Multimedia Hardware
Overview
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Printer technology
Scanning and digital cameras
Display devices
Other peripherals
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Printer Technology
The Technologies
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Dot matrix
Inkjet
Solid ink
Laser
Colour laser
Thermal wax transfer
Dye sublimation
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Dot Matrix
• Old technology - rarely used today
• Print-head consists of a column of 7 to 24 pins
• Selected pins strike the paper through
a ribbon
• Limited colour possible by using
multi-colour ribbon
• Problems with alignment of columns
• Relatively slow, every copy takes same time
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Inkjet
• Sprays a stream of CMYK ink through a set of fine
nozzles mounted in a disposable cartridge.
• Heat sometimes applied to aid the ink’s drying.
• Ink tends to bleed into ordinary paper.
• Special paper often used - adds to the expense.
• Resolutions of up to 1440dpi possible.
• Capital cost low, running costs high.
• Solid ink variations are now on the market.
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Laser
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Xerographic process
Charge transfer
Image on drum by laser
Toner
Heat to fix image
Resolutions up to 1200dpi possible
Higher capital cost, lower running cost
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Laser / Colour laser
• Colour possible by having 4 such mechanisms
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Thermal Wax Transfer
• Roll of plastic film coated with pigment-impregnated wax
• Roll alternates page-sized panels of CMYK
• Print head has thousands of heating elements which melt
the wax and transfer it to the paper
• Glossy appearance to prints & vivid colours
• Excellent results for OHP transparencies
• Fixed running cost per sheet
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Dye Sublimation
• Similar to thermal wax transfer - CMYK roll
• Differs in method of colour transfer to paper
– inks are opaque and make a solid dot
– dyes are transparent and merely tint the print media
– (varied) heat changes the solid dye directly into a gas
– immediate absorption by the polyester coating of the
print medium, producing a small coloured dot
– dyes can mix, so no dithering is required
– photographic quality results
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Scanning & Digital Cameras
Sampling
• Like sound, scanning and digital cameras are
further areas where an analogue signal is being
sampled.
• How does the Nyquist Sampling Theory apply?
– for frequency read resolution
– typically, some detail is lost => what is acceptable?
– for example, to record detail of 0.1mm, sample at
0.05mm (look at dpi details on scanner)
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Scanners
• Detects light bounced back from document, at
given colour depth and sample resolution
• Document type => colour depth / resolution
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text
b&w drawing
b&w photo
colour drawing
colour photo
full colour photo
bi-level, i.e. black & white
256 grey levels, higher resolution
256 grey levels, lower resolution
256 colours, higher resolution
256 colours, lower resolution
16.7M colours, lower resolution
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Beware the file size!
A4 page scanned as colour photograph
page size is approximately 81/3” x 112/3”
typical resolution is 150dpi (dots per inch)
81/3 x 150 x 112/3 x 150 = 2,187,500dots
colour depth typically 256 colours => 1 byte per dot
so, final file size is 2,187,500 / (1024 x 1024) = 2.1MB
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Consider the output path
• Screen resolution is typically 75-72dpi
• Inkjet printers are typically 360-720dpi
• Laser printers are typically 300-600dpi
BUT…
• beware of dithering which reduces effective
resolution
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Dithering
How do laser printers produce shades of grey?
Each dot is either black or white, so use groups of dots.
Consider a 2x2 block on a 300dpi laser printer:
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1
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Effectively producing 5 grey levels, but at 150dpi.
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Dithering problem
• If the pattern used for shades is too regular:
• Random dithering helps overcome this problem.
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Digital Cameras
• Television cameras generally use the vidicon
image-sensing tube
– similar to a CRT in construction
– light at the faceplate causes electrons to flow thru
– this is detected and generates the video signal
• Solid-state sensor arrays are now taking over
– CCD (the charge-coupled device)
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CCD
• Single chip manufactured on light-sensitive
crystalline silicon
• A rectangular array of photo-detector sites
• Light falling on a site causes loss of charge
• When the site is ‘inspected’, the current needed
to re-charge it is proportional to the amount of
light which has fallen on it - i.e. a grey level.
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Display Devices et al
Components
• Input device
– tablet/pen, mouse, keyboard, etc.
• Output device
– screen, printer/plotter, slide, film/video, etc.
