Additional Questions
Q.Describe different Display system Technologies
Ans.A variety of display system technologies are employed for decoding signals and compressed data for displaying acceptable renderings of the original images (images is used here in a generic sense to include full-motion video).
An important aspect of displaying images and full-motion video in windows is the capability to dynamically resize the window to suit the users preferences.
Resizing the window causes the number of pixels being displayed to change. As the window becomes larger, pixels per inch of the original rendering change. For example, if a video is originally shot in Quicktime using a window size of 640pixels by 480 pixels, it would display at normal resolution on a full VGA screen.
If reduced to a quarter-sized window on the same screen, both the x and y axes are scaled by a factor of two. However if the same video is displayed on an 8514A screen with a resolution of 1028 x 768 pixels, it will show at its original resolution in a window consisting of 640 pixels horizontally and 480 pixels vertically, that is, a window about two-thirds the size of the original rendering.
Scaling down to a smaller window is achieved by dropping pixels, but scaling up to a larger window (for example, a full screen size of 1024 x 768 pixels) requires adding pixels that do not exist in the originally captured image. Dynamic scaling becomes even more complex if the display window contains full-motion video playing at 30 frames/sec.
Combined graphics, imaging, and full-motion video applications require functionality for dynamic scaling which is addressed by mixing and scaling technologies in single-monitor architecture with substantially varying level of capabilities. For a VGA screen, these technologies include the following:
VGA mixing
In VGA mixing, images from multiple sources are mixed in the image acquisition memory. The image acquisition memory also serves as the display source memory. An image once captured, is fixed in memory and its position and size on screen is fixed.
VGA mixing with scaling
Use of scalar ICs allows sizing and positioning of images in predefined windows. The scaling and resizing causes the original image data to be lost in memory since the same buffer is used for the scaled and resized result. For most multimedia applications, this maybe fine. However, resizing the window causes the image to be retrieved again. For some applications this can be very disconcerting if the resizing does not appear to be a smooth operation.
Dual buffered VGA mixing/scaling
The loss of the original image is overcome by providing dual buffering. The original image is maintained in a separate buffer so that another round of scaling and resizing can be achieved dynamically. Double buffer schemes maintain the original images in a decompression buffer and the resized image in a display buffer.
Digital Signal Processor
Another emerging approach is the use of digital signal processors (DSPs) for display image processing that integrates graphics through an optional 34020-based daughter board equipped with VGA pass-through and, in some versions, with double buffering. On the input side, frame grabbers feature self-adjusting variable-scan input that accepts input from video cameras, VCRs, or RS-170/NTSC- or CUR/PAL-compatible still-video devices, scanning electron microscopes, and various other devices.
Daughter boards handle color NTSC, PAL, and RGB video cameras, variable-scan input from monochrome cameras, and digital video cameras. Over-the-top cables avoid bogging down the main system bus. High-performance boards incorporate a VGA feature connector and have their own backdoor bus for high-speed interboard transfers to and from accelerator boards or memory boards.
Q.What is CRT Monitor?
Ans. The cathode ray tube (CRT) is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets and others.
A cathode ray tube is a vacuum tube which consists of one or more electron guns, possibly internal electrostatic deflection plates, and a phosphor target. In television sets and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference. In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube, although electrostatic deflection is commonly used in oscilloscopes, a type of diagnostic instrument.
Q.Describe constructional details of CRT?
Ans.CRT is a large bottle shaped vacuum tube. The picture tube used in TV set is an example of CRT. An electron gun at the rear end of tube produces beam of electrons, which is directed towards the frond end of tube. The surface inside the front of tube is coated with phosphor material, which produces light when electron hits the screen.
The deflection system consist of the cathode ray tube consist of two pairs of pairs of parallel plates, referred as the vertical and horizontal deflection plates , the voltage applied to vertical plates controls the vertical deflection of the electron beam and the voltage applied to horizontal plates controls the horizontal deflection of the electron beam.
The color of light produced depends on different phosphor material. To produce color display as in the color TV sets, dots of Red, Blue and Green producing phosphor are used inside of screen in triangular pattern.
Separate electron beam is focused on each dot for different colors. By alternating the intensity rather of three electron beams, we can make 3-dot triangle to produce any desired color.
Color CRT
Figure Color CRT
The most common method to produce image on the CRT screen is to sweep the electron beam back and forth across the screen. When the beam reaches right side of the screen, it is turned off (blanked) and retraced rapidly back to left-hand side of the screen to start over.
