Video display unit (VDU)

A video display unit (VDU) is a computer peripheral device, like a TV set, that the computer sends information to. The VDU displays this information in the form of text or graphics (pictures) on a cathode ray tube.

A VDU operates like a normal TV set except the information is sent directly through a cable from the computer. There are many different kinds of VDUs and the choice of a VDU is determined by selecting:

*whether it's monochrome or colour

*the resolution

*the type of drive signal from the computer

Note: the high voltage inside a colour monitor can cause X-rays. Sit away from the screen if you're using the VDU for long periods of time.


Types of VDUs

The main consideration when selecting a VDU (or monitor) for a computer is whether the display is monochrome or colour. Monochrome monitors are cheaper than colour monitors and are suited mainly to showing text and high-resolution CAD and desktop publishing applications where a single colour is adequate. Colour monitors are the most common these days, and can display colour drawings, multi-layer CAD drawings and graphics-based software.

Another consideration is the resolution, which tells you how many pixels the monitor can display across and down the screen. The higher the resolution, the better the definition and the more expensive the monitor! As well, a monitor must match the video driver card inside the computer.

There are four common colour formats:

*Colour Graphics Adapter (CGA)

*Enhanced Graphics Adapter (EGA) 

*Video Graphics Array (VGA)

*Super VGA (SVGA).

Two less common formats are: *Multi-Colour Graphics Array (MCGA)

*Professional Graphics Adapter (PGA).

Two monochrome formats are: *Monochrome Display Adapter (MDA)

*Hercules Graphics Controller (HGC).

Most video adapter cards can operate in two modes: *alpha-numeric character mode

*graphics mode

In alpha-numeric character mode, individual characters are displayed on the screen in columns and rows (normally 80 x 25). In graphics modes, individual pixels are turned on to create an image. Table 1 lists the resolution and colour capabilities of the major video adapter cards.
TABLE 1: Video adapter cards
Video adapter Maximum resolution
Number of colours
HGC 720 x 348 
      On & off
CGA 320 x 200 (and 640 x 200)
      4 and (2)
EGA 640 x 350
PGA 640 x 480  
MCGA 320 x 200 (and 640 x 480)
      256 & (2)
VGA 320 x 200 (and 640 x 480)
      256 & (16)
SVGA 800 x 600 (also 1024 x 768 and 1280 x 1024)
      memory dependant

Super VGA cards either have 256K, 512K or 1024K bytes of memory (RAM) on the board (where K = 1024 and one byte = 8 bits). The more RAM on board, the greater the number of colours that can be displayed. About 300K of RAM is required on a super VGA card to display 640 x 480 x 256 colour format, so the card would be fitted with 512K of RAM. To display 256 colours in 1024 x 768 mode, about 768K bytes of RAM is required so it would be fitted with 1024K (or 1M) bytes of RAM. Higher resolution cards can usually emulate the lower resolution modes for software compatibility.


Multiscan and multisync monitors

Normally, a monitor is matched with the type of video display card driving it. For example, an EGA monitor would normally be driven by an EGA card. A multisync (or multiscan) monitor is one that is compatible with a range of video adapter cards. For example, a multisync monitor will work with a CGA, EGA or VGA card. Most multisync monitors have a high resolution and can also work with SVGA 800 x 600 and 1024 x 768 formats.

Monitor scan rates

A monitor produces an image on its screen by scanning an electron beam horizontally across the screen and vertically down the screen. The scanning frequencies (scanning rates) for each of the types of display adaptors are shown in Table 2. The unit for frequency is the Hertz (Hz) which in this case means the same as scans per second.

Table 2
Video adapter Horizontal frequency (kHz) Vertical frequency (Hz)
HGC 18.432
CGA 15.750
EGA 18.432 – 21.850
    50 – 60
PGA 30.480
MCGA 31.500
    60 – 70
VGA 31.500
    60 – 70
SVGA 15.750 – 48.000
    56 - 72
Analog and digital monitors
The terms analog and digital describe the type of video signal that the adaptor card sends to the monitor. Most colour cards supply separate red, blue and green signals (RGB) to the monitor as all colours are made up of varying proportions of these primary colours.

Digital video adapter cards produce red, green and blue signals that can be either ON or OFF, giving eight colours. An intensity signal doubles this to 16. Digital signals are referred to as TTL signals (Transistor-Transistor Logic, a family of digital ICs).

