Transcript Lecture 8

Types of Displays
• Two main types
– CRT (cathode ray tubes)
– LCD (liquid crystal display)
• OLED
• Related terms
– Monitor or screen
• A display is often called a “monitor” or “screen”
• However, the term “monitor” usually refers to the
entire box, where as “screen” often implies just a
sub-assembly within the box
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Layout for a display
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Pixels
• A Pixel is a “picture element”
– a single point in a graphic image
– A graphics display is divided into thousands (or
millions) of pixels arranged in rows and columns
– The pixels are so close together, they appear connected
– The number of bits used to represent each pixel
determines how many colours or shades of grey can be
represented
– For a B&W (black and white) monitor, each pixel is
represented by 1 bit
– With 8 bits per pixel, a monitor can display 256 shades
or grey or 256 colours (Note: 28 = 256)
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Display Size
• Usually specified in “inches”
• Value cited is the diagonal dimension of the
raster -- the viewable area of the display
• E.g., a 15” monitor
15”
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Resolution
• Resolution is the number of pixels on a
screen display
• Usually cited as n by m
– n is the number of pixels across the screen
– m is the number of pixels down the screen
• Typical resolutions range from…
– 640 by 480 (low end), to
– 1,600 by 1,200 (high end)
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Video RAM Requirements
• Total number of pixels is n  m
• Examples
– 640  480 = 307,200 pixels
– 1,600  1,200 = 1,920,000 pixels
• Video RAM required equals total number of
pixels times the number of bits/pixel
• Examples
– 640  480  8 = 2,457,600 bits = 307,200 bytes = 300
Kbytes
– 1,600  1,200  24 = 46,080,000 bits = 5,760,000 bytes
= 5,625 Kbytes = 5.49 Mbytes
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Video RAM (KB) Per Image
Bits per pixel
Resolution
8 bit
16 bit
65K colors
24 bit
16M colors
640 x 480
300
600
900
800 x 600
468.75
937.5
1406.25
1024 x 768
768
1536
2304
1152 x 1024
1152
2304
3456
1280 x 1024
1280
2560
3840
1600 x 1200
1875
3750
5625
See previous slide for calculations
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Dot Pitch
• Dot pitch is a measure of the diagonal distance
between phosphor dots (pixels) on a display
screen
• One of the principal characteristics that determines
the quality of a display
• The lower the number, the crisper the image
• Cited in mm (millimeters)
• Typical values range from 0.15 mm to 0.30 mm
• Note
– Dot pitch, as specified, is the capability of the display
– For a particular image, dot pitch can be calculated as…
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Dot Pitch Image Example
• Q: What is the dot pitch of an image
displayed on a 15” monitor with a
resolution of 640 by 480?
• A:
Dot pitch = 15 / 800 inches
= 0.01875 inches
= 0.01875 / 0.039 mm
= 0.481 mm
Notes:
1.
2.