If the beam is slowly swapped from top of the screen to bottom of the screen as it is swapped back and forth horizontally, the entire screen appears lighted. A display produced in this way is referred to as Raster display. To produce image, we turn on or off the electron beam as it sweeps across the screen.
Q.What is Scanning ?State its different of scanning?
Ans.One sweep from top to bottom of screen is called as field. Moving the beam from left to right and back is called as scanning, there are two types of scanning: -
1)Interlaced scanning
2)Non-interlaced scanning
Figure of Interlaced and Non – interlaced Scanning.
Interlace scanning
In interlaced scanning, the scan-lines for one field are offset or delayed and interleaved with the next field. After every other field, the scan-line repeats therefore two fields are required to make a complete picture or frame. The beam sweeps 262.5 times horizontally for each vertical sweep. To get better picture and avoid flickering, interlaced scanning is used.
Non-interlace scanning:
CRTs used for computer usually have non-interlace scanning. The vertical sweep rate is of 60HZ and 260 sweep lines per field as shown in the figure. In non-interlaced scanning, there is only one sweep, which sweeps from top to bottom. In non-interlaced scanning, the picture is comprised of one frame. Whether the CRT is used in TV monitor, video or computer monitor or terminals, there are certain basic circuits required to drive CRT, those are: -
1)Vertical oscillator to produce vertical sweep signal for beam.
2)Horizontal oscillator to produce horizontal sweep signal for beam.
3)Video amplifier to control intensity of electron beam.
4)A unit, which contains only basic driving circuitry, is referred to as video monitor.
Q.Draw Block diagram of Color monitor and describe its functioning
Ans.A heating element in a CRT heats the cathode and causes it to emit electrons which are accelerated and focused on a phosphor screen by means of high voltage grids. An image (raster) is displayed by scanning the electron beam across the screen. Since the phosphor’s luminance begins to fade after a short time, the image needs to be refreshed continually. In order to eliminate flicker, most monitors refresh the screen at a 60 Hz rate.
Fig. 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 and
3.Power supply.
The video signal is usually weak signal and thus requires amplification before the signal can be applied to the CRTs cathode. The amplification of the video signal is usually done in two stages. A low voltage amplifier, often called, a preamplifier, amplifies to high level signal.
In addition to amplification, the preamplifier also provides contrast and brightness control. CRT video amplifier provides high voltage amplification The CRT video amplifier is the second stage amplifier, it amplifies the preamplifier’s high level signal to a more High level 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.
Q.Describe Color CRT with neat diagram
Ans.Fig. shows a simplified cross view of a color CRT. A heating element heats up the cathodes. Heating the cathodes energizes the electrons in the cathodes and greatly aids in the emission of electrons.
A large DC potential, on the order of several hundred volts more positive than the cathode is applied at the second grid, G2. This causes the electron beam to be accelerated towards the screen. Since the beam emerging the cathode tends to diverge, a negative potential with respect to the cathode is applied at grid G1. By making G1 (also called control grid) more negative than the cathode, the electron beam begins to converge as shown in Fig.
This action is similar to beam focusing using an optical lenses. Furthermore, by modulating the potential difference between the cathode and the control grid, the beam intensity and hence the brightness level is modulated. Finally the beam is electrostatically focused on the screen by adjusting grid G3’s potential until the desired focus is achieved.
Q.Define dotpitch , resolution , frame rate ,screen size , refresh rate related to CRT monitor or factor to be checked before selecting a CRT monitor
Ans
Screen Size
The screen size is the measure of the diagonal length of the CRT. Since CRTs are measured diagonally from one corner of the case to the opposite instead of by their viewable area, a CRT will generally have a physically smaller display than that of an LCD with the same rated screen size. Screen size is one of the main variables that affects the price of a CRT monitor.
Refresh Rate
Refresh rate is the number of times per second the screen is redrawn expressed in hertz. Slow refresh rates will produce a noticable flicker. Higher refresh rates produce a picture with very little or no flicker that is easier on the eyes. A minimum of 75 Hertz is recommended.
Dot Pitch
Dot pitch is an important factor of the image quality of a monitor. Dot pitch is the measurement of the distance between two phosphor dots of the same color on a monitor. The smaller the dot pitch, the better image quality a monitor will have.
Resolution
Resolution is the number of pixels displayed on screen. The higher the resolution, the sharper and more detailed the picture will be.