Analog cards and monitors (such as the VGA type) allow the red, green and blue signals to have up to 64 different levels. This gives 64 x 64 x 64 colours (over 260,000).


Monochrome monitor/VDU operation
The picture on a television or computer monitor is produced by a beam of electrons projected towards the screen from the cathode, which is in the neck of the tube. The electrons are attracted towards the front of the screen by a very high positive voltage. The inside of the screen is coated with phosphor - a material that gives off light when struck by electrons. When the electron beam hits the screen, it creates a tiny spot of light.


Beam deflection

To produce a picture, the electron beam is scanned across the screen as shown in Figure 1. The beam is moved from left to right (scan), then quickly returned to the left of the screen (flyback). As well, the beam is moved down the screen, giving complete coverage of the phosphor surface. The resulting screen display is called a raster. The scanning of the beam is called deflection and is achieved by a set of electromagnetic coils called the deflection yoke. The yoke sits at the front of the neck of the tube and is driven by the horizontal and vertical deflection circuits.

As the electron beam is scanned across and down the screen, the intensity (or brightness) of the beam is varied by the video signal, giving a picture.


Block diagram

The three main signals required from a video adapter card to a monochrome monitor are the:

*video signal

*horizontal synchronisation (horizontal sync.)

*vertical synchronisation (vertical sync.).

The video signal controls the brightness of the beam as it is scanned across and down the screen. In a monochrome display, this signal can only have two brightness levels, on or off, determined by the pattern of characters to be displayed on the screen as stored in the memory on the video adapter card. The horizontal and vertical sync signals ensure that the scanning action of the electron beam down and across the screen is in synchronisation with the way the video adapter card reads the information for each line out of the video memory. This way a stable picture is displayed on the screen. Figure 2 shows the basic block diagram of a monochrome VDU.

 In Figure 2, the video signal is sent to the video amplifier stage where the signal is boosted so it can drive the tube. The amplified video signal is sent to the cathode of the tube and controls the number of electrons that reach the screen, and therefore the brightness of the display.

The vertical sync signal from the video adapter card informs the monitor that an entire screen has been displayed, and that it is time to deflect the beam back up to the top of the screen. That is, vertical flyback is initiated. The vertical sync stage detects the incoming vertical sync pulses and controls the vertical deflection of the beam to maintain the correct timing. For a VGA system, the frequency of this oscillator is between 60Hz and 70Hz. That is, about 70 complete screens are displayed every second.

The horizontal sync signal from the video adapter card informs the monitor when each scan line has been displayed, and that it is time to deflect the beam back to the left hand side of the tube (looking from the front). That is, horizontal flyback is initiated. The horizontal sync stage detects the incoming sync pulse and uses this to control the frequency of the horizontal oscillator. The frequency of the horizontal oscillator for a VGA system may be as high as 31.5kHz. That is, 31,500 horizontal scans per second.

The output of the horizontal oscillator is used to generate the high voltage needed by the picture tube. This voltage is called the Extra High Tension (EHT) and may be from 9,000 volts in a small monochrome monitor up to 30,000 volts in a colour monitor.


Colour Monitors
Colour monitors operate in a similar manner to monochrome monitors except the video adapter card produces three video signals: red, green and blue. These signals are amplified and sent to the three electron guns in a colour picture tube. The screen of a colour tube is coated with three different phosphors which generate red, green or blue light when struck by electrons.

The red, green and blue video signals activate the red, green and blue electron guns inside the tube, generating three electron beams. Each beam is arranged to hit its respective colour phosphor to produce the required colour image.


Colour monitor picture tubes

Monochrome VDU picture tubes mainly have either a green or amber phosphor. Colour tubes have three different coloured phosphors; red, green and blue, in tiny dots or rectangles on the inside surface of the screen. Every colour can be produced from a mixture of these three primary colours. For example, yellow is a mixture of red and green. Therefore, to display a totally yellow screen, only the red and green electron guns will be on, and only the red and green phosphor dots will be lit. There are three separate, but identical electron guns in the neck of a colour tube, called the red, green and blue guns. So each beam hits its colour phosphor only, a perforated metal screen, called the shadow mask, is fitted near the screen, inside the tube.

Colour tubes also have an assembly of ring magnets around the neck of the tube. These are used to adjust the purity and convergence of the tube. The purity adjustment ensures the beam from each gun is aligned to hit its colour phosphor dots through the shadow mask. Patches of wrong colours appear on the screen if the purity is out of adjustment.