• Graphics engine
– processor, memory, video control
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Simple Raster Architecture
Peripheral
Devices
CPU
System Bus
System
Memory
Frame
Buffer
Video
Control
Display
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CRT Display Device
• CRT - Cathode Ray Tube
– light is produced by impacting phosphor coated
surface with a beam of electrons
– phosphorescence decays through time depending on
the persistence of the phosphor
– refresh rate = no. of redraws/second
– critical fusion frequency (CFF) = minimum refresh
rate which avoids flicker
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Monochrome CRT Schematic
Evacuated tube
Horizontal & vertical
deflection plates
Phosphorescence
Phosphor coated
surface
Electron gun
Electron beam
Focus mechanism
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Colour CRT
• Colour is formed by combinations of different
coloured phosphorescence
– closely spaced red, green & blue dots
• Shadow-mask CRT
– aligned so that each of 3 electron beams can only
impact one phosphor group
• Most popular arrangement is delta-delta
– triangular placement of guns & phosphors
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Delta-Delta Shadow Mask CRT
Red
Shadow mask
Electron guns
Green
Blue
G R
B G
R B
G R
B
G R
B G
Electron beams
Phosphor dot (triad)
coated screen
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Interlaced Video
• Interlacing - a technique used to halve the
bandwidth from memory to video controller
– display frame in 2 passes (fields)
– odd scanlines only (1,3,5...) in 1st field
– even scanlines only (0,2,4...) in 2nd field
• Refresh rate is effectively halved
– field refresh of 50Hz interlaced has a frame
refresh rate of 25Hz
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Video Cards
• Issues to consider in specifying a video card
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resolutions supported
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colour depth
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interlacing / non-interlacing )
size & type of the on-board memory, e.g. 8MB SGRAM
(static graphics RAM)
– double-buffering
– hardware acceleration, e.g. 3D-AGP
– bus interface, e.g. PCI
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Colour Lookup Tables
• An indirection between the pixel value and the
displayed colour
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10, 50, 190
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Palette
• n-bit pixel => 2n simultaneous colour palette
– also means 2n entries in the CLUT
• CLUT permits wider choice of colours for this
palette depending on the number of output bits
– n output bits also implies 2n potential colours
• Example: 8-bit pixel & 24-bit output
– 256 simultaneous colours drawn from 16.7 million
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Output bits
The combination of output bits has important
implications; examples:
24-bits, 8 for each of RGB => max of 256 grey levels
8-bits, 3 for R, 3 for G, 2 for B => max of 4 grey levels
also non-linear 256 colour scale (more yellows)
9-bits, 3 for each of RGB => max of 8 grey levels
but now a linear scale of 512 colours
Sections of the CLUT are sometimes reserved.
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Other Peripherals
DVD
• Digital Versatile Disk
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high capacity replacement for CD-ROM
capacity of 5 GB per side
equates to ~130 minutes of MPEG-2 video + sound
high quality video (slightly better than S-VHS)
Dolby Digital 5.1 soundtrack (also surround sound)
multiple soundtracks possible (e.g. languages)
movies often region coded, e.g. 1=USA, 2=Europe
• New Formats
– HD DVD, capacity 15GB per side
– Blu Ray, capacity 25GB per side
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Mouse / Trackerball
• Freely rotating ball
• Motion detected in horizontal & vertical directions
=> turned into a relative motion value
• Can use light reflected off
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special mat instead of ball
• Additional signals from
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buttons (1, 2 or 3)
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Digitising Tablet
• Flat bed of a fine grid of horizontal and vertical
wires
• User has a puck, often with cross-hairs & buttons
• Puck has a coil of wire which sets up a field
• Field induces a current in horizontal/vertical wires,
depending on actual position (x,y) co-ord.
• Accuracy down to .001” - height possible also
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Light Pens
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The apparent ability to draw on the screen
Pen detects the CRT beam as it passes by
Timing determines the pen’s position on screen
Software does the drawing / selecting etc.
Pen can be radio-linked for cordless operation
• Alternative: use a stylus and touch sensitivity...
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Touch Sensitive Screens
• Two main systems:
– two membranes with slight separation, pushed together by the
finger and/or stylus
• hard to get fine resolution unless using a stylus
• prone to physical damage, wear, dirt
– grid of light beams criss-cross the screen, finger breaks the beams
vertically and horizontally
• more robust
• screen tends to get dirty from use
• Tablet PCs
– Hand writing recognition
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Voice Input & Output
• Voice input coming of age
• Low cost systems now in common use
– ViaVoice, SpeakEasy, etc.
• Strong need to train systems for several hours
• Training unnecessary for limited applications
• Voice output less of a problem
– similar idea to wave tables / samples
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The Future
• Voice driven input
– is it always appropriate?
• Gesture recognition
– necessary for greater recognition accuracy?
• Larger output displays
– desktop sized?
• Faster communications connections
– software on a vendor server; licence to use for time
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