How CRT tv works

You have seen how bulky and heavy old TVs or computer monitors were several years ago. You may even have one in your artic. However, today things are very different, you tv or computer monitor is a lot thinner and even be stuck on the wall. In the history of Tv, CRT tv took a lot of space and required considerable man power to move around. Here we are going to look at how those CRT tv with elongated back used to work.  since you know examples of CRT monitors look like, it is time to take a look at how CRT tv works. However, we shall discuss what cathode ray tube tv are.

What is a cathode ray tube tv

A cathode ray tube tv ( or a CRT tv) is a tv that uses cathode rays to create images on your screen. Such tv is created using a single or multiple cathode ray guns and fluorescent screens where your image is displayed. The Cathode ray tv  are dangerous because of the high level of radiation.

The glass tubes are coated with 2 to 4 kg of lead. The screen where your image is displayed is coated with high quantities of phosphor dust.  Some well cited researched also proved that manufactures used to add some traces of mercury in the device.  It is wise to recycle them instead of just throwing them away. Now lets take a look at how each of the components work.

Cathode ray tube (CRT).

Understanding cathode ray tubes is detrimental on understanding how CRT tv works. Let us start by defining the Cathode ray tube ; it is a type of electronic display screen that creates images from a video signal. It is a vacuum tube that has a phosphor screen where images are created once the electrons hit it.

Cathode ray tubes have a distinctively designed funnel shape.

How CRT tv works
The first CRT ever created and patented

What made those old tv long was the cathode tube that they used to create images on a phosphor screen. The as the name suggestively inform us, cathode rays or electrons were ejected from a surface and then projected on your phosphor screen. Here is how cathode ray tube works. The components of a cathode ray tube are electron gun and a phosphor screen.

Electron production

The process starts with production of electrons from the electron gun. The generated electrons are highly focused and then accelerated by high voltage. At the end of the vacuum tube is phosphor screen that grows when the high velocity electrons hit it. It has to be a vacuum tube so that electrons can move with no interference from air molecules and dust particles.  The image below simplifies who the CRT tube works.

Figure 2How the CRT tube works

Between the electron gun and the phosphor screen, there are several pairs of electrostatic deflection plates. The deflection plates have high voltage applied across it so that it deflect the high velocity electrons. At a carefully selected point, there has to be at least two pairs of electrostatic deflection plate for shorter tube. One pair of the plate is to deflect the electrons sideways (horizontally) while the other pair is used to deflect the electrons up and down (vertically). However, since the horizontal and vertical movement of the electrons is independent of each other, a pair deflection plates can be place anywhere along the tube.

The electron guns

You are right to think The electron gun  is where the electrons are generated in the CRT. Its assembly consist of accelerating anode, heater, focusing anode, cathode, pre-accelerating anode and grid. Electrons are emitted by the cathode, a cylindrical rode that has its end coated with barium oxide and strontium. The two coating compounds causes high emission of electrons. Electron gun is placed at the back of the tube. The other name for the gun is the cathode since it fires electrons only. Now we are half way through understanding how CRT tv works.

Electron acceleration

The freshly produced electrons are made to pass through a small grid that is made up of nickel material which has a hole.  During the design, the hole in the nickel material is centrally placed where it is coaxially with the CRT axis. This is a control grid and its positively charged. The pre-accelerating and accelerating anodes are also positively charged. To accurately focus the beam, focusing anode is used.

The accelerating and focusing the electrodes are shaped in a cylindrical manner and has a very small opening at their centres. Once your beam of electrons passes through the focusing and accelerating electrodes, it now enters the deflection region.  It does not matter whether you have the vertical or horizontal deflection plates first; you can place them in whatever order that pleases you.

Caution

You have heard it dangerous to open your CRT tv if you no knowledge? Yes, you should not try open it even when it not connected to power. The pre-accelerating and accelerating plates are each connected to 1500V. this voltage is high enough to send you to a hospital.  The focusing electrodes are connected to 500 V, still high enough to have you hospitalized.

How CRT tv works

3D image of the CRT

Electrostatic deflection plates

Each pair of deflection plate is able to produce uniform field of electrons in whatever direction the plates are oriented.

The phosphor screen

At the far end of the CRT is a square screen that is made up face plate. The screen is made up of fiber optics that have special attributes. The internal side of the screen is sprayed with a uniform coat of phosphor crystals.  The coating is meant to create a phenomenon known as cathodoluminescence; where electrical energy is converted into light energy.

This type of light you get to see once electrons hit the phosphor screen is called fluorescence.  When the power is turn off ( no more electrons produced) the phosphor crystals regains their originals state and in the process a quantum light is produced; the quantum light energy is referred to as the phosphorescence or persistence.

