How the Monochrome CRT TV Works!

1

Unique qualities of the device

The monochrome TV was firstly the most commonly used TV in the world, it supported only black and white pictures and had very limited channels of around UHF (compared to those today), for their time this was a lot. This TV was made available to the public around the 1950s, where it was electromechanical to begin with then moved to being a purely electrical device. This type of TV uses the CRT technology, however before this there where electromechanical TVs. Here we will focus on the most common type which was the CRT monochrome TV.

Features & Functionality

 

A black and white TV screen is coated with what’s known as a white phosphor, then the electron beam produced by the cathode ray tube and magnetic coils. This beam produces the reaction from the white phosphor as the electrons are shot at it one line at a time. To do this the magnetic coils change the direction of the electrons as they are emitted to create what’s known as a “raster scan pattern” across the screen.

 

The electron beam will shoot lines of electrons expanding across the width of the screen, dividing it up into layers. The figure below illustrates this:

45

 

The beam moves from left to right very quickly, with its intensity changing very quickly to create the contrast ratios on the screen between black and white. Because the lines are arranged so close together each TV will have around 480 lines creating the whole picture.

The common monochrome TVs used a technique known as interlacing when emitting a picture where the CRT will refresh the TVs lines at 60 frames per second, however doing this the TV will only construct half a line per frame, then the next half in the next frame. As these mare moving so fast the human eye cannot detect this function so it is plausible to use it when producing images for the user.

An alternative to interlacing is something called progressive scanning, which paints every line at 60 times per second, most computer monitors used this feature as it reduces the flicker you would normally see. Due to the screen paining around 525 lines 30 times a second (with a total of 15,750 lines per second) this creates the high pitch sound you hear when the TV is on.

How the TV produced the image was that the electron beam paining these lines would react with the phosphorus on the screen, this excites the phosphorus as it gives them enough energy to emit light. White phosphorus only glows bright white when electrons excite it, this brightness can be modulated to the signal that the TV is receiving so the electron beam can switch from a high or low intensity electrons to be fired at the white phosphorus.

3

 

The white phosphorus glowing in stages of brightness will form contrast ratios between lines on the screen, these levels of contrast ratios produce the picture you could see upon the picture.

This type of TV also has what’s known as the burn in picture, this is a serious drawback. This happens when a single image is left on the screen for a long period of time, the phosphorus would begin to burn and lose their ability to illuminate, and creating a shadow of the burned image forever forged into the screen, or sometimes it will just go a dark black colour when seriously burned. This will eventually spread across the screen until it is unusable and needs to be replaced by the user, this effects both colour and monochrome CRT TVs.

 

The most defining functions of the monochrome TV are:

  • Only 1 electron beam was used to produce the picture, as there is only levels of brightness which can be utilized.
  • Only a single layer of white phosphorus is used on this type of TV screen, which only allows it to produce black and white images.
  • There is no shadow mask located inside of the of the monochrome TVs

 

Colour CRT TV

4

 

Unique qualities of the device

 

The colour TV was first introduced to the market in 1960s and took off with a very large market, as this was a totally new way to broadcast TV shows more efficiently and was much more durable than the colour electromechanically driven TV. These TVs are still being sold even today however the market is much smaller as they are an old technology.

Features & Functionality

 

This TV has the same operations as the monochrome CRT TV as it uses the same technology, with some minor differences:

  • Three electron beams are used here, one for each layer of the TV
  • The screen is coated with three layers of phosphorus which will emit three different wavelengths of light when excited by the electron beams, which are red, blue, and green lights.

    How this works is each electron beam will fire at the different area where the layers are located on the screen, these layers are tightly compacted into what’s known as pixels. The three electron beams will hit these different colour layers in the phosphorus and just like the black and white TV will force them to emit different contrasts of that colour so it creates a new colour.

  • Inside of the tube close to the phosphor is a thin metal sheet called the shadow mask, this mask is designed to spate the colours in the phosphorus, and has many tiny holes the electrons can pass though.

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s