Electronic Displays (5.11) PDF

Electronic Displays (5.11) Learning Objectives 5.11.1.1 Describe the principles of operation of cathode ray tubes used in modern aircraft (Level 2). 5.11.1.2 Describe the principles of operation of light emitting diodes used in modern aircraft (Level 2). 5.11.1.3 Describe the pr...

Electronic Displays (5.11) Learning Objectives 5.11.1.1 Describe the principles of operation of cathode ray tubes used in modern aircraft (Level 2). 5.11.1.2 Describe the principles of operation of light emitting diodes used in modern aircraft (Level 2). 5.11.1.3 Describe the principles of operation of liquid crystal displays used in modern aircraft (Level 2). 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 207 of 285 CASA Part 66 - Training Materials Only  Light Emitting Diodes in Aircraft LED Fundamentals The Light-Emitting Diode (LED) is one type of optoelectronic device. It was developed to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on panels. An LED, when forward biased, produces visible light. The light may be red, orange, yellow, green, blue, white or ultraviolet, depending on the material used to make it. LEDs emitting a non-visible light in the infrared part of the radiation spectrum are also available. These LEDs are invaluable in detection applications when used in conjunction with infrared detector components. The LED is designated by a standard diode symbol with two arrows pointing away from the cathode, indicating light leaving the diode. The LED operating voltage is low, about 1.6 V forward bias and generally about 10 mA. The life expectancy of the LED can be very long, over 100 000 hours of operation. A LED and its schematic symbol To turn on an LED, you must determine the anode and cathode connections. If you look closely at the red plastic base of an LED, you will notice a flat spot. The wire that comes out beside the flat spot must connect to the negative side of a power supply, and the other wire to the positive side. LEDs will be damaged if connected to power supplies rated at over 2 V. LEDs require resistors to limit current when used with power supplies rated at over 2 V. To avoid damaging your components, always check the supplier’s or manufacturer’s literature for this information. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 208 of 285 CASA Part 66 - Training Materials Only  Peak Wavelength Single-Coloured (Monochromatic) LEDs The colour of light is the way we perceive its wavelength. The light radiation spectrum is expressed as the wave length in nanometres (nm). Unlike incandescent lamps that produce light over a wide spectrum (of which visible light is only a small segment), LEDs emit light over only a relatively small part of the radiation spectrum. Peak wavelength is the technical method of defining the colour emitted by the LED (the wavelength of the emitted light). Typical figures range from 450 nm (blue) through 535 nm (green), 585 nm (yellow), 620 nm (orange) and 700 nm (red), up to 950 nm (infrared). Generally the output is not at one precise wavelength, but is distributed over a narrow range; a graph of intensity against wavelength would show a peak at the specified wavelength. Aviation Australia Light radiation spectrum is expressed in "nanometres" (nm) The peak wavelength for any LED component is determined by the chemical make-up of the semiconductor substrate rather than the current or power dissipated. It makes no difference if the LED is in a coloured or clear package. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 209 of 285 CASA Part 66 - Training Materials Only  Multi-Coloured and Bi-Coloured LEDs Within the multi-coloured LED epoxy package are two separate reverse-parallel semiconductor chips, each producing a different single colour. At any instant of time, only one of the LED chips can emit light, and which one depends on the direction of current flowing through the component. Only one series resistor is required, and is calculated using the same formula for calculation of series resistors for LEDs. Aviation Australia Bi-coloured LEDs Bi-coloured LEDs can produce a third colour that is a product of mixing the two primary colours. For example, a red and green bi-coloured LED can produce a yellow light. The simplest method to achieve this is to operate the LED from an AC voltage source. This results in each of the primary colour chips operating during their respective half cycles of the alternating flow of current; however, the human eye perceives the rapidly flickering red and green lights as a constant yellow. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 210 of 285 CASA Part 66 - Training Materials Only  Tri-Coloured LEDs Within the LED epoxy package are two separate semiconductor chips that each produce a different colour. A common lead from the two semiconductor chips is connected internally to produce a three- terminal component. Both common cathode and common anode types are available. Aviation Australia Tri-coloured LEDs Used simply, these components provide a selectable two-colour light source by switching the voltage between the two semiconductor chips. Alternatively both semiconductor chips can be operated simultaneously to mix the two primary colours. Only one series resistor is required provided that both semiconductor chips are never operated simultaneously. Otherwise it is essential that each chip is protected by its own dedicated resistor. Seven-Segment LED Display LEDs are used widely as power-on indicators of current and as displays for pocket calculators, digital voltmeters, frequency counters, etc. In calculators and similar devices, LEDs are typically placed together in seven-segment displays. This type of display uses seven LED segments, or bars (labelled A through G in the diagram), which can be lit in different combinations to form any number from 0 through 9. Aviation Australia 7-segment LED display 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 211 of 285 CASA Part 66 - Training Materials Only  The schematic shows a common-anode display. All anodes in a display are internally connected. When a negative voltage is applied to the proper cathodes, a number is formed. For example, if negative voltage is applied to all cathodes except that of LED E, the number 9 is displayed. If the negative voltage is applied to all cathodes except LED B, the number 6 is displayed. Aviation Australia Common Anode seven-segment display with Common Cathode (CC) and Common Anode (CA) schematic diagrams There are two types of LED seven-segment displays: Common Cathode (CC) and Common Anode (CA). In the common cathode, all the cathodes of the seven segments are connected directly, and in the common anode, all the anodes of the seven segments are connected. Shown above is a common anode seven-segment display. When working with a CA seven-segment display, power must be applied externally to the anode connection that is common to all the segments. Then, applying a ground to a particular segment connection (A–G) will cause the appropriate segment to light up. An additional resistor must be added to the circuit to limit the amount of current flowing through each LED segment. The Common Anode seven-segment display diagram shows the instance when power is applied to the CA connection and segments B and C are grounded, causing these two segments to light up. A common cathode seven-segment display is different from the common anode version in that the cathodes of all the LEDs are connected. To use this seven-segment display, the common cathode connection must be grounded and power must be applied to the appropriate segments in order to illuminate them. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 212 of 285 CASA Part 66 - Training Materials Only  Alphanumeric LED Display Alphanumeric LED displays operate similarly to seven-segment displays and typically use 16 segments. Alphanumeric LED display Dot Matrix LED Display The more flexible display which is commonly used to produce the full alphanumeric range is the 35- dot matrix in which LED dies are mounted in a 7 × 5 array. With these types of displays, the range of applications increases to a much higher level compared to use cases for the seven-segment display. Dot matrix LED display 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 213 of 285 CASA Part 66 - Training Materials Only  The alphanumeric LED display may be an older technology, but its uses are still numerous, even with the wide array of new technologies available for displaying information. Its biggest advantage is cost. It is hard to find a cheaper design solution for displaying small amounts of information. Another very good quality is its durability. The only pitfall is the possibility of a disconnection because, in effect, the diode is nothing more than an electrical connection. These displays are generally very simple to program. The logic to decode incoming data for the display is so basic that a designer can buy a desired decoder fairly cheaply if it does not come with the display. Also, because of its simplicity, required maintenance is minimal. Organic LEDs OLED (Organic Light-Emitting Diode) is a flat light-emitting technology made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. OLEDs can be used to make displays and lighting. Because OLEDs emit light, they do not require a backlight and so are thinner and more efficient than LCD displays (which do require a white backlight). Aviation Australia Basic structure of a bilayer OLED OLEDs work in a similar way to conventional diodes and LEDs, but instead of using layers of n-type and p-type semiconductors, they use organic molecules to produce their electrons and holes. A simple OLED is made up of six different layers. On the top and bottom are layers of protective glass or plastic. The top layer is called the seal and the bottom layer the substrate. Between those layers are a negative terminal (sometimes called the cathode) and a positive terminal (called the anode). Finally, between the anode and cathode are two layers made from organic molecules called the emissive layer (where the light is produced, which is next to the cathode) and the conductive layer (next to the anode). 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 214 of 285 CASA Part 66 - Training Materials Only  Aviation Australia Layers of an OLED OLED vs LCD OLED displays have the following advantages over LCD displays: Lower power consumption Faster refresh rate and better contrast Greater brightness and a fuller viewing angle Exciting new types of displays that we do not have today, like ultra-thin, flexible or transparent displays Better durability and operation in a broader temperature range Lighter weight due to a very thin screen and even the ability to be 'printed' on flexible surfaces. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 215 of 285 CASA Part 66 - Training Materials Only  Liquid Crystal Displays Polarisation Light is made up of electromagnetic radiation (waves). Light waves can travel in any direction or have any orientation. Aviation Australia A polarised filter passes light waves with one orientation A polarised filter allows only light travelling in one position to pass through. It is made of parallel micro-sized slits that block out all but one position of wave. Polarisation is the process of causing light to vibrate in one plane only. Cross-polarising lenses will stop light altogether. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 216 of 285 CASA Part 66 - Training Materials Only  Liquid Crystal Liquid crystal is an organic substance that has both solid crystalline and liquid characteristics within certain temperature ranges. Unlike liquid substances, liquid crystal demonstrates a crystalline structure and related refraction characteristics. Depending on the crystalline state, various refractions are possible. Aviation Australia Light passes through the liquid crystal when power is applied Calculators, digital watches, portable word processors and notebook PCs all use nematic liquid crystals which change their structure with the application of electric voltage. Liquid Crystal Reorientation When molecules with such a characteristic are brought into a sufficiently strong electrical field, they tend to align themselves in the direction of the field. Originally the orientation is almost flat. When an electrical field with direction E is applied (represented in red), there is a force T (represented in green) that tends to align the molecule parallel to the field. When the field is strong enough, the molecule will be almost parallel to the field. Aviation Australia Liquid crystal reorientation with electrical field 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 217 of 285 CASA Part 66 - Training Materials Only  Liquid Crystal Displays A Liquid Crystal Display (LCD) consists of two plates of glass, sealed around the perimeter, with a layer of liquid crystal fluid between them. The liquid crystal layer is a few microns thick. The layer thickness of liquid crystal is approximately 1/10 the thickness of an average human hair. Construction of liquid crystal display Transparent, conductive electrodes are deposited on the inner surfaces of the glass plates. The electrodes define the segments, pixels or special symbols of the display. Next a thin polymer layer is applied on top of the electrodes. The polymer is etched with channels to align the twist orientation of the Liquid Crystal (LC) helix-shaped molecules. Finally, polarising films are laminated to the outer surfaces of the glass plates at 90° angles. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 218 of 285 CASA Part 66 - Training Materials Only  LCD display with two polarising films at 90 degrees Normally two polarising films at 90° should be dark, preventing any transmission of light, but due to the ability of LC to rotate polarised light, the display appears clear. When AC voltage is passed through the LC, the crystals within this field align so that the polarised light is not twisted. This allows the light to be blocked by the crossed polarisers, making the activated segment or symbol appear dark. Basically, LCDs operate from a low-voltage (typically 3 to 15 V RMS), low-frequency (25 to 60 Hz) AC signal and draw very little current. They are often arranged as seven-segment displays for numerical readouts. The AC voltage needed to turn on a segment is applied between the segment and the backplane. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 219 of 285 CASA Part 66 - Training Materials Only  Aviation Australia 7-segment LCD display with common back plane The backplane is common to all segments. The segment and backplane form a capacitor that draws very little current as long as the AC frequency is kept low. It is generally not lower than 25 Hz because this would produce visible flicker. LCDs draw much less current than LED displays and are widely used in battery-powered devices such as calculators and watches. An LCD does not emit light energy like an LED, so it requires an external source of light. An LCD segment will turn on when an AC voltage is applied between the segment and the backplane, and will turn off when there is no voltage between the two. Rather than generating an AC signal, it is common practice to produce the required AC voltage by applying out-of-phase square waves to the segment and backplane. For one segment, a 40-Hz square wave is applied to the backplane and also to the input of a Complementary Metal-Oxide-Semiconductor (CMOS) 4070 exclusive-OR gate. The other input to the XOR is a CONTROL input that will control whether the the segment is ON or