Few years ago, most of the computer screens were CRT monitors. The main element of the screen was CRT.
Working of CRT
- Some CRT monitors (computer screens) use cathode ray tubes (or CRT), which are glass vacuum tubes into which an electron gun emits a flow of electrons guided by an electrical field towards a screen covered in small phosphorescent elements.
- The electron gun is made up of a cathode, a negatively charged metallic electrode, and one or more anodes (positively charged electrodes).
- The cathode emits the electrons attracted by the anode. The anode acts as an accelerator and concentrator for the electrons, forming a flow of electrons aimed at the screen.
- A magnetic field guides the electrons from left to right and from top to bottom. It is created with two electrified X and Y plates (called deflectors/ Deflection plates) which send the flow horizontally and vertically, respectively.
- Electrons that pass through the plates then hit the phosphor coating which is on the inside of the glass screen. When the particles hit the phosphor, they immediately light up – causing the light to shine through the front of the monitor, thus making up the picture on the screen.
- There are three differently coloured phosphours for each pixel (known as phosphor triads), and depending on which phosphor the electron hits, that’s which colour the pixel will light up.
CRT Colour Monitor Block Diagram
Figure 2 shows a simplified block diagram of a color CRT monitor. The entire circuitry within the monitor can be grouped into three main categories:
- Video signal processing and amplification,
- Horizontal/vertical deflection and synchronizing,
- Power supply.
Working of CRT Monitor
Video signal processing and amplification
- A transmission line or a coaxial cable carries the video signal from the host computer to the monitor. The video signal is usually a 1 VPP signal and thus requires amplification before the signal can be applied to the CRT’s cathode.
- The amplification of the video signal is usually done in two stages. A low voltage amplifier, often called a preamplifier, amplifies the 1 VPP signal to a 4±6 VPP signal. In addition to amplification, the preamplifier also provides contrast and brightness control.
- Contrast control allows the user to vary the gain of the video amplifier. Increasing the contrast for instance increases the video signal’s level and thus causes the lighter portions of the raster to be brighter than the darker portions. The result is a sharp picture with contrasting light and dark.
- Brightness control on the other hand allows the user to change the brightness of the raster by varying the DC offset of the video signal. Increasing brightness in effect makes both the light and dark portions of the image brighter. Most preamplifiers also provide DC restoration or black level clamping which makes the brightness control possible.
- The CRT video amplifier is the second stage amplifier; it amplifies the preamplifier’s 4±6 VPP signal to a 40±60 VPP signal that the cathode requires to energize each phosphor dot on the screen.
- In a color monitor, there is a trio of red, green and blue phosphor dots. Together, each trio constitutes a picture element, often called a pixel for short. The light emitted by the phosphor dot is proportional to the number of electrons striking the phosphor.
- Thus, by modulating the voltage of each of the three cathodes in a color monitor, the corresponding phosphor dot is energized at varying intensities, thereby producing various shades of color.
Horizontal/vertical deflection and synchronizing
- Once all lines on the screen are traced, the beam moves from the bottom to the top during the vertical retrace interval.
- The composite video signal contains the horizontal syncpulse which is repeated at the horizontal scan rate. The horizontal scan rate may be anywhere from 15 kHz to 240 kHz depending on the resolution of the monitor.
- The vertical sync pulse is much wider than the horizontal sync pulse and occurs at the end of the raster, i.e., after all lines in the frame have been traced. The vertical sync pulse is repeated at a 60 Hz rate.
- For proper operation, a sync separator separates the horizontal and vertical sync pulses from the composite video signals.
Power Supply
- Power supply unit creates different power signals applied on anodes and the plates of CRT.
Characteristics of CRT Monitor
Pixels:
It is the pictorial elements i.e. individual dot on the screen.
Dot Pitch:
Dot pitch is the distance measured between two same coloured consecutive pixels. It is measured in millimeters. The less space between pixels, the better the image quality.
Resolution:
It is defined as the potential pixels present onto the screen.
Resolution= Total no. of horizontal pixels X Total no. of vertical pixels
Horizontal scanning frequency:
It is the frequency at which the monitor will repaint the horizontal lines that make up an image. It is measured in KHz.
Vertical scanning frequency/ Vertical refresh rate:
It is the frequency at which the monitor will repaint the whole screen. It is measured in Hz.
Frame Rate:
It is used to show the no. of times a screenful information produced per second on the monitor. Higher the frame rate, less flicker problem.
Video bandwidth:
It is the highest input frequency a monitor can handle. It is measured in MHz. Higher the video bandwidth better the image quality
Bandwidth= Horizontal pixels X vertical pixels X Frame rate
Interlaced scan & Non-interlaced scan
Today, two different techniques are available to render the video: interlaced scanning and Non-interlaced scanning.
Which technique is selected will depend on the application and purpose of the video system, and particularly whether the system is required to capture moving objects and to allow viewing of details within a moving image.
Interlaced scanning
- Interlaced scan-based images use techniques developed for Cathode Ray Tube (CRT)-based TV monitor displays, made up of 576 visible horizontal lines across a standard TV screen.
- Interlacing divides these into odd and even lines and then alternately refreshes them at 30 frames per second.
- The slight delay between odd and even line refreshes creates some distortion. This is because only half the lines keep up with the moving image while the other half waits to be refreshed.
Non-interlaced scanning/ Progressive Scanning
- Non-interlaced scanning, as opposed to interlaced, scans the entire picture line by line every sixteenth of a second. In other words, captured images are not split into separate fields like in interlaced scanning. Computer monitors do not need interlace to show the picture on the screen.
- It puts them on one line at a time in perfect order i.e. 1, 2, 3, 4, 5, 6, 7 etc. so there is virtually no “flickering” effect. As such, in a surveillance application, it can be critical in viewing detail within a moving image such as a person running away. However, a high quality monitor is required to get the best out of this type of scan.
Advantages of CRT Monitors
- Less expensive – Although LCD monitor prices have decreased, comparable CRT displays still cost less.
- Better colour representation – CRT displays have historically represented colors and different gradations of color more accurately than LCD displays. However, LCD displays are gaining ground in this area, especially with higher-end models that include color-calibration technology.
- More responsive – Historically, CRT monitors have had fewer problems with ghosting and blurring because they redrew the screen image faster than LCD monitors. Again, LCD manufacturers are improving on this with displays that have faster response times than they did in the past.
- Multiple resolutions – If you need to change your display’s resolution for different applications, you are better off with a CRT monitor because LCD monitors don’t handle multiple resolutions as well.
- More rugged – Although they are bigger and heavier than LCD displays, CRT displays are also less fragile and harder to damage.
Disadvantages of CRT Monitors
- They have a big back and take up space on desk.
- The electromagnetic fields emitted by CRT monitors constitute a health hazard to the functioning of living cells.
- CRTs emit a small amount of X-ray band radiation which can result in a health hazard.
- Constant refreshing of CRT monitors can result in headache.
- CRTs operate at very high voltage, which can overheat system or result in an implosion.
- Within a CRT a strong vacuum exists in it and can also result in a implosion.
- They are heavy to pick up and carry around.