Aquadag

During the production, focusing, deflection and transmission of electrons, there are secondary electrons that produced as well. To remove such electrons, an aquadag is used. It is a sea of aqueous solution of graphite. This is done to ensure the tube is at electrical equilibrium.

How the CRT tv works

Now we already know how the CRT screen is made to emit the light.  We shall now take a look at how the tv works.  Your CRT tv do not paint all the image at once on the screen. However, the image is painted in sections.  For most of the CRT tv, image painted started on the top of the screen moving downward each pixel at time. In the figure below, the solid lines represent lines that beam of electron paints from right to left or vise vasa.

Once a line has been completed, the beam moves back to right (assuming paint starts from right to left). When the beam has completed covering the whole screen, the process starts all over again. This is done 60 times per second.  The vertical movement of the beam is referred to as vertical retrace and the horizontal movement of the beam is referred to as horizontal  retrace.

On a typical tv, there are slightly less than 500 such lines.

 

How images are created on the screen is process that has to be accurately controlled using signals.  The signals are received from an antenna or a video player. The antenna receives the radio signals from the source. Today, antennae are hardly used since people either uses satellite dishes or fiber optic cable if you are using a cable tv.

Three electron guns

If you have a color tv, it has three electron guns one for each of the primary colors, red, blue and green. However, the initial CRT tv were black and white only so only one electron gun was used.

Interlacing

A technique called interlacing is used to paint an image on your screen. This means that an image, motion one or static one, is painted 60 times each second. The beams paints the lines on the screen from top to bottom. However, each time painting is done, only half of the lines are painted. we can simplify it this way, during the first of 60 painting, only the odd number lines are painted. then on the second of 60 painting the rest of the even number lines are painted. so, you will have the odd number lines painted 30 times every second and so are the even number lines. It is described as horizontal interlacing.  It is possible to have the lines be drawn from top to bottom while moving left to right. That is, the lines are drawn vertically instead of horizontally. This is called vertical interlacing.

There is another technique of painting the horizontal lines called progressive scanning. Unlike the interlacing, progressive scanning paints all lines 60 times every second.

Black and white tv

you have to understand that there are two classes of CRT TVs, the colored tv and black & white tv. Before we are done on how CRT tv works, lets are discuss these two categories.

The signal has pictures and sound from station of your choice. The signals is split into picture signal and sound signal. You should know that instead of anodes, CRT tv uses magnetic coil to deflect the electrons in a method known as raster scan pattern. This pattern has the image painted left to right from top to bottom.

When you are using a video signal from a DVD player, the signal is transmitted to the tv using the yellow jack of the RCA from the VCR input. The horizontal retrace signals are created in pulses where each gap between a rise and a fall is 5 microseconds long. The pulse alternates  from 0 v to 0.5 volts or 2.0 volts. When the signal processors detects a 0.5 volts, it create a black spot, when a 2.0 volts is detected a white spot is created. The bandwidth of black and white tv is roughly 3.5 MHz.

Color tv

There are three ways that a color tv is different from a black and white tv. Firstly, there are three electron guns in a color tv while there is just one in the black and white tv. Secondly, there is an addition component of the screen called shadow mask.

The mask has precisely perforated small holes that align with the phosphor dots called pixels. And finally, the screen is coated with three phosphor sheets instead of just one sheet. The three phosphors are green, blue and red. The dots can be seen under magnification once you monitor or tv is turned on.

A white light is produced by firing the three beams together. when creating a blue light, the blue beam has to be fired. This goes for the rest of the two colors.

Signals for color tv

It is just like a black tv signal only that there is an extra chrominance signal superimposed on it. Instead of having a bandwidth of just 3.5MHz, an additional 3.579545 MHz signal is added. This is added to add color burst.

The chrominance indicates the color that need to be displayed where its amplitude determines the color saturation.  This is an oversimplification of the whole process. It is complicated because each color has to be displayed in its unique way.

All CRT tv were analogue tv. You need to know that digital tv decodes signal is a slightly different way. You also ought to know that cable and satellite tvs work in a different manner. Now that you know how CRT tv works, lets take a look at the modern television sets.

Modern tv

Today, technology allows your tv be very large, as large as 75 inches. The old CRT tv hardly exceeded 29 inches at best. They were small and their design encouraged power wastage.  Today with have the LCD, plasma tv, OLED tv and LED tv. While the signal decoding may work in a similar manner for all digital tvs, LCD, plasma tv, OLED tv and LED tv uses different techniques to display images that you see on your screen.

All the types tv mentioned above are an improvement on the CRT tv. In general, how CRT tv works is the same principle used by modern tv sets. However, they use a light source instead of electron beams. There are screens that you can place on your wall like a sticker, the OLED screens. They are translucent, foldable and most power efficient screen of them all. Its disadvantage is that it very expensive to produce and hence they are prohibitively expensive.

 

 

 

 

 

 

 

 

 

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