Z = (6402 + 4802)1/2 = 800
1 mm = 0.039 inch
Z
480
640
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Dot Pitch Illustrated
Pixel
0.481 mm
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Dot Pitch Image Table
Display Size
Resolution
14”
15”
17”
19”
21”
640 x 480
0.45
0.48
0.54
0.61
0.67
800 x 600
0.36
0.38
0.44
0.49
0.54
1024 x 768
0.28
0.30
0.34
0.38
0.42
1152 x 1024
0.23
0.25
0.28
0.32
0.35
1280 x 1024
0.22
0.23
0.27
0.30
0.33
1600 x 1200
0.18
0.19
0.22
0.24
0.27
Note: Dot pitch figures in mm (millimeters)
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Colour Displays
• CRT displays
– each pixel is composed of three superimposed dots: red,
green, and blue
– Hence, RGB display
– The three dots are created by three separate beams
– Ideally, the three dots should converge at the same
point, however, in practice there is a small amount of
convergence error, and this makes the pixels appear
fuzzy
• LCDs
– Colour is created by filtering/blocking different
frequencies of light
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CRT Display
Rev: Fig 9.21 pg 267 ff
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Operation of a CRT Display
• A CRT display contains a vacuum tube
• At one end are three electron guns, one each for
red, green, and blue
• At the other end is a screen with a phosphorous
coating
• The three electron guns fire electrons at the screen
and excite a layer of phosphor
• Depending on the beam, the phosphor glows,
either red, green, or blue
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Color Transformation Table
G
Figure 9.17 Use of a color transformation table
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B
Figure 9.20 Display example: (a) desired display, (b) video memory contents, (c)
color palette table, (d) color signals
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Operation of an LCD
• Two sheets of polarizing material with a
liquid crystal solution between them
• An electric current passed through the
liquid causes the crystals to align so that
light cannot pass through them
• Each crystal, therefore, acts like a shutter,
either allowing light to pass through or
blocking the light
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Active-Matrix Display
• A type of liquid crystal display in which the
image is refreshed more frequently than in
conventional (passive matrix) displays
• Most common type of active-matrix display
is known as TFT (thin-film transistor)
• The terms active matrix and TFT are used
interchangeably
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Video Interfaces (1 of 2)
• Composite video
– Definition: a video interface in which all the colour and
sync information is contained in one signal
– Contrast with RGB
– TVs in North America use composite video
• RGB (Red, Green, Blue)
– Definition: a video interface in which the red, green,
and blue signals, and the horizontal and vertical sync
signals, are separate
– Computer monitors use RGB
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Video Interfaces (2 of 2)
• S-video
– A technology for transmitting video signals over a
cable by dividing the video information into two
separate signals: one for colour (chrominance, C), and
one for brightness (luminance, Y)
– Also called Y/C video
– Televisions (internally) are designed for separate
luminance and chrominance signals
– Computer monitors are designed for separate red,
green, and blue signals
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RGB Video Standards
• A variety of standards exist for delivering
RGB signals to a video display monitor
• Developed and consolidated by VESA
(Video Electronics Standards Association)
• Examples
– VGA – video graphics adapter
– SVGA – super-VGA
– XGA – extended graphics adapter
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VGA/SVGA/XGA Pinouts
Pin
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Signal
1
Red
2
Green
3
Blue
4
ID bit 2
5
Ground
6
Red return
7
Green return
8
Blue return
9
-
10
Sync return
11
ID bit 0
12
ID bit 1
13
Horizontal sync
14
Vertical sync
15
-
DE15 connector
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S-video Pinouts
Pin
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Signal
1
Ground
2
Ground
3
Y (luminance)
4
C (Chrominance)
4-pin mini-DIN connector
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Plan
• Printers
• Scanners
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Printers
• Main types:
– Impact
– Laser
– Ink jet
Impact
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Impact vs. Non-Impact
• Impact printers physically transfer a dot or
shape to the paper
• Include dot-matrix, belt, & solid line printers
• Non-impact printers spray or lay down the
image
• Impact printers remain important because
they can print multi-part forms (eg: carbon or
NCR copies)
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Printers
• Main types:
– Dot matrix (sample impact)
– Laser
– Ink jet
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How it works
( Impact Type Dot-Matrix )
A print-head moves back-and-forth in front of forms
(paper) on which characters or graphic images are transferred.
The print-head contains numerous wires, typically from 9 to
24. Each wire is part of a solenoid-like unit. An electrical
pulse applied to the solenoid creates a magnetic field which
forces the wire to move briefly forward then backward. As
the wire moves forward, it strikes a print ribbon containing
ink. The impact transfers an ink dot to the paper. The paper
is supported from behind by a platen. (a hard flat piece)
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Dot Matrix Print Head
One print wire
Print wires
(e.g., 12)
Front view
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Side view
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Dot Matrix Impact Printing
Paper
Print
wire
Platen
Ribbon
Paper
Side view
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Side view
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Front view
Illustration
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Creating a Gray Scale
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Specifications
• cps
– characters per second
– Varies by quality of print (e.g., draft vs. final (NLQ))
• lpm
– lines per minute (related to cps)
• Forms
– Maximum number of layers of paper that can by printed
simultaneously
– Specified as n-part forms (e.g., 4-part forms)
• mtbf
– Mean time between failure (e.g., 6000 hours)
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Dot Matrix Printer Example
Specifications
• 800 cps
• 400 lpm
• 6-part forms (max)
FormsMaster 8000 by Printek, Inc.
http://www.printek.com
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Printers
• Main types:
– Dot matrix
– Laser
– Ink jet
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Operation of a Laser Printer
•
Four steps
–
–
A laser is fired in correspondence to the dots to be printed. A spinning
mirror causes the dots to be fanned out across the drum.
