Monochrome CRT

Article No: 3

By far the most common type of monitor used with multimedia desktop systems is based on the picture tube called the Cathode Ray Tube (CRT). It is essentially a vacuum sealed glass tube containing two electrodes inside it, the cathode (K) or negative electrode and the anode (A) or positive electrode.

The front face of the tube is coated with a chemical compound called phosphor arranged in the form of a rectangular grid of a large number of dots. The material phosphor has a property of emitting a glow of light when it is hit by charged particles like electrons. The phosphor dots called pixels, short for picture elements, are responsible for producing an image on the monitor screen. Other positive electrodes are called grids (G1, G2, G3) are located near the cathode. Near the neck of the CRT are two coils of wire known as the deflections coils. Electron beams generated from the cathode are made to hit the phosphor dots generating spots of light and thereby producing the image.

Figure: Monochrome CRT

A CRT capable of producing a black and white image on the screen is called a monochrome CRT. A high voltage of the order of 18KV is maintained between the cathode and anode. This produces the beams of electrons, known as cathode rays, from the cathode and anode. The beam of electrons is controlled by the three other positive terminals. The control grid (G1) helps to control the amount of electrons in the beam and thereby determine its strength, the accelerating grid (G2) provides acceleration to the electrons in the forward direction and focusing grid (G3) focuses the beam to a single point X on the screen ahead, so that the diameter of the beam is equal to the diameter of a single dot of phosphor.

As the beam hits the phosphor dot, a single glowing spot of light is created at the center of the screen. This spot of light is known as a glowing pixel. When current flows through the deflection coils, the electrical field produced interacts with the electron beam thereby deflecting it from its original path to the another point Y. One of the coils called the horizontal deflection coil moves the beam horizontally across the screen and the other coil called the vertical deflection coil moves the beam vertically along the height of the screen. When both these coils are energized in required proportions the electron beam can be moved in any direction thus generating a single spot of light at any point on the CRT screen.

Raster Scanning

To draw an image on the screen the electron beam starts from the upper left corner of the screen and sequentially moves over each pixel row from left to right. This is referred to as forward trace. During the phase the electron beam is sometimes switched ON and sometimes remains OFF. When the beam is ON, the pixels over which the beam remains OFF do not get a dose of the electrons and therefore remains dark. At the end of each horizontal line the beams get switched OFF and retraces diagonally to the beginning of the next row. This phase is referred to as horizontal retrace. The switching OFF saves power and avoids activation of unwanted pixels. At the beginning of the next line it is again to the switched ON and begins the next trace. The process continues until the beam reaches the lower right corner of the screen, after which it is again switched OFF and moves diagonally back to the starting point. This is referred to as vertical retrace. The entire process from beginning to end is called raster scanning.

To display an image on the screen, as the electron beam moves over the pixel rows, some of the pixels are turned ON or activated while the other pixels remain OFF or deactivated. Activated pixels are those over which the electron beam is switched on and therefore these pixels emit a glow of light. Over the other pixels the electron beam remains switched OFF and so these pixels remain dark. The image is recognized on screen by the pattern of glowing pixels over a dark background of deactivated pixels.

Frames and Refresh Rate

The electron beam is said to produce a complete frame of picture when starting from the top-left corner it moves over all the pixels and returns back to the starting point. The human brain has the capability to hold on to the image of an object before our eyes for a fraction of a second even after the objects has been removed from before our eyes. The phenomenon is called persistence of vision.

As the beam moves over each pixel, the glow of the pixel dies down although its image persists in our eyes for sometimes after that. So if the beam can come back to the pixel before its glow has completely disappeared, to us it will seem that the pixel is growing continuously. It has been observed that we see a steady image on the screen only if about 50 – 60 frames are generated on the screen per second, i.e. the electron beam should return to its starting point within 1/50 th or 1/60 of a second. The monitor is then said to have a refresh rate of 50 or 60 Hz. A monitor with a refresh rate of less than 50 Hz produces a perceptible flicker on the screen which is caused by the previous image fading from the eye retina before the next image could be generated. Such flickering monitors should be avoided as it creates a strain on the eye. In practice the frame refresh rate is usually determined by the frequency of the mains electric supply which is either 50 Hz in Europe and 60 Hz in America.

Color CRT

The working principle of a color CRT is similar to that of a monochrome CRT, except that here each pixel consists of three colored dots instead of one and is called a traid. These processed phosphors produce lights of colors red, green and blue (in short, RGB) and are called primary colors. These are so called because it has been experimentally observed that these three colored lights can combine in various proportions to produce all the other colors that we see. Corresponding to the three dots there are also three electron beams from the electrode (also called electron gun), each of which falls in the corresponding dot. As each of the three beams hits the corresponding dot in various intensities, they produce different proportions of the three elementary colored lights which mix together to create the sensation of a specific color in our eyes. Our eyes cannot distinguish the individual dots but see their net effect as a whole. For example, red and blue in equal proportions produce a color called magenta; blue and green likewise produce cyan and green and red produce yellow.