OFF. Aviation Australia Applying out-of-phase square waves to the segment and backplane via XOR gate 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 220 of 285 CASA Part 66 - Training Materials Only  When the CONTROL input is LOW, the XOR output will be exactly the same as the 40-Hz square wave, so that the signals applied to the segment and backplane are equal. Since there is no difference in voltage, the segment will be off. Aviation Australia XOR control input signal is low turns segment off When the CONTROL is HIGH, the XOR output will be the INVERSE of the 40-Hz square wave, so that the signal applied to the segment is out of phase with the signal applied to the backplane. As a result, the segment voltage will alternatively be at +5 V and -5 V relative to the backplane. This AC voltage will turn on the segment. Aviation Australia XOR control input signal is high turns segment on 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 221 of 285 CASA Part 66 - Training Materials Only  Driving a Seven-Segment LCD This same idea can be extended to a complete seven-segment LCD. The Binary Coded Decimal (BCD) segment is ON or OFF. A BCD-to-seven-segment decoder/driver supplies the CONTROL signals to each of seven XORs for the seven segments. In general, CMOS devices are used to drive LCDs for two reasons: (1) they require much less power than Transistor–Transistor Logic (TTL) and are more suited to battery-operated applications where LCDs are used, and (2) the TTL LOW-state voltage is not exactly 0 V and can be as much as 0.4 V. This produces a DC component of voltage between the segment and backplane that considerably shortens the life of an LCD. Aviation Australia A BCD to 7-segment decoder/driver to control a seven-segment LCD 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 222 of 285 CASA Part 66 - Training Materials Only  Reflective LCD Liquid crystal materials emit no light of their own. Small and inexpensive LCDs are often reflective, which means to display anything they must reflect light from external light sources. Look at an LCD watch: The numbers appear where small electrodes charge the liquid crystals and make the layers untwist so that light is not transmitting through the polarised film. LCDs are common in watches due to the low electrical power demands of the LCD panel. This panel is composed of two polarisers that transmit light in perpendicular directions, a mirrored surface and a layer of liquid crystal material that is sandwiched between two electrically conducting glass plates. The liquid crystal material used is of the so-called twisted nematic type. Aviation Australia When light passes through both polarisers with no voltage on electrodes as the liquid crystals are not aligned - segment is clear The liquid crystal molecules in all segments of the panel are precisely aligned in the absence of an applied voltage. Therefore, the entire panel appears silvery because light passes through both polarisers, reflects off the mirrored surface and then passes back through both polarisers again. If the surfaces of the glass plates that will be in contact with the liquid crystal molecules are ribbed, the liquid crystal molecules orient in the direction of the ribbing. Molecules have been oriented in the direction in which the adjacent polariser transmits light, and the intervening molecules gradually rotate their relative orientation to accommodate the 90̊ change from one polariser to the other. This gives a silvery appearance to the panel. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 223 of 285 CASA Part 66 - Training Materials Only  Operation of a LCD The initial alignment of the liquid crystal molecules is lost when a voltage is applied to a segment of the panel. That segment will then appear black against a silver background. When a voltage is applied to a segment of the display, the precise alignment of the liquid crystal molecules is lost. This results in the polarised light from the first polariser not being rotated by the required 90° to align with the second polariser. The second polariser blocks the passage of light and causes that segment of the panel to appear black. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 224 of 285 CASA Part 66 - Training Materials Only  Backlit LCD A backlit LCD functions in the same manner as normal-type LCDs (including the reflective type) except it uses backlighting. Most computer displays are lit with built-in fluorescent tubes above, beside and sometimes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. On its way through filters, liquid crystal layers and electrode layers, a lot of this light is lost – often more than half. Backlit LCD 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 225 of 285 CASA Part 66 - Training Materials Only  Greyscale LCDs If we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a greyscale. Most displays today offer 256 levels of brightness per pixel. Controlling the voltage applied to an LCD segment in very small increments can vary the light levels in each segment or pixel. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 226 of 285 CASA Part 66 - Training Materials Only  Colour LCDs An LCD that can show colours must have three sub-pixels with red, green and blue colour filters to create each colour pixel. Through the careful control and variation of the voltage applied, the intensity of each sub-pixel can range over 256 shades. Combining the sub-pixels produces a possible palette of 16.8 million colours (256 shades of red × 256 shades of green × 256 shades of blue). LCD that can show colours must have three sub pixels with red, green and blue colour filters 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 227 of 285 CASA Part 66 - Training Materials Only  Additive Colour Mixing Additive colour mixing is the combination of projected beams of coloured light to form other colours. Many find this model hard to understand, simply because it does not work like anything you have learned about before. But since additive colour mixing is how the eye (and electronic displays) produce colour, it is important to know about. It uses light to create colours – shining red, green and blue (additive primary colours) together to obtain white in the overlap of all three. Conversely, white light can be split into colours (e.g. prisms, rainbows). You get cyan, magenta and yellow in the other overlaps. These are additive secondary colours. Black is the absence of light when dealing with additive colours. Aviation Australia Additive colour mixing is how the eye (and electronic displays) produce colour LCD Sub-Pixels LCD sub-pixels - Red, Green and Blue stripes arranged in a regular matrix 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 228 of 285 CASA Part 66 - Training Materials Only  Red, green and blue stripes are arranged in a regular matrix. The gaps between pixels (the shadows) are for the drive circuits and wires. The black lines are significant in that they cannot have their colour changed, yet they take up space. Getting as close as possible to the right colours in the right places is what image display quality is all about. Note how sub-pixels (magnified) are controlled to obtain colour and clarity. Colour LCDs Colour displays use an enormous number of transistors. For example, a computer that supports resolutions up to 1024 × 768 has 2,359,296 transistors etched onto the glass (multiply 1024 columns by 768 rows by 3 sub-pixels). If there is a problem with any of these transistors, it creates a ‘bad pixel’ on the display. Bad pixels may appear black (no operation of any colour sub-pixel) or bright (where one or all of the colours are fully on), producing a bright single primary colour or bright white pixel. Luís Flávio Loureiro dos Santos, CC BY 3.0 Close-up view of LCD monitor showing variations or