How Television Works – Lights, Camera, Action!

Many people simply take their television for granted. They just assume that when they turn it on, picture and sound appear like magic. When the channel gets snowy, they hit the side of the television to get it working again.

Amazingly, sometimes, it does.

But have you ever wondered how your favourite television program gets to your house from a broadcast station thousands of miles away? How about from a satellite dish? Just what is this technology that spends so much time entertaining and enlightening us that it could be considered a part of the family?

Take a minute to read this article, and what you find out about your television may surprise you.

The Heart and Soul of Your Television

At the center of the standard projection-style television set is the all-important Cathode Ray Tube (CRT). You may have heard of a CRT before, but what exactly does it do? The answer to that is surprisingly simple and amazingly complex at the same time.

In science, the term “cathode” means a negative terminal. The opposite of a cathode is an “anode”, which means negative terminal.

In a CRT, the Cathode is a heated filament that creates electrons. The Ray is the stream of electrons that emanate from the cathode filament, and the Tube is the large glass vacuum (otherwise known as your picture screen) that houses the entire thing. It’s that simple.

Now it’s time for the complicated part.

The cathode, or negative-charged heated filament, creates negative charged electrons in a stream (the Ray part of the CRT, remember?). These electrons are compacted by a focusing anode terminal and an accelerating anode terminal, which causes the electron stream to be compacted into a tight beam travelling at a high rate of speed.

This electron beam is then directed at the inside of the flat screen (your viewing screen) at the opposite end of the tube. The flat area is coated in a thin layer of phosphor, a substance that lights up when it is struck by the electron stream.

The inside of the picture tube is also given a thin coating of conductive material, ensuring that all those electrons have somewhere to go after they hit the phosphors.

Now, if this was all there was to your CRT, all you would see when you turned the television on would be a tiny little circle of light, much like that tiny light in the center of old television sets when you turn them on or off. The electron beam needs to cross every pixel of phosphor on the screen to show an effective picture, and it does this with the help of steering coils.

A steering coil is a series of copper wires wrapped around the CRT to make an electromagnet. These electromagnets can create two separate electromagnetic fields within the CRT, one vertical and the other horizontal.

The electrode beam can be manipulated by changing the strength of these electromagnetic fields because it carries a negative charge. Using an X and Y axis style grid, the beam can be moved to point at any part of the picture screen.

Monochrome

For the first 50 years of television, the most common television sets in people’s homes were black and white sets. This was because black and white was the technology standard of the day, while colour sets were unreliable at best. During the 1940s and 1950s, even if one could afford a colour television, it was disheartening to turn it on and realize that most broadcasts were black and white transmissions only.

This is not the case today. In the age of High Definition Television (like Sony HDTV) and digital broadcasting, black and white television screens have been relegated to mostly cheap security video monitors and camping televisions.

There may always be a market for black and white or monochrome television technology simply because it is so easy use (by television standards, at any rate).

The CRT in a monochrome television is pretty much the same as a colour television, but with a few important exceptions. Where the phosphors of a colour set are bunched in groups of blue, green, and red, the phosphors in a monochrome set are all white. That means that when an electron beam is directed at them they give off white light. To get black, the phosphors are simply switched off.

This balance of white phosphors and black ones are able to create the many soft “greys” of a monochrome set. By manipulating the amount of light the white phosphors give off and which phosphors are turned off, a monochrome CRT can reproduce a perfect black and white version of any image.

Colour

In a colour CRT, things work just a little differently. Instead of a single electron beam, as in a monochrome CRT, there are actually three beams. Since the phosphors are grouped into blue, green, and red pixels (take a close up look at your television screen and you can see them), the colour CRT is constructed to direct beams at each group.

By varying the intensity of the three beams or turning one or more of them off completely, thousands of colour schemes can be achieved. When all three of the pixels are struck simultaneously with the same level of power, then the pixels mix to form white light. When they are all shut off, a black effect is achieved.