The drum is photosensitive. As a result of the laser light, the drum
becomes electrically charged wherever a dot is to be printed.
The drum rotates to the next line, usually 1000th or 1600th of an inch.
Photosensitive
drum
Laser
Spinning
mirror
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Top View of Rotating Mirror
Drum
Rotating Mirror:
This one has eight
faces
Laser light source
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Operation of a Laser Printer
2.
As the drum continues to rotate, the charged part of the drum passes
through a tank of black powder called toner. Toner sticks to the drum
wherever the charge is present. Thus, the pattern of toner on the drum
matches the image.
Toner
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Operation of a Laser Printer
3.
A sheet of paper is fed toward the drum. A charge wire coats the
paper with electrical charges. When the paper contacts the drum, it
picks up the toner from the drum
Charge
wire
Paper
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Operation of a Laser Printer
4.
As the paper rolls from the drum, it passes over a heat and pressure
area known as the fusing system. The fusing system melts the toner to
the paper. The printed page then exits the printer.
As the same time, the surface of the drum passes over another wire,
called a corona wire. This wire resets the charge on the drum, to
ready it for the next page.
Corona
wire
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Fusing
system
Specifications
• ppm
– Pages per minute
– Typically 4-10 ppm
• dpi
– Dots per inch
– Typically 600-1200 dpi
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Laser Printer Example
Laserjet 5000 Series from Hewlett Packard Co.
(http://www.hp.com)
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Printers
• Main types:
– Dot matrix
– Laser
– Ink jet
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Background
• Inkjet technology was developed in the
1960s
• First commercialized by IBM in 1976 with
the 6640 printer
• Cannon and Hewlett Packard developed
similar technology
• Also called bubble jet
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How it works
Characters and graphics are 'painted‘ line by line to from a pattern of
dots as a print head scans horizontally across the paper. An ink-filled
print cartridge is attached to the inkjet's print head. The print head
contains 50 or more ink-filled chambers, each attached to a nozzle. An
electrical pulse flows through thin resistors at the bottom of each chamber.
When current flows through a resistor, the resistor heats a thin layer of ink
at the bottom of the chamber to more than 900 degrees Fahrenheit for
several millionths of a second . The ink boils and forms a bubble of
vapour. As the vapour bubble expands, it pushes ink through the nozzle to
form a droplet at the tip of the nozzle. The droplet sprays onto the paper.
The volume of the ejected ink is about one millionth that of a drop of
water from an eye-dropper. A typical character is formed by an array of
these drops 20 across and 20 high. As the resistor cools, the bubble
collapses. The resulting suction pulls fresh ink from the attached reservoir
into the firing chamber.
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Plan
• Printers
• Scanners
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How it works
A scanner works by digitizing an image. A scanning mechanism
consists of a light source and a row of light sensors. As light is reflected
from individual points on the page, it is received by the light sensors and
translated to digital signals that correspond to the brightness of each point.
Colour filters can be used to produce colour images, either by providing
multiple sensors or by scanning the image three times with a separate
colour filter for each pass. The resolution of scanners is similar to that of
printers, approximately 300-600 dpi (dots per inch).
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Scanners
• Three main types
– Flatbed
– Sheet-fed
– Handheld
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Flatbed Scanner Example
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Sheet-fed Scanner Example
OfficeJet Series 700 from Hewlett Packard Co
(http://www.hp.com)
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Handheld Scanner Example
QuickScan GP Bar Code Scanner from PSC, Inc.
(http://www.pscnet.com)
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