RGB pixels 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 229 of 285 CASA Part 66 - Training Materials Only  Cathode Ray Tube Thermionic Emission – Edison Effect Thomas Edison discovered the principle of thermionic emission as he looked for ways to keep soot from clouding his incandescent light bulb. Edison placed a metal plate inside his evacuated bulb along with the normal filament. He left a gap, a space, between the filament and the plate. He then placed a battery in series between the plate and the filament, with the positive side towards the plate and the negative side towards the filament. When Edison connected the filament battery and allowed the filament to heat until it glowed, he discovered that the ammeter in the filament-plate circuit had deflected and remained deflected. He reasoned that an electrical current must be flowing in the circuit – even across the gap between the filament and plate. In the following diagram the heated filament causes electrons to boil from its surface. The battery in the filament-plate circuit places a positive charge on the plate (because the plate is connected to the positive side of the battery). The electrons (negative charge) that boil from the filament are attracted to the positively charged plate. They continue through the ammeter and the battery, and back to the filament. The electron flow across the space between filament and plate is the application of the a basic electrical law – unlike charges attract. © Aviation Australia Thermionic emission – Edison effect 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 230 of 285 CASA Part 66 - Training Materials Only  Remember, Edison's bulb had a vacuum so the filament would glow without burning. Also, the space between the filament and plate was relatively small. The electrons emitted from the filament did not have far to go to reach the plate. Thus, the positive charge on the plate was able to attract the negative electrons. The key to this explanation is that the electrons were floating free of the hot filament. It probably would have taken hundreds of volts to move electrons across the space if they had to be forcibly pulled from a cold filament. Such an action would destroy the filament and the flow would cease. The application of thermionic emission that Edison made in causing electrons to flow across the space between the filament and the plate has become known as the Edison effect. You will remember that metallic conductors contain many free electrons, which at any given instant are not bound to atoms. These free electrons are in continuous motion. The higher the temperature of the conductor, the more agitated the free electrons and the faster they move. A temperature can be reached where some of the free electrons become so agitated that they actually escape from the conductor. They ‘boil’ from the conductor's surface. The process is similar to steam leaving the surface of boiling water. Heating a conductor to a temperature sufficiently high to cause the conductor to give off electrons is called thermionic emission. Cathode Ray Tube Older television technology (prior to the LED and plasma flat screens) used the cathode ray tube (CRT). This technology was commonly used to present the picture on the screen of a television set. Aviation Australia Cathode Ray Tube (CRT) 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 231 of 285 CASA Part 66 - Training Materials Only  CRTs are used in more applications than just television. They have one function that cannot be duplicated by any other tube or transistor, namely, converting electronic signals to visual displays such as pictures, radar sweeps or electronic wave forms. All CRTs have three main elements: an electron gun, a deflection system and a screen. The electron gun provides an electron beam, which is a highly concentrated stream of electrons. The deflection system positions the electron beam on the screen, and the screen displays a small spot of light at the point where the electron beam strikes it. The front of the tube is phosphor-coated and when electrons hit it, light is emitted on the other side (the side where you are sitting). Electron Gun The cathode of the electron gun in the CRT is required to emit electrons in a concentrated beam. In electron tubes, the cathode is cylindrical and emits electrons in all directions along its entire length. To produce a highly concentrated electron beam the cathode is fitted with a small-diameter nickel cap. The closed end of the cap is coated with emitting material. This type of construction 'fires' the electrons in one direction. CRT electron gun Initially the emitted electrons are leaving the cathode at different angles. If these electrons were allowed to strike the screen, the whole screen would glow. The purpose of the electron gun is to concentrate the electrons into a tight beam. A special grid in the form of a solid metal cap with a small hole in the centre is used. The grid is placed over the emitting surface of the cathode and charged negatively in relation to the cathode. The blue dotted lines represent the direction of cathode- emitted electron repulsion. This repulsion forces emitted electrons to the centre and out through the hole in the grid. Consider an electron leaving the cathode in an upwards direction. Its path will be curved due to electrostatic repulsion. These curving electron paths result from the negative potential of the grid coupled with the high positive potential of the anode. The potential of the anode attracts electrons out of the cathode-grid area towards the screen. The grid potential may be varied to control the number of electrons allowed to go through the control- grid opening. The brightness or intensity of the display is determined by the number of electrons that strike the screen, the control grid is used to control the brightness of the CRT. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 232 of 285 CASA Part 66 - Training Materials Only  CRT schematic showing electron gun with control grid, focus anode and accelerating anode The proper name, brightness control, is given to the potentiometer used to vary the potential applied to the control grid. The control grid actually serves as an electron lens. It is this electronic lens that you adjust when you turn up the brightness control on your TV set. Notice that the effect of the grid is to focus the electron beam at point P. After passing point P, the electrons start to spread out, or diverge, again. Therefore, it becomes necessary to provide some additional focus that forces the electrons into a tight beam again. This is done by two additional positively charged anodes. Generally, the focusing anode is charged a few hundred volts positive with respect to the cathode. Electrons emitted by the cathode are attracted to the focusing anode. This is why they travel through the small hole in the grid. The second electrode, called the accelerating anode, is charged several thousand volts positive in relation to the cathode. Any electrons approaching the focusing anode will feel the greater electrostatic pull of the accelerating anode, will be bent through the opening in the focusing anode and will travel into the area labelled D. Because the accelerating anode is cylindrical, the electrostatic field radiating from it is equal in all directions. Electrons in this section are pulled in all directions at once, forcing the electron to travel down the centre of the tube. Then, the electron is accelerated into the accelerating anode. Once electrons pass the midpoint (point E), the electrostatic attraction from the front wall of the accelerating anode causes the beam to accelerate towards the screen. At point F equal electrostatic attraction on either side of the opening focus the beam through the small opening in the front of the anode. From there the electron travel in a tight beam until it strikes the screen (point S). 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 233 of 285 CASA Part 66 - Training Materials Only  The CRT Screen The inside of the large end of a CRT is coated with a fluorescent material that gives off light when struck by electrons. This coating is necessary because the electron beam itself is invisible. The material used to convert the electrons’ energy into visible light is a phosphor. Many different types of phosphor materials are used to provide different coloured displays and displays that have different lengths of persistence (duration of display). The CRT screen can suffer from the effects of secondary emission. In order to reach the screen, electrons from the cathode are accelerated to relatively high velocities. When these electrons strike the screen, they dislodge other electrons from the material of the screen. If these secondary emission electrons are allowed to accumulate, they will form a negatively charged barrier between the screen and the electron beam, causing a distorted image on the CRT screen. To control secondary emission in CRTs a special coating called aquadag is applied to the inside of the tube. This coating is composed of a conductive material, such as graphite, and has the same high- positive potential applied to it that is applied to the accelerating anode. The highly positively charged aquadag performs two functions: 1. it attracts the secondary emitted electrons and removes them and 2. it aids in the acceleration of electrons towards the screen Aquadag coating on the inside of the CRT control secondary emission and accelerate the electron beam to phosphor coated front screen 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 234 of 285 CASA Part 66 - Training Materials Only  CRT Operation Review Fundamental CRT operating principles: Electrons are emitted from a specially constructed cathode and are fired towards the front of the CRT The number of electrons that leave the area of the cathode is determined by the potential on the grid to achieve brightness control The grid concentrates the emitted electrons into a beam The electron beam is focused and accelerated towards the screen by two electrodes: the focusing anode and the acceleration anode The electron beam strikes the screen and causes a bright spot to appear at the point of impact due to a phosphor coating Any electrons released by secondary emission are removed from the tube by the aquadag coating CRT Electron Beam Deflection At this point, there is an electron beam creating a bright spot in the centre of the CRT screen. To produce a picture on the screen the spot must be moved to various positions on the screen. In a TV set, for example, the spot is moved horizontally across the CRT face to form a series of tightly packed lines. As each line is displayed, or traced, the electron beam is moved vertically to trace the next line as shown in the illustration below. The production of the image starts at the top of the tube and ends when the last line is traced at the bottom of the CRT screen. Because the beam is swept very quickly across the CRT and the phosphor continues to glow for a short time after the beam has moved on, you do not see a series of lines, but a continuous picture. Moving or manipulating electron beam around the CRT screen is called deflection 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 235 of 285 CASA Part 66 - Training Materials Only  As you should know, there are two ways to move an electron (and thus an electron beam): with either a magnetic or an electrostatic field. Because of this, there are three possible ways to move or deflect an electron beam in a CRT: magnetically, electromagnetically and electrostatically. All three ways are used in electronics. In general, though, electrostatic and electromagnetic deflections are used most often. Your TV set, for example, uses electromagnetic deflection. Electron Beam - Electrostatic Deflection Electrostatic deflection use the basic principle of opposites attract, and likes repel. In the image view A shows an electron travelling between two charged plates, H1 and H2. Before the electron reaches the charged plates, called deflection plates, it is directed towards the centre of the screen. In view B, the electron (negative charge) has reached the area of the deflection plates and is attracted towards the positive deflection plate, H2, while being repelled from the negative deflection plate, H1. As a result, the electron is deflected to the right on the inside of the screen. Viewing from the front of the screen will show the spot of light on the left side of the CRT face. This is represented in view C. Electron travelling between two deflection plates, towards front of CRT 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 236 of 285 CASA Part 66 - Training Materials Only  Horizontal Deflection of CRT Electron Beam A spot of light on the left side of the CRT screen, however, is no more useful than a spot of light in the centre of the screen. To be useful, this spot will have to be converted to a bright line, called a sweep, across the face of the CRT screen. Horizontal deflection of CRT electron beam - sweep controlled by variable resistors R1 and R2 controlling potential on deflection plates In view A, five electrons are emitted in sequence, 1 through 5, by the electron gun. The right deflection plate, H2, has a large positive potential on it, while the left plate, H1, has a large negative potential on it. Thus, when electron 1 reaches the area of the deflection plates, it is attracted to the right plate while being repelled from the left plate. In view B, electron 2 has reached the area of the deflection plates. However, before it arrives, R1 and R2 are adjusted to make the right plate less positive and the left plate less negative. Electron 2 will still be deflected to the right, but not as much as electron 1. In view C, electron 3 has reached the area of the deflection plates. Before it arrives, R1 and R2 are adjusted to the midpoint. As a result, both plates have 0 V applied to them. Electron 3 is not deflected and simply travels to the centre of the CRT screen. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 237 of 285 CASA Part 66 - Training Materials Only  In view D, electron 4 has reached the area of the deflection plates. Notice that R1 and R2 have been adjusted to make the right plate negative and the left plate positive. As a result, electron 4 will be deflected to the left. Finally, in view E, the left plate is at its maximum positive value. Electron 5 will be deflected to the extreme left. What you see when facing the CRT is a bright luminous line, as shown in view E. While this description dealt with only five electrons, in reality the horizontal line across a CRT face is composed of millions of electrons. Instead of seeing five bright spots in a line, you will see only a solid bright line. In summary, the horizontal line displayed on a CRT or on the face of a television tube is made by sweeping a stream of electrons rapidly across the face of the CRT. This sweeping action, or scanning, is performed by rapidly varying the voltage potential on the deflection plates as the electron stream passes. Vertical Deflection of CRT Electron Beam A CRT can be used to graphically and visually plot an electronic signal, such as a sine wave. This is done by using a second set of deflection plates called vertical-deflection plates. In normal usage, the horizontal plates sweep a straight line of electrons across the screen from left to right while the signal to be displayed is applied to the vertical deflection plates. Horizontal Cathode Ray Tube HORIZONTAL DEFLECTION PLATES De ection Circuits VERTICAL DEFLECTION TRANSFORMER CRT CRT VERTICAL DEFLECTION PLATES (A) (B) Aviation Australia CRT horizontal and vertical deflection plates The box on the left of the CRT labelled ‘horizontal deflection circuits’ is an electronic circuit that will duplicate the actions of R1 and R2 used earlier in making up a horizontal line. The output of the vertical deflection transformer is applied to the vertical-deflection plates. The signals applied to the vertical plates are 180° out of phase with each other. Thus, when one plate is attracting the electron beam, the other is repelling the electron beam. The external circuitry has been removed and only the CRT and its deflection plates will be shown, as in view B. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 238 of 285 CASA Part 66 - Training Materials Only  Cathode Ray Tube Time T 1 T2 T3 T4 T 5 +v Repel Attract 0v -v 4 VERTICAL DEFLECTION 1 3 5 PLATES 2 +v 0v -v Attract Repel (C) Aviation Australia CRT electron beam displacement with deflection plate potentials In view C, the sine wave in the centre of the screen is the signal that will be displayed as a result of the two 180° out-of-phase sine waves applied to the vertical-deflection plates. The five spots on the centre sine wave represent the five electrons used to explain horizontal deflection. Only now these electrons will be deflected both vertically and horizontally. Time lines T1 through T5 represent the time when each like-numbered electron reaches the area of the deflection plates. As the electron beam is swept or deflected horizontally, we will not discuss horizontal deflection. Just remember that from T1 to T5, the electron beam will be continuously moved from your left to your right. At T1, the sine waves applied to both vertical-deflection plates are at their null points, or 0 V. As a result, electron 1 is not vertically deflected and strikes the CRT at its vertical centre. At T2, the sine wave applied to the top plate is at its maximum negative value. This repels electron 2 towards the bottom of the CRT. At the same time, the sine wave applied to the bottom plate is at its most positive value, causing electron 2 to be attracted even farther towards the bottom of the CRT. Remember, the beam is also being moved to the left. As a result, electron 2 strikes the CRT face to the right of and below electron 1. At T3, both sine waves applied to the vertical-deflection plates are again at the null point, or 0 V. Therefore, there is no vertical deflection and electron 3 strikes the CRT face in the centre of the vertical axis. Because the electron beam is still moving horizontally, electron 3 will appear to the right of and above electron 2. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 239 of 285 CASA Part 66 - Training Materials Only  At T4, the sine wave applied to the top vertical-deflection plate is at its maximum positive value. This attracts electron 4 towards the top deflection plate. The upwards deflection of electron 4 is increased by the negative-going sine wave (at time 4) applied to the bottom deflection plate. This negative voltage repels electron 4 upwards. Thus, electron 4 strikes the CRT face to the right of and above electron 3. Finally, at T5, both input sine waves are again at 0 V. As a result, electron 5 is not deflected vertically, only horizontally. (Remember, the beam is continually moving from right to left.) Deflection Coils Deflection coils may also be used in CRTs. These are electromagnets and serve the same purpose as deflection plates. N NECK OF CRT VERTICAL DEFLECTION ELECTROMAGNETIC COILS CRT ELECTRON BEAM MAGNETIC FIELD ELECTRON BEAM FIELD S HORIZONTAL DEFLECTION OF ELECTRON BEAM Aviation Australia Electron beams in a vacuum tube can be moved when subjected to electromagnetic fields 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 240 of 285 CASA Part 66 - Training Materials Only  Magnetic Deflection In order to ‘trace out’ a luminescent display, it is necessary for the spot of light to be deflected about the horizontal and vertical axes, and for this purpose a beam deflection system is provided. A moving electron constitutes an electric current, so a magnetic field will exist around it. This occurs in the same way that a conductor experiences a deflecting force when placed in a magnetic field so an electron beam can be forced to move when subjected to electromagnetic fields acting across the space within the vacuum tube. Deflection coils provided around the neck of the tube Coils are therefore provided around the neck of the tube and are configured so that fields are produced horizontally (x-axis field) and vertically (y-axis field). The deflection coils are then connected to the source signal that generates the required display and the electron beam can be deflected left/right and up/down depending on the polarities produced by the coils. Aviation Australia Electron beam can be deflected left/right and up/down depending on polarities of the deflection coils 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 241 of 285 CASA Part 66 - Training Materials Only  Summary of CRT Operation Aviation Australia A schematic diagram and a pictorial representation of a CRT Heater – heat source for cathode. Cathode – source of thermionic emitted electrons; outer surface is coated to ensure that electron emission is roughly unidirectional. Control grid – controls number of electrons that will be fired or emitted. Focusing anode – attracts electrons from grid and focuses into beam. Accelerating anode – accelerates electrons towards front of tube. Vertical-deflection plates – moves electron beam up and down screen. Horizontal-deflection plates – moves electron beam horizontally across screen. Electromagnetic coils may also be used for horizontal and vertical deflection of the electron beam. In most equipment using CRTs, including TV, electronic signals are supplied to these plates/coils to trace or paint a horizontal line. Aquadag coating – positively charged, eliminates effects of secondary emission and aids in acceleration of electrons towards screen. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 242 of 285 CASA Part 66 - Training Materials Only  Screen – phosphor coated, glows when struck by electrons. CRTs operate on two principles: Thermionic emission Electrostatic/electromagnetic attraction and repulsion. CRT components 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 243 of 285 CASA Part 66 - Training Materials Only  CRT Handling and Disposal Safety There are certain safety precautions you should follow when you work with or handle cathode ray tubes. The glass envelope of a CRT encloses a high vacuum. Because of its large volume and surface area, the force exerted on a CRT by atmospheric pressure is considerable. The total force on a 10-in. CRT may exceed 4000 lb. Over 1000 lb is exerted on the CRT face alone. When a CRT is broken, a large implosion usually occurs. The face of the CRT is coated with a chemical coating that is extremely toxic. If you are unfortunate enough to experience an accidental implosion of a CRT and are nicked by one of these fragments, seek immediate medical aid. When handling a CRT, you should take the following precautions: Avoid scratching or striking the surface of the CRT. Do not use excessive force when you remove or replace a CRT's deflection yoke or socket. Do not try to remove an electromagnetic-type CRT from its yoke until you have discharged the high voltage from the CRT's anode connector (hole). Never hold the CRT by its neck. Always set the CRT with its face down on a thick piece of felt, rubber or smooth cloth. Always handle the CRT gently. Rough handling or a sharp blow on the service bench can displace the electrodes within the tube, causing faulty operation. Wear safety glasses and protective gloves. Disposal of CRTs One additional handling procedure you should be aware of is how to dispose of a CRT properly. Neck and connector of a CRT 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 244 of 285 CASA Part 66 - Training Materials Only  Note: Be sure to wear safety goggles. Place the CRT face down in an empty carton and cover its side and back with protective material. Carefully break off the plastic locating pin from the base by crushing the locating pin with a pair of pliers. Brush the broken plastic from the pin off the CRT base. Look into the hole in the base where the locator pin was. You will see the glass extension of the CRT called the vacuum seal. Grasp the vacuum seal near the end with the pliers and crush it. This may sound a little risky, but it is not. The vacuum seal can be crushed without shattering the tube. Once the seal has been crushed, air will rush into the tube and eliminate the vacuum. Coloured CRTs A colour CRT has three electron guns. Each one can direct an electron beam at the screen, which is coated with three different types of phosphor material. On being bombarded by electron beams, the phosphors illuminate in each of the three primary colours: red, green and blue. The beam from a particular electron gun assembly must be able to strike only the small areas or dots of phosphor of one colour. It is not a beam colour that determines the colour displayed on the screen (each gun just fires electrons), and it is the colour of the phosphor dot that is illuminated. A colour CRT has three electron guns. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 245 of 285 CASA Part 66 - Training Materials Only  CRT Screen Shadow Mask The tricky part is to make sure each electron gun can hit only the phosphor dots for its colour. This is done by arranging three dots of different colours in groups, called triads. Since there are now three electron beams coming from three separate guns, each hits the phosphors from a slightly different angle. A thin sheet called the shadow mask is suspended in front of the phosphor dots on the screen. The shadow mask has one hole for each triad and is arranged so that each beam can ‘see’ only the phosphor dots for its colour. CRT shadow mask and triad of phosphor dots The shadow mask is a thin, perforated sheet suspended in front of the phosphor dots. There is one hole in the shadow mask for each phosphor triad. Since the three electron beams go through the shadow mask holes from slightly different angles, each beam can light up only the dots for its colour. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 246 of 285 CASA Part 66 - Training Materials Only  Simultaneous Picture Formation Directly facing the scene is a panel holding enough light-sensitive electronic devices, in this case 25 photocells, to be able to cover the whole scene. The light accepted by each photocell is converted into a separate electrical signal, and the more intense the light, the greater the amplitude of the signal. These signals are conveyed, each by a separate wire, to a bank of 25 amplifiers. The amplifiers are necessary to increase the signals to a strength adequate for feeding into an array of 25 more lines connected to a distant receiver. The link may involve direct contact, as for closed circuit television, or transmitted communication, as for standard commercial television. Aviation Australia Simultaneous picture/image formation from capture to presentation At this point, the cost of the system is becoming exorbitant. The provision of 25 amplifiers, each adjusted to provide exactly the same characteristics as the others (gain, bandwidth, etc.), is difficult enough, but if transmission is involved, 25 separate frequencies to carry each of the signals will be required, together with an appropriate aerial system and communications arrangement. At the receiver, a similar arrangement of amplifiers for every line in the link will be required. Each of the signals is then fed to a lamp focused on its area of the viewing screen. The larger the amplitude of the signal leaving each amplifier, the brighter the lamp it controls will shine, and the stronger the beam of light which the lamp throws onto the viewing screen in the corresponding position. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 247 of 285 CASA Part 66 - Training Materials Only  Picture Resolution The inability to resolve fine detail is, of course, principally due to the comparatively small numbers of lamps and photocells used in the system. If the numbers of both were substantially increased, the resolution of the system would be somewhat improved. A revised letter H could, for example, be made up of 100 areas of illumination instead of only 25. This would involve a corresponding increase in the number of photocells, lamps, lines and all the other paraphernalia involved in the system. However, the end result would be very little improvement in the picture resolution. Clearly, for a TV system to be practical, some other technique must be found. Sequential Scanning The disadvantage of the primitive simultaneous image reproduction system can be greatly reduced by using the peculiar property of the eye called persistence of vision. If signals from the photocells in the camera are not transmitted simultaneously, but sent one after the other in very rapid succession, persistence of vision in an eye at the receiving end will create the illusion that the image observed there is really made up of a large number of simultaneously produced areas of illumination which are free of flicker. Aviation Australia Sequential scanning - signals from individual photocells are transmitted sequentially (one after other) in very rapid succession The method of transmitting the photocell signals in rapid succession is called sequential scanning. It is a basic principle behind all television systems. Every one of the 25 photocells in the transmitter is connected to a fast-moving scanning switch which picks up, one after the other, signals corresponding to the brightness level of the area of the scene which each individual photocell is watching and feeds it to a common amplifier. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 248 of 285 CASA Part 66 - Training Materials Only  Connections to the switch are arranged so that signals are collected from the photocells in sequence, top line first from left to right, then down to the second line from left to right, and so on from top to bottom. When the switch reaches photocell 25 in the bottom right corner, it flies back very rapidly to number 1 and begins the cycle all over again. The output from the common amplifier is fed to an aerial and transmitted as a modulated radio frequency signal. At the receiver, the signals picked up are amplified and then connected to another switch, synchronised with that in the transmitter, which in turn is connected to a bank of 25 lamps arranged in the same geometric pattern as the photocells. The brilliance of lamp 4 is thus controlled by the signal produced from photocell 4, and so on. The area of the viewing screen illuminated by each lamp corresponds to the area of the transmitted scene viewed by the corresponding photocell. One obvious advantage of this system over the more primitive one is that the equipment needed is much reduced. Another is that normal radio communication methods can be applied to the single link between transmitter and receiver. A third is that a large number of channels can be served without serious bandwidth problems. The key to the system is perfect synchronisation of the scanning switch in the transmitter with the lamp selector switch in the receiver. Without it, heavy distortion of the reproduced image will occur. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 249 of 285 CASA Part 66 - Training Materials Only  Scanning Raster In the picture tube of the TV receiver, the spot is made to move across the screen rapidly from left to right, and at the same time in a series of horizontal lines from top to bottom. When it reaches the bottom right corner of the screen, it is returned very quickly to the top left corner, and the scanning cycle is repeated. If this sequence is repeated fast enough, about 50 times per second, the whole screen will have been scanned and the fluorescence of the top line will have been renewed by a second scan before the light of the first scan has had time to fade in the eye of the viewer. The image presented will be a series of parallel lines of light running very close together almost horizontally across the screen. This presentation, parallel lines of light with no picture content, is known as the scanning raster, in which the number of lines has been much reduced for greater clarity. It should be noted that during the unwanted right-to-left and bottom-to-top movement of the spot, called the flyback periods, the electron beam is suppressed altogether and produces no trace. Aviation Australia Scanning raster - Screen of receiver (TV), the spot or electron beam moves across screen very rapidly from left to right, and in a series of horizontal lines from top to bottom The scanning raster movement may be seen as similar to the movement of the eye in reading the printed page. It starts at the top left corner of the page and scans fairly slowly from left to right, then flies back very quickly to the left side to start again on the next line. It will also be obvious that the page-scanning rate is much lower than the line-scanning rate. Notionally, there will be 50 sets of line scans per second. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 250 of 285 CASA Part 66 - Training Materials Only  The raster, in its normal state, produces only a series of parallel lines of light running horizontally across the screen and presents no picture detail at all. However, the CRT modulator grid voltage can be varied by the picture signal in order to vary, or modulate, the brightness of the raster. It can be taken from the brightest level, called Peak White, to the darkest, called Black, very quickly indeed. Interlace Scanning It appears at this stage that normal TV practice is to cause the beam to scan the screen at no less than 50 times per second so that flicker of the image is avoided. However, a picture repetition frequency as high as 50 times per second would call for a very wide frequency bandwidth for transmission of the video signal, and would therefore greatly restrict the number of channels which could be accommodated within a given frequency band. It would also make receiver design more complex, leading to increased receiver cost to the user. An ingenious way of getting around the bandwidth difficulty has been found. It is called interlaced scanning. Instead of the target in the camera tube and the screen in the picture tube being scanned in consecutive lines, the beam is first made to scan all the odd-numbered lines, in their proper order, followed by all the even lines, down to the penultimate line of the raster. Aviation Australia Interlace scanning - Beam is driven horizontally and vertically by two circuits called timebases From there it flies back to the beginning of line 1 and the process is repeated. Thus the picture is comprised of two 'fields', one for the odd lines and one for the even lines. This technique allows the picture frequency to be lowered to 25 times per second, reducing the bandwidth problem. The interlacing causes each line, apart from top and bottom, to be formed between two which are fading. This cheats the eye into thinking the picture is being produced at twice the rate. The result is a continuous picture without flicker. The half-lines at top and bottom allow flyback co-ordination without the necessity for special timing elements. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 251 of 285 CASA Part 66 - Training Materials Only  The spot is driven horizontally and vertically by two circuits called timebases. The Line Timebase moves the spot from left to right, and the Field Timebase from top to bottom. The video signal itself resets the timebases at the appropriate times to make the spot fly back from right to left and from bottom to top. This part of the raster scanning is called, appropriately, the flyback. The spot is turned off, or 'blanked' during flybacks. The Field Timebase scans much slower than the Line Timebase. At the beginning of a frame, the two timebases are both reset so that the spot is at position A. Both timebases then start scanning. The video signal triggers line flybacks at the appropriate time until the end of the first field (odd) is reached. Interlaced raster example Halfway through the last line (B), the field timebase is reset. The spot then flies back to C and continues with the remainder of the line, then scans all the even lines in a similar manner. Because the field timebase was reset halfway through a line, the even field naturally interlaces between the lines of the odd field. When the spot reaches the end of the final line (D), both timebases are reset and the spot flies back to A, ready for the next frame. This diagram gives the impression that the scan lines are at an angle. That is because so few lines are shown for clarity. In fact, the angle is much less than 1° in reality and is easily corrected by twisting the coils on the tube that deflect the electron beam. The diagram also shows that the visible part of the screen is smaller than the area of the scanned raster. This is because it actually takes several line-scans for the frame timebase to fly back to the beginning (rather than the instantaneous paths shown for clarity in the diagram). The slight 'overscan' on the lines themselves also allows the spot time to stabilise before starting its active scan. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 252 of 285 CASA Part 66 - Training Materials Only  Interlacing, then, is an effective way of reducing the flicker in the picture without increasing the amount (known as 'bandwidth') of transmitted information. The problem is it can sometimes produce an undesirable vertical 'bounce' on images that have strong horizontal detail. As an example, look at the illustration, which shows a magnified capital F as it may appear on the screen. Aircraft CRTs In an aircraft CRT, the symbol generators control the picture painted on the screen, controlling the firing of the electron beams at the appropriate coordinates on the screen grid to paint single-colour pictures which are refreshed about 50 times a second. Aviation Australia Aircraft CRTs Some aircraft have only one or two CRTs, while others with a full glass cockpit system may use six or more. Modern aircraft display systems may alternatively be LED- or LCD-type displays. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 253 of 285 CASA Part 66 - Training Materials Only  The electronic instruments that make up a full glass cockpit come in four main types: Electronic Attitude Director Indicators (EADIs) Electronic Horizontal Situation Indicators (EHSIs) Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralised Aircraft Monitoring system (ECAM) Head-Up Display (HUD) or Head-Up Guidance System (HGS). Care of Electronic Instrument Displays Electronic displays and instrument panel lighting should, whenever possible, be turned down from full brightness to prolong life. Screens of electronic instruments should be cleaned carefully to avoid scratching. Typically only side- to-side action is permitted using only a soft lint-free cloth and an approved cleaning detergent/agent. Avoid touching display screens with your fingers, which introduces smudges. 2023-03-03 B1-05a Digital Techniques / Electronic Instrument Systems Page 254 of 285 CASA Part 66 - Training Materials Only