In both colour and monochrome CRTs, the phosphor pixels are separated by a device known as a shadow mask. A shadow mask is simply a filter devised to keep the electron beams from spilling into other pixel groups. A shadow mask is like a metal plate covered with rows of tiny holes. Each tiny hole lines up with a pixel grouping.

And They’re Off!

In standard televisions, these beams race across the picture screen, lighting up and combining phosphor colours as they go. They travel in a straight line from the left side of the screen to the right side of the screen, and when they reach the end they race back to the left side. Only this time they start at a slightly lower position, right underneath the original line.

This is known as a “Raster Scan”, and it continues in this pattern all the way down to the bottom of the screen. Then it goes back to the top left corner and starts again. When the beam is activating phosphors in a line it is on, and it switches off when it starts a new line or begins another pattern. The left to right movement is known as “horizontal retrace”, while the movement from the bottom to the top of the screen is known as “vertical retrace.”

Because these lines are tiny and spaced so close together, your brain blurs them into a single image.

The average television uses an interlacing technique where every other line is painted first, and then the remaining unpainted lines are completed on a second sweep. In this way the entire screen is covered in two sweeps and is completed 30 times a second.

Another pattern, known as a “Progressive Scan”, paints each line one after the other with no spaces, and completes the screen 60 times a second. At these speeds, your brain doesn’t see a series of still images spaced fractions of a second apart.

It sees a uniform, moving image.

As a side note, that high pitched whine that some people hear coming from their television is actually the broadcast signal, moving the electron beam across the screen at over 15,000 times a second!

Tune in and Drop Out

Ok, so you see how a picture is recreated in a Cathode Ray Tube, but how does it get to your television set?

Television broadcasting is based on a signal, much like radio. However, a monochrome television signal is comprised of three pieces, and a colour signal is comprised of four.

The different pieces of a television signal are:

Intensity Information
Intensity information is used to tell the television how bright or dark the phosphors need to be at a given point.
Horizontal Retrace Signals
Horizontal retrace signals tell the television when it is time to move back to the left side of the screen after is has completed a line.
Vertical Retrace Signals
Vertical retrace signals occur in the broadcast signal 60 times per second, and tell the television to move the beam back from the bottom-right side to the top-left side so it can paint a new image.
Chrominance Signal
A Chrominance Signal is added to a monochrome signal to add colour to the image. Essentially, it is a separate frequency that tells the television how to modulate the electron beams so that they will produce the right colour combination for each pixel grouping. This part of the signal is ignored by old monochrome televisions.

So, if you take these four parts, add another frequency modulation for sound, and a vestigial sideband, and you have a full television signal. Your television takes in all that information, interprets it, and turns it into your favourite programs. Sounds like a lot of work, right?

Now think about that effort going into all those annoying commercials you see on television. Sheesh!

This is the basic principle behind image reception for televisions, and all the signals you receive basically operate in this same fashion, from using an antenna to pick up the Saturday night hockey game to receiving satellite broadcasts on your high-definition widescreen television set.

A High Definition Future

The television industry has been lit up in the past few years with a growth of new broadcast and reception ideas. As we speak, high definition television and digital surround sound technology are quickly becoming the norm. With every passing day new markets and broadcast opportunities are opening up across the globe.

The advent of the internet is also doing for television what it has done for computers. It is a very real possibility that one day soon you will use your television set to watch sitcoms, rent movies or sporting events, do your groceries, check your mail, do on-line banking or surf the internet, or even order a pizza for supper.

The possibilities are endless. That may be the ultimate genius of television. More than just a tool for couch potatoes, it could one day be an integral part of how society runs.

Of course, if you don’t like it, you can always turn your television off.

For now.

About The Author

Bill Schnarr is a freelance writer for hire providing valuable tips and advice for consumers purchasing HDTV reviews and ratings, HDTV receivers and home theater systems. His numerous articles offer moneysaving tips and valuable insight on typically confusing topics.

This article on "How Television Works" reprinted with permission.