The convergence adjustments ensure that the three beams come together at the one group of phosphor dots. If the convergence is out of adjustment, objects on the screen will appear to have fringes of colour around them. Purity and convergence adjustments should not be attempted without the appropriate test equipment and service manuals.


The VGA monitor
The VGA monitor is a high-resolution, analog colour monitor with a maximum resolution of 640 pixels across the screen and 480 pixels down the screen. That is, the image on one entire screen is made up of 640 x 480 — over 300,000 dots! The VGA monitor can only display signals from a VGA adapter card, but the adapter card can usually emulate CGA and EGA signals in a form that the VGA monitor can display. The five important signals from the video card are: *red video

*green video

*blue video

*horizontal sync

*vertical sync.



The VGA monitor Model

The following refers to the detailed block diagram for a VGA computer monitor. The block diagram is shown below (also in Section 3).

RGB signal path

The RGB colour signals from the video adapter card inside the computer are analog. These signals need amplifying and are then sent to the respective gun. The RGB signals therefore take separate but identical paths to the three cathodes at the tube. Each RGB signal is first fed to a buffer stage to prevent the monitor loading the adapter card. The signals are then fed to a video driver and blanking stage to amplify the signals. This stage is blanked during horizontal and vertical flyback of the three beams so that retrace lines are not displayed on the screen as the beams retrace. The amplified red, green and blue video signals are sent to individual video output stages to drive the three cathodes in the colour tube.


Why sync pulses are needed

Vertical and horizontal sync signals (in the form of pulses) are sent by the video card to ensure the video information is synchronised with the scanning action of the VDU. That is, the card sends the video information for each line as each new line is scanned on the screen. If there was no synchronisation between the video card and monitor, the picture would not be stable on the screen.


The vertical section

The vertical sync pulses from the sync interface stage are sent to the vertical oscillator, causing the frequency of this oscillator to lock in to the frequency and phase of the incoming vertical sync pulses, normally between 60Hz and 70Hz. This is the rate at which the screen is scanned from top to bottom. The output of the vertical oscillator is sent to a pulse shaper stage to get the correct wave shape. This stage supplies a signal to the high-power vertical output stage which produces the relatively large current needed to drive the vertical part of the deflection yoke. The sawtooth shape of this waveform is such that the magnetic field generated in the yoke causes the electron beam to be scanned at a steady rate down the screen and then to fly back quickly, ready to display the next screen. As well, a blanking pulse is taken from the vertical output to the RGB drive amplifiers to blank retrace lines.

The horizontal section

The horizontal sync section is more complex, but operates in a similar manner to the vertical sweep section, only at a much faster rate. The horizontal sync pulses are fed from the sync interface to a pulse shaper and then to a horizontal phase detector. This stage compares the frequencies of the incoming horizontal sync pulses to that of the horizontal oscillator. Like the vertical oscillator, the horizontal oscillator will run without external sync pulses and a frequency close to, but rarely the same as that of the sync pulses. If there's a difference, the phase detector varies the frequency of the horizontal oscillator to equal that of the sync pulses, (normally 31.48kHz).

The output of the horizontal oscillator goes to the driver stage where the signal is shaped and amplified so it can drive the output stage. This stage produces the current waveform in the yoke that generates the required magnetic field to deflect the electron beam across the screen. The horizontal output pulses (called flyback pulses) are used to blank the screen during retrace.


High voltage output stage

The high voltage required in the tube to accelerate the electron beam towards the front of the screen is in the order of 26,000 volts and is referred to as the extra high tension (EHT). This voltage is generated by taking pulses from the horizontal output stage and boosting them with a device called an EHT transformer. The EHT transformer and its associated circuitry develops the EHT which is connected via a special high tension cable to the anode of the tube through a hole called the ultor.


 Brightness and contrast controls

Brightness and contrast controls are fitted to most monitors and form part of the video and tube drive circuits. Both controls perform a very different task and are usually adjusted together to get the best display. The brightness control setting determines the operating point of the picture tube. At one extreme, the picture will appear whiter, at the other it will be dark. That is when the whites are whiter, the blacks are whiter also.

The contrast control varies the amount of amplification (or gain) of the video amplifier. At maximum, the whites are whiter and the blacks are blacker. In summary, the brightness control sets the illumination of the screen, and the contrast control sets the difference between white and black. These controls are in the video section.