These colors, magenta, cyan and yellow, produced by mixing equal proportions of primary colors, are called secondary colors. All these three colors in equal proportion produce the color called white, their absence leads to the color sensation called black.

A perforated screen called a shadow mask prevents the beams from falling in the gap between the dots. As the electron beams sweep across the shadow mask, it gets heated up due to the electron bombardment on it. Expansion due to heating may disturb the alignment of the perforated holes with the phosphor dots, thereby causing the mask to lose its utility. To prevent this, the mask is made up of a special alloy of iron and nicked called invar which does not expand on heating.

Interlacing

Ordinarily a monitor of a refresh rate of less than 60 Hz, would produce a perceptible flicker on the screen as each frame requires more than 1/60^th of a second to be created. Interlacing is a technique by which monitors of lower refresh rates can be made to produce images comparable in quality to that produced by monitors of the higher refresh rates. To reduce the flicker each frame produced by a raster scan is split into two halves. Each of the halves is known as field. The first field is made up of only odd-numbered pixel rows, i.e.1, 3, 5, . . . and is called the odd field. The second field is made up of even-numbered pixels rows i.e. 2, 4, 6 . . . and is called the even field.

Since each field is made up of only half the total number of pixel rows, generating a field takes only half the duration as for generating frame, i.e. 1/6^th second for a 30 Hz monitor. The image is created on the screen by displaying the first field for half the frame duration and then the second field for another half frame duration such that the pixel rows of each field fit between the pixel rows of the other field. Due to persistence of vision this kind of arrangements leads to a smooth blending of the rows of each field and helps to reduce the flickering effect prominent in monitors of low refresh rates. However it should be remembered that a 60 Hz non-interlaced monitor is always preferable to a 30 Hz interlaced monitor as the former produces a better quality and a stable picture. Modern monitors are mostly non-interlaced and should be preferred over interlaced monitors for purposes related to multimedia.

Monitor Specifications

The following important specifications of a monitor play decisive roles in determining picture quality.

Refresh Rate (or Vertical Scan Rate)

This is defined as the number of frames displayed by a monitor in one second and measured in a unit called Hertz(Hz). Typical values lie within the range 60-70 Hz.  A monitor having refresh rate of 60 Hz is capable to generating 60 frames per second.

Horizontal Scan Rate

This is defined as the number of horizontal lines displayed by the monitor in one second. It is measured as the product of the refresh rate and the number of the horizontal lines on the screen, and expressed in Hertz (Hz). For a monitor having a refresh rate of 60 Hz and 600 horizontal lines on the screen, the horizontal scan rate equals 36 KHz.

Dot Pitch

This is defined as the shortest distance between two neighboring pixels (for monochrome monitors). It is usually of the order of 0.25 mm to 0.4 mm.

Pixel Addressability

It is defined as the total number of pixels that can be addressed on the screen. It is measured by the product of number of horizontal pixel rows and the number of pixels per row. Modern monitors have pixel addressabilities of 640 x 480 or 800 x 600 or higher.

Aspect Ratio

This is defined as the ratio of the horizontal number of pixels (number of pixels along a horizontal row) to the vertical number of pixels (number of horizontal rows of pixels). For a computer monitor or a TV screen the ratio is fixed and equals 4:3. For certain display media like movie theatres and high definition TVs the ratio is 16:9.

Monitor Size

The size of the monitor is defined as the longest diagonal length of the monitor. Standard computer monitors are usually between 15 inches to 20 inches in size.

Resolution

This is defined as the total number of pixels per unit length of the monitor in the horizontal directions. It is expressed in a unit called dots per inch (dpi). Standard monitors usually have a resolution between 72 dpi and 96 dpi.

Color Depth

This is a measure of the total number of colors that can be displayed on a monitor and depends on the total number of varying intensities of the electron beam of a CRT. A monitor with a color depth of 8 bits can display of 2⁸ or 256 colors.

Modern color monitors usually have a color depth of 24 bits implying that they can display upto 2²⁴ or 16.7 million colors. These are also known as true color monitors as these many colors are sufficient to represent the true color of real world objects.

Problem 1

A 15 inch monitor with an aspect ratio of 4:3 has a pixel addressability of 800 x 600. Calculate its resolution.

Let the width and height of the monitor be 4x and 3x.

Since the diagonal is 15” inch in length, we have by property of a right angled triangle,

(4x)² + (3x)² = 15²

Solving for x we get x = 3. So width of the monitor “w = 12”

Resolution r = number of pixels along width / width = 800/12 = 66.67 dpi (or number of pixels in inch)

Problem 2

A monitor can display 4 shades of red, 8 shades of blue and 16 shades of green. Find out the color depth supported by the monitor.

Each pixel is capable of displaying 4 x 8 x 16 or 512 colors.

To display 512 different colors, requires a color depth of 9 bits since 512 = 2⁹

Hence, the color depth supported by the monitor is 9-bits.

Compiled By: Chaudhary Amit V.