The purpose of the analog video signal is to display moving pictures onto a video screen. Todays flat screen monitors and TV sets are quite different from the CRT displays from the past but the CRT and it’s circuit’s capability defined almost all of the parts of the analog video signal. Today’s digital video is based on the analog video signal parameters to make it compatible with existing systems. So by knowing how analog video worked and why it worked will help in understanding the digital video signal.

 

Introduction to Television

In this first chapter we cover television sets and television monitors and how they produce a picture on their screens. The effect of the electrical circuits, inside the television set to produce the picture on the television screen will be gone over. Even though we will avoid going into the details of the electronics it would be very helpful to have a basic understanding of electronics in general.

 

TELEVISION SETS AND MONITORS

First lets describe the differences between a television set and a television monitor. First a television set has a tuner for receiving over the air broadcasts, this is what you are adjusting when you change channels. The tuner receives the video and audio signals that are transmitted by the television station. Television sets also have a demodulator to turn the received video and audio signals into what is called base band signals. The audio signal is sent to an audio amplifier and 
from there to a speaker. Where as the video signal is sent to the video circuitry for display on the television screen. Some television sets will accept audio and video inputs from connectors on it's back, these would be called television monitors.

 

A television monitor only has video circuits for displaying the television picture on its the screen. It accepts the base band video input and may be able to select between 2 or 3 different video inputs.

Although displaying the video signal is the only function a monitor performs it does it very accurately, generally a television monitor is capable of displaying a much higher quality picture with truer colors than a television set.

 

 

 

 

 

Television monitors receive the video signal from a connector on the back of the monitor. This means that to use a television monitor you must also have something that generates a video signal that you can connect to the monitor ( like a DVD Player, a video camera or a separate tuner with a video output ). Since we are only concerned with video for now we will only describe how a video monitor operates.

 

 

HOW A TELEVISION MONITOR WORKS

The cathode ray tube is the device that actually makes the light variations on its screen that we see as a television picture. After I describe the actions that take place inside the cathode ray tube you will be better able to understand the reasons for the different component of the television signal that will be described later on. The reason I am starting with how your television set works to reproduce a picture instead of a television camera (where most television signals begin ), is that most people are more familiar with television sets (and television monitors) than television cameras . I think it will be easier for most people to understand something that she or he is already familiar with.

 

CATHODE RAY TUBE

The CRT ( Cathode Ray Tube ) consists of a glass envelope with a vacuum inside. It has a thin neck at one end and a wide flat screen at the other. Inside this neck is the "ELECTRON GUN ASSEMBLY" and the inside of the screen is coated with "PHOSPHOR". Attached to the outside of the neck is the "YOKE". All of these components functions will be described in this chapter.

PHOSPHOR

Starting with the Phosphor on the inside of the screen, it is a substance that reacts to an electron (electrical) charge. When an electron charge makes contact with the Phosphor the Phosphor emits light. The more electrons (and thus the electrical charge) that come in contact with the Phosphor the brighter the light is that's emitted. This Phosphor is what actually creates the light you see on the screen of your television set. The Phosphor is connected to the high voltage anode (connection) on the outside of the CRT.

 

ELECTRON GUN ASSEMBLY

The electron gun assembly is where the electrons that strike the phosphor come from. The gun assembly consists of a heater that is used to heat a piece of metal called a cathode. This cathode is covered with a substance that has an abundance of free electrons (electrons that can be broken away from their atoms). When the cathode is heated the electrons are more easily freed.

 

 

 

SCANNING

Inside the CRT the electrons we are using to make the Phosphor emit light are made into a very tight beam. This tight beam of electrons will only strike a very small area of Phosphor at a time and by moving it we could use it to write or draw on the inside of the screen thus creating an image of light on the face of the CRT.

 

To move the beam around the screen the Yoke is used to deflect the beam with its magnetic fields. The Yoke is made up of two electromagnets, one for horizontal and one for vertical movement.

 

 

 

 

 

 

In order to cover the entire screen with the electron beam it is swept across it in a zig- zag pattern. In a short period of time the entire screen will be covered by the beam moving in this zig pattern. After the electron beam has passed the Phosphor's light starts to fade, the Phosphor has to be constantly refreshed ( re-scanned ) in order to keep a picture on the screen.

 

While the beam of electrons is sweeping across the Phosphor on the inside of the screen the number of electrons in the beam can be changed so that the brightness of the light coming from the screen changes along with the number of electrons. So as the electron beam is swept across the inside of the CRT's screen the number of electrons in the beam keeps changing and as this happens the brightness of the point on the screen that the electron beam is striking changes along with it. This means that all the points on the screen can each have a different brightness level. So that when you see all these points together on the screen they form a television picture made up of black, white and gray areas.

