How it Works

The broadcast station transmits RF waves into the air.  The signal contains all the components of what you see and hear on your TV  .  .  .  video and sound.  Your TV picks the very faint signals up, amplifies them, separates the various sub-signals, and sends them to your screen and speaker.  

Although all you see is a flat screen - it is actually the front of a large, cone-shaped glass vacuum tube (CRT - Cathode Ray Tube).  The screen is supercharged at around 30,000 volts.  The back end has 3 guns for Red, Green, and Blue.  The high voltage screen is illuminated by a pinpoint beam of electricity, which slams into the faceplate, lighting up a phosphor lining that coats the inside of the glass.  At the other end of the CRT, the gun carries the actual picture signal, and the signal causes the voltage to vary, which varies the intensity of the beam, which varies the brightness of the phosphor on the screen.

The following picture tells the whole story.  It is a monochrome (black and white) TV.  

In reality, 29.97 frames are displayed every second - or 59.95 fields (see the next section).

Color TV works the same way, except there are 3 guns (RGB - Red, Green, and Blue) that send 3 beams to the screen.  The beams themselves are colorless, but the color TV screen is composed of thousands of tiny RGB dots, arrange in traingles.  The corresponding beams light up the colored dots on the screen - incredible that they can be so accurate as to hit the correct colored dots since they are so small that you can't seem them without a magnifying glass. 

It is a bit more complex when you consider the actual number of visible scan lines, how much resolution is available, etc.  

There are 525 horizontal lines in one frame, 262.5 per field. The first 40 lines, 20 from each field, are reserved for retrace, synch, and other non-image information, leaving 485 lines. The frame around the picture tube cuts off a few more (cropping), and if the video includes closed captioning, line 21 of each field is used.  This reduces the number of visible lines to approximately 480 - give or take a couple of lines.

Here is info from RECOMMENDATION ITU-R BT.656-4 (Digital Television - which uses the same screen scan lines technology and the same broadcast signal), that shows there are indeed 20 lines of vertical retrace per field: