Some discussion of the raster manufacturing plant will be of value to us here. The horizontal, high voltage and vertical sections of the TV provide the light source on the phosphor screen just as the lamp in a movie projector provides light on the movie screen. While the raster is necessary, it is boring to sit and stare at it, unless there is a picture. The picture information in a movie projector comes from the film obstructing the light and producing light spots and dark spots on the screen.
The picture information inserted into the raster comes through the air from the TVstudio in the form of electromagnetic pulsing waves. These invisible forces are intercepted by your TV antenna, funneled down your antenna wire and fed into the antenna terminals of your TV set. The TV processes these tiny bits of electronic information and injects it into the electron ray in the picture tube. The ray then produces light spots and dark spots on the TV screen.
Let’s examine how a scene at the TV studio can get out into the air.
A TV camera takes a moving picture of the programming at the studio. The photo sensitive plate inside the camera’s picture tube is electrically altered by the light that is focused on it. An electron gun similar to the one in your picture tube, fires a ray through a yoke, scanning the photographic plate with a 15,750 CPS horizontal )rate and a 60 cycle vertical rate.
The ray hits either dark spots or light spots according to the scene that is coming through the lens. A wire leads out from the plate. If the ray hits a light spot a small voltage is developed and a small voltage emerges from the wire. If the ray hits a dark spot a heavier voltage is developed.
The signal voltage represents each picture frame in terms of millions of tiny spots ranging from light to dark. The signal voltage at this point is ready to be processed for beaming out on the air waves.
This is where the carrier wave comes in. Each TV station is known by its frequency. Each TV station has a different frequency. As you flip your channel selector you are tuning in a different station each time. The station’s designated frequency or carrier is developed by an oscillator set to run at that frequency in the transmitter. It is called the carrier wave because the picture signal is impressed on it for transmission to your set.
The carrier wave is invisible. It exists at the same time as we do but in different dimension. While we have length, breadth and height, a carrier wave has amplitude and frequency, both electromagnetic characteristics. If we graph it, it looks like “A”.
In addition to the video we also must have audio. Thus there is a microphone near the camera. It picks up the sound of the scene. The sound is also impressed on the carrier wave, but in slightly different fashion so the sound and picture voltages won’t mix. While the picture signal affects the height or amplitude of the carrier the sound affects the frequency. The frequency as we mentioned before is one of the carriers’ dimensions and is represented by the distance between the hills and valleys shown on the drawing. Since the audio is varying the frequency, it is said to be frequency modulated (FM).
The video signal, on the other hand, is amplitude modulated (AM) because it affects the other carrier dimensions’ amplitude. The fact that the picture and sound are respectively AM and FM helps keep them from interfering with each other.
In addition to transmitting sound and picture, another passenger must be loaded aboard the carrier wave: sync.