 

You could think of it as if you were moving a pencil across a sheet of paper, from left to right and back again. At the same time you also kept each line close together as your hand moved down the paper. This way you will eventually cover the entire sheet of paper. And as you were making this zig-zag pattern on the sheet of paper you could either press down hard for a darker line or you could press more lightly for a lighter or grayish line or even lift the pencil for no line at all. In this same way the number of electrons in the beam control the brightness produced on the screen. By putting either the maximum number of electrons into the beam for the brightest parts of the picture or some lesser amount for the gray parts of the picture or turning off the electrons to produce black on the screen.

 

 

SOME OF THE DETAILS

The beam of electrons we have been talking about comes from the " ELECTRON GUN ASSEMBLY " in the neck of the CRT. The electron gun has a heater, cathode and control electrodes (grids) just like a normal vacuum tube. The control electrodes work to focus the electrons into a very small beam. This beam of electrons is attracted to the screen because the inside of the screen is charged with a very high voltage ( black & white CRT's use about 12,000 volts and color CRT's use about 24,000 volts ). With this large voltage difference the electrons, from the electron gun assembly, are attracted to the screen and thus to the Phosphor.

The Yoke is attached to the neck of the CRT on the outside and it serves to deflect the electron beam across the screen by using two electromagnets. One electromagnet is aligned so that it will move the beam back and forth horizontally while the other one is aligned to move the beam up and down vertically. These two electromagnets work together to cause the electron beam to cover the entire screen.

 

The Yoke is controlled by the sweep or scanning circuits inside the monitor. These circuits supply two different voltages to the yoke that causes it's magnetic fields to change. One of the voltages is connected to the horizontal electromagnet and the other is connected to the vertical electromagnet. These voltages are constantly changing which keeps the magnetic fields coming from the yoke changing and thus keeps the beam of electrons moving. The sweep circuits must keep the electron beam accurately positioned on the screen at all times in order to produce a high quality television picture. To do this the sweep circuits receive synchronizing information contained within the video signal itself, but this is covered in the next section.

 

As the scanning starts, the horizontal electromagnet moves the beam from left to right and back again very quickly, over 15,000 times a second. At the same time the vertical electromagnet pulls the beam down from the top of the screen and then returns it back to the top, slowly, just 60 times a second.

This causes the beam to make a zig-zag zigzag pattern on the screen. Because the vertical electromagnet pulls the beam down much more slowly, than the horizontal electromagnet all the horizontal lines on the screen are scanned very close together. So close in fact that you cant see that they are separate lines unless you look very closely at your television screen.

 

HORIZONTAL LINES AND VERTICAL FIELDS

One sweep of the electron beam across the screen from left to right and then back to the left side where it started from is called a " HORIZONTAL LINE " and the electrical signal that causes this to happen is also called a HORIZONTAL LINE. A complete sweep of the screen from top to bottom and then back to the top again is called a " FIELD ". It is called a field because within a sweep of the screen from top to bottom there are many horizontal lines being swept across the screen from left to right and back again, this makes for a FIELD of horizontal lines.

 

To get the maximum use out of each line most of the time that the line takes up is used for picture information and a very small amount of time is spent on returning the beam back to where it started. This last step is called HORIZONTAL RETRACE time and the electron beam is turned off during the retrace time so you wont see that retrace line on the screen. The same is true for the vertical scanning, in that, when the beam reaches the bottom right corner of the screen the electron beam quickly returns to the top right corner of the screen with the electron beam turned off so you wont see it.

The time it takes for the beam to reach the top left corner of the screen while the beam is turned off is called VERTICAL RETRACE time. The time it takes for the beam to scan from the top of the screen to the bottom and back to the top again is called a VERTICAL FIELD. When the electron beam scans the next field each horizontal line is positioned in the space between the lines of the previous field. So, within the time it takes to show two vertical fields you will see twice as many horizontal lines, all of which are filling in the spaces left between the lines from the field before. Each whole television picture is scanned two times, first all the odd numbered lines are scanned in one vertical field, then all the even numbered lines are scanned in the next vertical field. After the even lines are scanned the beam goes back and starts with the odd lines again.

 

This method is called "INTERLACED SCANNING" because of the way the lines from one field are fitted in between the lines from the field before it. When two fields are shown one after the other like this we call them a "VIDEO FRAME". One frame of video has within it all the information needed to make one complete picture and one field has exactly half the visual information needed to make a frame. The main reason the picture is segmented like this, with 2 FIELDs making a FRAME or a whole picture, is so the number of pictures shown per second can be great enough that the picture on the screen won't appear to be flickering. In other words it is done this way so you don't notice that there is a completely new picture being reproduced on your screen every sixtieth of a second. Because the top of the screen is growing darker as the beam of electrons is scanning the bottom of the screen, and vice -verse. The people who developed this system decided to break the picture up into 2 halves ( odd lines and even lines ). Then you have twice as many scanning from the top to the bottom of the screen per second and twice as many pictures on your screen per second.

