Multimedia Engineering
  NTSC Theory

Television is a system of communication consistent with many models that have been developed in science. We have completed the transition, for the most part, from analog technology where television stations must transmit from station to receiver over five million bits of video information every second just to achieve a medium resolution picture, to digital television and high definition television, where as many as 19 million bits per second may be used to display very high resolution images and multi channel multi lingual sound. Understanding how pictures and sound become an electronic signal at both ends of the process is still important. Remember, in nature, video and audio are analog. The human ear and the human eye are implementing both analog and digital electro-mechanical principles in their operation.

Understanding how any camera captures and stores its images is very helpful in comprehending how images can be analyzed and converted to other forms. Film cameras expose the negative film to a controlled amount of light, whereas digital cameras expose the CCD (charge coupled device) to a controlled amount of light. In both cases, there are costs involved in both systems. Moving the information along the path in both cases costs time. Also, the more vacuum tube or solid state devices involved in the path means the addition of distortion in every instance. Some of these losses in time and quality can not be ignored. Loss of information accuracy or content is a cost that might later prove to be troublesome, just as in the additonal cost of moving freight on the rails or in the sky if pieces of the cargo get dropped or lost. Knowing which information can be moved with no taxing effect is important. That is the secret to compression. Cameras in television must be able to convert the image coming through the lens to either a series of changes in light intensity and color represented electronically (analog) or a grid of data (digital) where each location on the viewing screen has intensity and color data.

Anyone who has ever examined a piece of motion picture film realizes quickly that movies are made up of single pictures. The industry standard of 24 frames per second applies to both the motion picture camera as well as the motion picture film projector.. Because the human eye and brain together fill the empty space between images, the illusion of actual motion is created. This is called the "persistence of vision." All television systems, analog and digital, employ this phenomenon. We see moving pictures, but in reality all that is there is a series of light, dark and colored dots. The analog video camera sees the object as a series of dots and scans them with an electron gun, and projects them onto a photosensitive surface where they are converted into electrical information which is transmitted to the receiver. CCD cameras, both analog and digital, have already converted the light information to electrical information at the output of the CCD. This information in managed and processed right at the camera and can be transmitted to another digital device on the camera (viewfinder) or away from the camera. In the analog case, at the receiver the electron gun in the CRT (cathode ray tube) rescans these dots on the inside of the picture tube. This is simply reversing the electrical information back to an image.

To briefly explain the scanning technique used by much of the world's television broadcasters up until the digital transition, and still in use today in some places in the world and even in the U.S. on LPTV stations and translators, was developed by the National Television System Committee (NTSC) [a.k.a. "No Two Same Colors] is not of much interest to most. Although NTSC video standards had become a noose around the neck of implementing broadcast of advanced video signals, it is no longer the standard and will no longer be an impediment to improving video transmission.

In film, each frame is a unique photograph. In NTSC video, a fairly complex but organized method of scanning was used in order to accurately reproduce images. The scanning of the image, in both the camera and the television receiver, was done by an electron beam moving in horizontal lines across the target image (camera end) or screen (receiver end). Simultaneously, the electron beam moved down to the bottom of the image, where it was then sent to the top to scan again. The horizontal lines were scanned alternately. The odd lines are scanned in the first pass, the even lines in the second pass. This process is called "interlacing" and was developed to reduce flicker, an artifact of the scanning process. Each scan of the image is called a field and involves half of the total horizontal lines (262.5 to be exact). Two complete scans are required to accumulate all 525 lines, and this is called a "frame." A total of 60 fields are scanned each second (or 30 frames per second. Interlacing techniques have carried over to digital video formats as well.

To accurately reproduce the image being scanned by the camera, both camera and television receiver must be be scanning the same part of the image or "picture" at the same time. This syncronization applies both to the horizontal as well as vertical motion of the electron beam. At the end of each horizontal line, the beam returns to the left side of the screen. This is called horizontal retrace and is controlled by the horizontal sync pulse. At the end of one field (262.5 lines), the beam must be sent to the top of the screen. This is called vertical retrace and is controlled by the vertical sync pulse. During the intervals that the beam is undergoing horizontal or vertical retrace, the electron beam must be turned off in the camera and receiver. This is called blanking and is controlled by sending a voltage level below picture level to signal the scanning beam to turn off. The turned off beam continues to move to it's new position (sometimes referred to as invisible retrace) before being signalled to turn back on. There are also a series of equalizing pulses inserted during vertical blanking which causes the video fields to begin at the proper points to achieve interlacing. This is basic NTSC video scanning. Although effectively obsolete, it is important to understand this method in order to fully comprehend how much is involved in capturing and converting a picture into a series of data lines or points that can be electrically represented in an orderly fashion so that it may be reproduced after traveling through a wire or through the air.