Power supply

The power supply provides power to the various sections within the VDU by converting the 240V AC mains to a suitable DC voltage. The power supply section is easily located by following the mains cable.


The super VGA multisync monitor
The following is brief summary on multisync monitors. Read the section on VGA monitors first as much of that information applies to the multisync type.

A multisync monitor is one that is compatible with many adopter cards. That is, a multisync monitor will work with a CGA, EGA, VGA or SVGA video adopter card. The main differences between these adapter cards is the frequency of the horizontal and vertical sync pulses and the type of video signals they supply to the monitor (analog or digital). A multisync monitor operates with all these different systems, making their circuitry quite complex.

The operation of the video amplifiers and deflection circuits in a multisync monitor are similar to the VGA monitor. The difference is the way a multisync monitor recognises which type of video standard it's receiving and how it switches modes to accommodate each type.


Horizontal sync

The range of horizontal frequencies a multisync monitor must be able to lock in to is shown in Table 2, Section 1. The video/sync interface circuit sends these horizontal sync pulses to a pulse width correction circuit and a horizontal phase position circuit which work together to do four jobs:

*To supply the rest of the horizontal circuit with a constant width sync pulse.

*To supply the sync decoder with an adjustable pulse width.

*To supply the horizontal section with the correct frequency sync pulses.

*To supply a one second pulse that blanks the raster and stabilises horizontal control circuits when the monitor switches between video modes.

As well, electronic components are switched in and out of the horizontal oscillator to match its frequency for each different type of video card. Once the horizontal phase detector receives the horizontal sync pulses, synchronisation is maintained in a similar way to normal monitors.


The sync decoder

The sync decoder detects the type of video standard being received and controls the other parts of the VDU system so they respond accordingly. It controls the video interface block, via the colour mode block, and the vertical size section to ensure that the entire screen is filled. It also sets up the horizontal output section so it can operate properly at each different frequency. For example, if the incoming horizontal sync pulses are at a frequency of 31.48kHz, the sync detector recognises this as a VGA signal and sets up the video section to operate with an analog RGB video signal, the horizontal section to operate at 31.48kHz, and the vertical section so the vertical scan fills the screen.


The vertical scan section

The vertical section needs to be able to operate at frequencies between about 40Hz and 70Hz. To do this, the vertical section has a phase detector circuit, much like that used in the horizontal section circuits to lock in to the frequency of the incoming vertical sync pulses.


 Review questions

These questions will help you revise what you have learnt in Section 1.

1.What is meant by the term 'computer peripheral'?

2.Name three computer peripherals that mainly send data to a computer.

3.Name three computer peripherals that mainly receive data from a computer.

4.Name three computer peripherals that equally send and receive data in a computer system.

5.List two peripheral devices that are used along with a keyboard to operate computer software.

6.List two peripheral devices that send data over the phone lines.

7.**Which peripheral device is normally used to import a diagram into a desktop publishing program?

8.Give two basic differences between a hard disk and a floppy disk.

9.What is the main use of a printer in a computer system?

10.What is the main use of a plotter?

11.How does a plotter produce its output?

12.List three different types of printers commonly used in computer systems.

13.Of the printers referred to in the previous question, which one gives the highest quality print? The lowest quality print?

14.What type of printer gives good quality text, but has very limited graphics capability?

15.What is meant by the term 'resolution' as applied to a printer? Give a typical figure for a laser printer.

16.What advantage does an inkjet type printer have compared to a dot-matrix printer, other than print quality?

17.Briefly explain how dot-matrix, inkjet and laser printers produce printed copy dot-matrix:





18.What advantage does a laser printer have over other printers, apart from print quality?


19.What is the main advantage of a dot-matrix printer compared to other types of printers? What's the main disadvantage?


20.What mostly determines the resolution of a dot-matrix printer?


21.What is meant by the terms portrait  and landscape when referring to the orientation of print on a printed page? Use sketches to answer.

portrait landscape

22.What type of printer is best suited to heavy and continual use such as in an office?

23.What type of printing mechanism is used in most portable, battery powered printers?

24.Describe the basic difference between serial and parallel data transfer.

25.What peripheral devices always use serial data transfer?

26.Which type of data transfer method is the fastest, serial or parallel? Why?

27.What three things should you know about a VDU when selecting it for a computer?

28.There are a number of video standards used by IBM type computers. List four of these.

29.What is the main difference between the standards referred to in the previous question?

30.Of all the standards you've listed, which one is best suited to displaying high quality graphics?