Even though each field has only half the visual information needed to make a complete picture ( a FRAME ), you don't notice it because the fields are shown one right after the other. And your eyes retain the pictures for a moment so you see only a complete picture. This also helps movement on the screen to look like continuous motion and not a series of still pictures ( which they are).

 

COLOR TELEVISION SYSTEM

In order for you to understand how a color television monitor produces color pictures on it's screen you must understand how colored light can be added together. This is known as a " ADDITIVE COLOR SYSTEM ". If you shine two different colored lights on a white background the two colors will add together and create a new and different color. Now if you were to change the intensity of the lights independent of each other you could change the color being projected on the white background by adding more of one color and less of the other or vice -verse. By adding a third color to this system you can get even more colors by adding them together in different amounts.

 

This is what is known as an additive color system and the same principals are used in the color television system. Color television uses "RED", "GREEN" & "BLUE" as our three colored lights. These colors are called the "PRIMARY" colors because by using them together they can make almost any color, but no combination of these three colors will make any one of the original primary colors.

 

It is important to remember that the television screen is a source of light and is not reflecting light like most things we see are. In order to make color pictures we need to change the phosphor on the inside of the screen from one that only produces white light and put three different colored phosphors on the inside of the screen. Then when we excited them with three different electron beams so that they each produce light, the three colored lights will combine to make one color as they reached our eyes.

To excite the three different colored phosphors there are three electron guns in the neck of the CRT, one for each color. To change the intensity of the individual colors the number of electrons in the beam that corresponds to that particular colored Phosphor must change.

These three different colored Phosphors are grouped together in small clusters of three. Each cluster contains all three colors and is considered a single point of light since it takes all three of the colors to make any one color.

 

These clusters are arranged all over the screen, there are many thousands of these clusters on the screen. The individual points of Phosphor making up the clusters are very small. From a distance you see the picture as a whole but if you look very closely at the screen you can see the individual points of color when they are on.

Now to prevent the three electron beams from hitting the wrong colored Phosphor as they are continuously moved across the screen a "SHADOW MASK" is placed very close to the Phosphor on the inside of the screen. This shadow mask covers the spaces in between the groups of colored Phosphor. For example, as the three electron beams are moved across the screen the shadow mask prevents the red electron beam from striking any other colored Phosphor than the red Phosphor, the same is true for the other electron beams as well. The Yoke of the color CRT moves the three electron beams across the screen just like it does in the black & white CRT.

 

IN REVIEW

The television picture is seen on the screen of the CRT. Covering the inside of the screen is a substance called Phosphor which when struck by electrons emits light. In the neck of the CRT is the electron gun assembly which emits the beam of electrons, the beam is attracted to the Phosphor on the inside of the screen because of a very high voltage which is connected to the inside of the screen. As the beam of electrons strike the Phosphor it emits light and and as the number of electrons in the beam changes the intensity of the light coming from the Phosphor changes along with it. To make a picture on the screen the beam of electrons is swept across the entire screen. The beam covers the screen in a zigzag pattern, from left to right and from top to bottom. This is accomplished with the use of two electromagnets which deflect the electron beam just as it leaves the electron gun assembly. There is one electromagnet for moving the beam horizontally and one to move it vertically, together they are called the Yoke.

A complete scanning of the screen from top to bottom is called a FIELD and this makes one half of a complete television picture. When the next FIELD occurs it completes the picture and the two FIELDs are called a FRAME. The two fields can make a complete picture because your eyes retain the information from the first field for a moment while the second field is being scanned on the screen. Using two fields to make a frame or a whole picture is called Interlaced Scanning.

For color television there are groups of three different colored phosphors covering the inside of the CRT's screen. In the neck of the CRT there are three electron gun assemblies, one for each colored Phosphor. As the number of electrons change in each of the beams the intensity of the light coming from the Phosphor the beam is striking changes along with it. The colored lights coming from the three phosphors mix together as they reach our eyes and we see the result of this mixture as a single color. The three primary colors are RED, GREEN & BLUE. By mixing these colors together in different amounts you can create almost any color.

 

Covering the Phosphor on the inside of the screen is a shadow mask, it covers the Phosphor but it has lots of tiny little holes in it to let the electron beams through to strike the Phosphor that they are suppose to. As the three beams move across the screen the shadow mask prevents any of the beams from striking any of the colored Phosphor until the three beams are lined up to strike only the particular colored Phosphor they were intended to. The yoke on a color CRT works in the same way as it does on a black & white CRT to move the three beams across the screen and make a series of color image that we see as moving pictures.

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