Multimedia Hardware

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Overview

• Printer technology

• Scanning and digital cameras

• Display devices

• Other peripherals

Printer Technology

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The Technologies

• 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

• 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

Scanning & Digital Cameras

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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– text bi-level, i.e. black & white– b&w drawing 256 grey levels, higher resolution– b&w photo 256 grey levels, lower resolution– colour drawing 256 colours, higher resolution– colour photo 256 colours, lower resolution– full colour photo 16.7M colours, lower resolution

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Beware the file size!

A4 page scanned as colour photographpage 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:

Effectively producing 5 grey levels, but at 150dpi.0 1 2 3 4

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• If the pattern used for shades is too regular:

• Random dithering helps overcome this problem.

Dithering 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.

Display Devices et al

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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

CPU

SystemMemory

FrameBuffer

VideoControl Display

System Bus

PeripheralDevices

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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

Electron gun

Focus mechanism

Horizontal & verticaldeflection plates

Phosphor coatedsurface

Electron beam

Phosphorescence

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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

R B

G RR

RB

B

B

G

GG

Electron gunsShadow mask

Phosphor dot (triad)coated screen

Green

Red

Blue

Electron beamsG

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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– resolutions supported )– colour depth )- related– 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

2310, 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 levelsalso non-linear 256 colour scale (more yellows)

9-bits, 3 for each of RGB => max of 8 grey levelsbut now a linear scale of 512 colours

Sections of the CLUT are sometimes reserved.

Other Peripherals

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DVD

• Digital Versatile Disk– 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

special mat instead of ball• Additional signals from

buttons (1, 2 or 3)

031

63

95127

159

191

223

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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

• 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