colors the human eye can see).
Next, each color signal controls its own electron beam inside the CRT that will excite either the red, green or blue phosphor on the back side of the tube, depending on which color is needed at any given spot to create the image. (Figure 2) The electron beams actually "draw" a picture on the screen from left to right and from top to bottom.
Figure 2. The Inner workings of a CRT
The technology relies on a phenomenon of human vision called the "phi phenomenon". The phi phenomenon is a perceptual illusion where multiple still images are combined by the brain and computed as motion. A CRT draws an image once every 1/60th of a second, while your brain combines these still images and perceives motion. The whole process is much the same as how a film projector works.
Now that you understand the basics about how a CRT display produces an image, we can get more detailed about NTSC video. We'll start with the more complex definition of NTSC video and break it down - one piece at a time.
NTSC Video is a 4:3 aspect ratio, interlaced, analog signal with 525 total lines of resolution.