31.What is a 'multisync' VDU?

32.What is meant by the term 'pixel'?

33.List three major specifications for a VDU.

34.List three major specifications for a printer.



Review questions

These questions will help you revise what you have learnt in Section 2. You may not have learnt all the topics in these questions so do the groups you have learnt.

Questions 1 to 10 are about computer monitors.

1.What is meant by the term raster?

2.What is the name of the electromagnetic device that causes an electron beam to move inside the picture tube of a VDU?

3.Which functional block in a VDU drives the device referred to in question 2 to deflect the beam across the screen?

4.Which functional block in a VDU drives the device referred to in question 2 to deflect the beam down the screen?

5.Show with a sketch the scanning action of the electron beam in the picture tube of a VDU.

6.How is the image displayed on a VDU synchronised to the output of the video adapter card?

7.Why is a high voltage needed for the picture tube in a computer monitor?

8.What terminal of the picture tube is the video signal connected to?

9.Briefly explain how an image is produced on the screen of a monochrome VDU.

10.What are the primary colours in a colour VDU?

Questions 11 to 20 refer to a dot-matrix printer.

11.What is the device called that controls the operation of a dot-matrix printer?

12.What is the main reason for random access memory (RAM) in a dot-matrix printer?

13.What is the term that refers to the communication that occurs between a computer and a printer during printing?

14.What are the two reasons for using ROM in a dot-matrix printer?

15.What is the difference between ROM and RAM?

16.Apart from computer commands, how else can the printer be controlled?

17.In the block diagram of a dot-matrix printer, there are two buses connecting the blocks together. What are the usual names of these?

18.In a parallel interface to a printer, how many lines carry the data for each character?

19.There are at least two motors in a typical dot-matrix printer. What are their functions? What is the type of motor?

20.How are the motors in a dot-matrix printer controlled?

Questions 21 to 25 refer to a laser printer.

21.How is the laser beam in a laser printer moved?

22.What is a laser beam?

23.What is the purpose of the corona wires in a laser printer?

24.How is the toner transferred from the drum to the paper in a laser printer?

25.How is the toner transfer to the paper made permanent in a laser printer?


Questions 26 to 30 refer to a mouse.

26.In terms of connection, what are the two main types of computer mouse?

27.In terms of operation, what are the two main types of computer mouse?

28.When a mouse is moved, what signals does it produce?

29.Where does the electrical power for a mouse come from?

30.In general terms, what is the purpose of the switch buttons on a computer mouse?

Questions 31 to 35 refer to a modem

31.When a modem is sending data, what does it do to make the binary ones and zeros different?

32.Why is isolation needed between a modem and the phone lines?

33.What is meant by the term 'Hayes compatible' when referring to a modem?

34.What is meant by the term 'full duplex' when referring to modem operation?

35.What is the main purpose of a modem?


Review questions

These questions will help you revise what you have learnt in Section 3.

1.List the three primary colours used in a VDU.

2.What is the name of the device that ensures the electron beams from the three guns in a colour picture tube strike their correct phosphor?

3.What is meant by the term 'purity'?

4.What is meant by the term 'convergence'?

5.Where are the purity and convergence adjustments in a colour VDU?

6.Is a VGA monitor analog or digital?

7.List the signals required to operate a VGA monitor.

8.What is meant by 'blanking' in a VDU?

9.What do the vertical and horizontal output blocks drive in a VDU?

10.Which block drives the EHT stage in a VDU?

11.What is a typical value for the EHT voltage in a VGA monitor?

12.Which two blocks supply signals to the horizontal phase detector block in a VGA monitor?

13.List the blocks in a VGA monitor that process the video signals.

14.What is meant by the term 'degaussing'?

15.How many inputs are there to the sync interface block in a VGA monitor?

16.What is the difference between a microprocessor and a microcontroller?

17.List the three buses found in a typical dot-matrix printer.

18.Apart from the computer interface, what other inputs affect the operation of a dot-matrix printer?

19.What is the purpose of the character ROM in a dot-matrix printer?

20.List three internal sensors of a dot-matrix printer.

21.What type of connector is generally used as a parallel interface in a dot-matrix printer?

22.What type of connector is generally used as a serial interface in a dot-matrix printer?

23.What are the usual front panel indicators of a dot-matrix printer?

24.Of the indicators referred to in Question 23, which one is unlikely to be controlled by the CPU?

25.What does the self-test routine do in a typical dot-matrix printer?

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