10. Computer Peripherals – Part II

1. 10. Computer Peripherals – Part II Chapt. 10

2. Plan • Tape drives • Optical disks • Displays

3. Rationale • Magnetic tape is used as secondary storage when… • Offline storage is acceptable or preferred • Capacity requirements exceed that of floppy disks • Sequential access is adequate • Magnetic tape is used for… • Off-site data preparation • Backup

4. Terminology • Backup • The act of copying files to a second medium (typically disk or tape) as a precaution in case the first medium fails • It is important in computing to “backup your files regularly”

5. Types of Tape Drives • Two types: • Reel-to-reel • Used on mainframe computers • Cartridge • Used on PCs • In either case, the tape can be removed from the drive (i.e., the tape drive supports offline storage) • When a tape is loaded in a tape drive and is ready to be accessed, the tape is mounted

6. Reel to Reel Tape Drive

7. Tape Reels

8. Tape Reel Specifications • Reel diameter: 10 ½” • Tape width: ½” • Tape length: 2400 feet • Number of tracks: 9 • Drive has nine read/write heads • 9 bits of data are read/written at a time (8 data + parity) • Each group of nine bits is called a frame • Datadensity/capacity • 1600 frames/inch  2400 x 12 x 1600 = 46,080,000 bytes/reel • 6250 frames/inch  2400 x 12 x 6250 = 180,000,000 bytes/reel

9. Nine-track Tape Layout Physicalrecord Inter-recordgap Track 1 ½” Track 9 1 byte of data (8 data bits + parity)

10. Tape Cartridge

11. Types of Tape Cartridges • QIC (Quarter Inch Cartridge) • DAT (Digital Audio Tape)

12. QIC (Quarter Inch Cartridge) • Pronounced: quick • Introduced in 1970s • Popular format for backing up personal computers • Two general classes • Full-sized, 5¼” (also called “data cartridge”) • Mini-cartridge, 3½” • Capacities up to 10 GB

13. DAT (Digital Audio Tape) • Tape width: 8 mm or 4 mm • Uses helical scan technique to record data (like VCRs) • Capacities to 24 GB (4 mm) or 40 GB (8 mm)

14. Plan • Tape drives • Optical disks • Displays

15. Operation • Uses light generated by lasers to record and retrieve information • Information is stored by varying the light reflectance characteristics of the medium • Available in read-only (CD-ROM) and read/write formats

16. CD-ROM • CD-ROM stands for “compact disc, read-only memory” • Evolved from audio CDs • Disk size: 120 mm (5¼”) • Capacity: 550 MB

17. CD-ROM vs. Magnetic Disk

18. CD-ROM Data Organization • 270,000 blocks of 2048 bytes each (typically) • 270,000  2048 = 552,960,000 bytes • Extensive error checking and correction (e.g., bad regions of the disk flagged) • Substantial overhead for error correction and identifying blocks • Capacity can be as high as 630 MB

19. CD-ROM Drive Spec’s

20. Pits and Lands (1 of 2) • Data are stored as “pits” and “lands” • These are burned into a master disk by a high powered laser • Master disk is reproduced mechanically by a stamping process • Data surface is protected by a clear coating • Data are read by sensing the reflection of laser light • A pit scatters the light • A land reflects the light

21. Land Pit Reflectedlight Scatteredlight Laser Laser Laser Pits and Lands (2 of 2)

22. Land Pit Transparent protective layer Prism Light detector Laser diode More detail CD-ROM Read Process

24. Laser Diodes

25. Read/Write Variations • WORM (Write Once, Read Many) • Magneto Optical

26. WORM Disks and Drives • WORM = Write-once, read many • Also called CD-R, for CD Recordable • Begin with blank CDs • WORMs drives are used to write the CD • The write process is irreversible • Many standards, some disks may be read on standard CD-ROM drive, others may not • Applications • Infrequent data distribution • Small quantities • For large quantities, cheaper to have CR-ROMs manufactured

27. Magneto Optical • Disk may be written, read, and rewritten • Write process is preformed at high temperature • Combines features of optical and magnetic technology • Data are stored as a magnetic charge on the disk surface • During reading, the polarity of the reflected light is sensed (not the intensity)

28. Plan • Tape drives • Optical disks • Displays

29. Types of Displays • Two main types • CRT (cathode ray tubes) • LCD (liquid crystal display) • 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

30. 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

31. 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

32. CRT Display

33. Operation of an LCD • Two sheets of polarizing material with a liquid crystal solution between them • An electric current passes through the liquid, causing 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

34. Liquid Crystal Display

35. 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)

36. 15” 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

37. 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)

38. 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

39. Video RAM (KB) Per Image See previous slide for calculations

40. Aspect Ratio • Aspect ratio is the ratio of the width to height of a display screen • For a 640 by 480 display, the aspect ratio is 640:480, or 4:3 • Related terms • Landscape • The width is greater than the height • Portrait • The height is greater than the width

41. 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…

42. Dot pitch = 15 / 800 inches = 0.01875 inches = 0.01875 / 0.039 mm = 0.481 mm Notes: Z 480 • Z = (6402 + 4802)1/2 = 800 • 1 mm = 0.039 inch 640 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:

43. Dot Pitch Illustrated Pixel 0.481 mm

44. Exercise – Dot Pitch Image • Q: What is the dot pitch of an image displayed on a 19” monitor with a resolution of 1,280 by 1,024? Skip answer Answer

45. Exercise – Dot Pitch Image Answer • Q: What is the dot pitch of an image displayed on a 19” monitor with a resolution of 1,280 by 1,024? • A: Dot pitch = 19 / 1639.2 inches = 0.01159 inches = 0.01159 / 0.039 mm = 0.30 mm Note: Z = (12802 + 10242)1/2 = 1639.2

46. Dot Pitch Image Table Note: Dot pitch figures in mm (millimeters)

47. Dot Pitch Example • Q: A 19” CRT monitor, made by Vapour Hardware Inc., has a dot pitch specification of 0.28 mm. What is the highest resolution at which this monitor will reliably operate? • A: 1,280 by 1,024 Discussion: Just read the value above from the dot pitch image table. For a 19” monitor, a 1,280 by 1,024 image has a dot pitch of 0.30 mm. The monitor’s dot pitch specification exceeds this, therefore, the monitor will work fine at this resolution. However, a 1,600 by 1,200 image on a 19” monitor requires a dot pitch of 0.24, which is beyond the capability of the example monitor.

48. Exercise – Dot Pitch • Q: A 17” CRT monitor has a dot pitch specification of 0.30 mm. What is the highest resolution at which this monitor may operate? • A: Skip answer Answer

49. Exercise – Dot Pitch Answer • Q: A 17” CRT monitor has a dot pitch specification of 0.30 mm. What is the highest resolution at which this monitor may operate? • A: 1,024 by 768

50. Interlacing • Interlacing is an image drawing technique whereby the electron guns draw only half the horizontal lines with each pass • The odd lines are drawn on the 1st pass, the even lines are drawn on the 2nd pass • A non-interlaced imaged is completely drawn in one pass • Let’s see…