Resolution is expressed in the number of horizontal and vertical picture elements, or pixels. More resolution means quality of image is good.
Resolution = Total number of Horizontal pixels X Total number of vertical pixels.
Frame Rate
How many times a screenful of information is produced per second is called frame rate.
Q.List and Describe issues related to display performance
Ans.For multimedia applications, the three main factors that affect performance are:
1. Network bandwidth
2. Decompression or decoding
3. Display technology
NetWork BandWidth
Network bandwidth is important for any client-server operation where the data is resident on a LAN-connected server and is being retrieved for display or playback. If the network bandwidth is not sufficient it is seen as a delay in the display of data records or an image. In the case of video, the effect is more severe. The playback becomes choppy and can actually become incoherent if the video bandwidth is insufficient to sustain the minimum required data rate. All video playback systems allow for variations in network bandwidth and attempt to scale the image accordingly while trying to maintain a constant sound stream.
Decompression or Decoding
To optimize network bandwidth and storage capacity, most designs store and transfer compressed data. For example, images may be compressed to CCITT Group 4 or 5,and video may be compressed to Motion JPEG or MPEG 2 standards.
The compressed data must be decompressed in the PC or the workstation before being displayed. Once again, while in the case of an image poor decompression performance causes an irritating delay for the user.
In the case of full motion video, poor decompression performance causes the same effect as poor network bandwidth. Loss of isochronicity in playback makes the video difficult to watch and comprehend.
Display Technology
The final component is the performance of the display technology itself. The time it takes to print a window can vary with the display hardware in use. Full-motion video playback requires repainting a window at a very fast rate. The data in that window can be changing at the rate of 30 or 60 frames/second
Q18. Describe Different terminologies used is display devices. (SP1.1.1, S2009-Q2.3.3)
- Triad
A triad consist of set of red, green and blue phosphors arranged in a triangle. The red gun excites red phosphor, the green gun excites blue phosphor, the blue gun excites blue phosphor. This triad produces a single color which is a combination of three excited phosphors.
- Pixels
A pixel is made of a triad. Pixels are arranged in an array of rows. Each row forms a scan line. Pixel resolution is the combination pixels in each row and number of rows for a given screen.
- Convergence
when the red, green and blue beams fire simultaneously, the beam excites all three phosphors in a pixel to generate white color. If there is a slight deviation of the beams due to magnetic misalignment of the electron trajectory, then the pixel could have red, blue or green color around the edge of the pixel and the beams are said to have poor convergence. If all three beams are perfectly aligned at all pixel spots on the screen, the monitor has excellent convergence.
- Pin cushioning
When the vertical edges of the displayed image curve inside to from concave edges, the screen is said to be pin cushioned. Pincushion term comes from the typical look of pincushion. Most monitors are factory aligned to minimize pincushion distortion and do not provide any adjustment for this.
- Barrel distortion
Barrel distortion is opposite of pin cushioning. The vertical sides of the sides of the display area curve outwards with convex edge.
- Drift, jitter, swim
These effects appear as unwanted motion in horizontal lines. Drift is caused when some electronic component drifts over time and the image moves up in a very slow motion. Periodically, it snaps back into place.
Jitter is caused due to unwanted electronic signals that cause the image to jump at a high rate .
Swim is another form of drift where a sort of shadow image seems to move from top to bottom; it is caused be noise in electronic signals or noise generated locally in display electronics.
- Roping
Roping causes straight lines to appear twisted or helical. This is caused by poor convergence as successive pixels in the line show different edge colors.
- Anti-glare Screen
A glossy monitor screen can cause reflections generated by florescent lights and other lights. The reflection can interfere with screen images and make the screen hard to read. To minimize these reflections, screens are coated with silicon compound. However, screen image may suffer as the coating reduces image contrast and sharpness. Multicoating technology for monitor screens are also used to reduce glare
- Shadow mask
A shadow mask has tiny holes and is located just behind the screen. The holes in the mask correspond to triad locations on the screen. The purpose of the shadow mask is to guide the electron beam to strike one of the three phosphor in a triad.
- Slot mask:- A slot mask is used in Sony Trinitron Picture Tubes. The purpose of the slot mask is the same as the shadow mask that is to guide the electron beam. The mask is made of vertical wires which create the slots. The dot pitch in case of the slot mask is spacing between these slots.
- Video bandwidth
Video bandwidth is the highest frequency at which the pixels can be input to the monitor. It is measured in MHz