Topic 14 - Digital technology

Question 1

Convert between binary numbers and decimal numbers

Answer 1

Question 2

What is meant the most significant bit and least significant bit in binary numbers?

Answer 2

Most significant bit is the largest power (first number)

Least significant bit is the smallest power (last number)

Question 3

Describe the function and properties of LPs

Answer 3

• The musical information is stored in a groove cut into the plasit record by a sharp stylus
• The shape of the wiggles in the groove is an analogue representation of the musical signal
• Loud sounds are large wiggles
• Quiet sounds are small wiggles
• The closer together the wiggles are, the higher the frequency of the sound
• The LP is played back by placing another stylus into the groove and rotating the record at constant speed
• The groove spirals inwards so the needle moves faster when it is playing on the edge of the LP
• The groove is more squashed in the centre of the LP
• The stylus vibrates and produces an electrical signal, which is played through an amplifier and speakers

Question 4

What are the disadvantages of LPs?

Answer 4

• Every time the LP is played, the stylus slightly damages and changes the shape of the groove
• Dust and smoke particles settle in the groove
• These cause unintended vibrations of the stylus
• Unwanted electronig noise is added to the signal, heard as a background hissing sound
• The LP may also bend

Question 5

Describe the function and properties of cassette tapes

Answer 5

• Music is recorder on a long thin plastic tape coated with a fine powder of magnetic material
• The material becomes magnetised during recording
• A chain of tiny permanent magnets is produced on the tape in a pattern which represents the original sound
• The magnetising is done by the recording head, which consists of a small electromagnet
• The information to be recorded is sent as small, changing magnetic field which magnetises the tape particles in the same pattern as the current in the coil
• To retrieve the information, the tape is pulled past the electromagnet
• The pattern of magnetisation produces a varying magnetic field in the core of the head
• This nduces a varying electric current in the coil
• The current is amplified and output through a loudspeaker

Question 6

Describe the function and properties of floppy disks

Answer 6

• A magnetic form of data storage
• Data stored in patterns of magnetic particles arranged in concentric rings
• The reader is able to access data on any track without having to search sequentially through the other tracks
• Example of direct access storage devices

Question 7

What are direct access storage devices?

Answer 7

Storage devices in which bits of data are stored at precise locations, enabling the computer to retrieve information directly without having to scan a series of records

Question 8

Describe the function and properties of hard disks

Answer 8

• A stack of platters which spin at high speed
• The platters are rigid and coated with iron oxide particles
• A head (electromagnet) has reversible polarity
• The head is used to read and write on the disk surface
• In one polarity the head aligns the magnetism in a tiny area of the disk so that information is stored as binary one (‘on’)
• In the opposite polarity the information acts as a binary zero (‘off’)
• To read the data, the platters spin past the head, which induces a current in the electromagnet
• If dust or smoke gets into the drive, the disk surface becomes irreversibly damaged

Question 9

Describe the function and properties of CDs

Answer 9

• An optical storage medium
• Data is stored as pits and lands, which are moulded into a thin layer of transparent plastic
• The dents are covered with a thin layer of reflective aluminium
• The pits and lands are arranged one after another in a spiral track, starting from the centre
• The disc rotates at about 500 revolutions per second
• A low power laser beam reads the data with its series of pits and lands
• The pattern of bits forms a coded signal of 16 binary numbers
• Each number represents one feature of the sound wave
• The laser starts reading from the centre and moves outward along a radial line
• There is no mechanical contact between the laser and the CD (no damage done on the CD)
• The laser reads the pits at a constant rate so that it doesn’t misread information
• The rotation of the CD is slowed down as the laser reader moves outward to ensure constant rate

Question 10

What is an optical storage medium?

Answer 10

A storage device that uses light (laser) to read and/or write data

Question 11

Describe the function and properties of DVDs

Answer 11

• Similar to CDs
• Capable to store seven times as much data as CDs
• Uses a shorter wavelength laser
• The laser allows smaller pits to be etched on the surface and gives a greater storage capacity
• The track length is twice as long as the track length of CDs
• DVD+R DL is a format that contains two layers of pits (the top layer is semi-reflective allowing light to pass to the bottom layer)

Question 12

How is the interference of light used to recover information stored on a CD?

Answer 12

• The laser beam is shone on the CD and reflected back onto a detector
• Reflection depends on whether it falls on a pit or a land
• When the laser is entirely incident on a pit or a land, all the waves in the reflected beam are in phase. Constructive interference takes place and a strong signal is detected
• I the laser is on a pit and part on a land, there is path difference between the two parts of the laser beam. Destructive interference occurs and there is no signal produced
• For a laser beam of wavelength λ, the path difference is λ/2
• As the CD rotates, the signal received by the detector changes as the beam travels from pit to land to pit
• This produces digital signals of 1s and 0s varying according to either constructive or destructive interference (also the lengths of the pits and lands)

Question 13

What is the pit depth on a CD if the path difference is λ/2?

Answer 13

λ/4

Question 14

What are the advantages of storing information digitally instead of analogically?

Answer 14

1. Quality
   - Retrieving analogue data affects the quality of the data (LPs, magnetic tapes)
   - Retrieving digital data does not degrade the data and the data is identical each time
2. Reproducibility
   - Can be easily copied
3. Retrieval speed
   - Much more rapid than that of analogue data
4. Portability
   - Larger amounts of data can be stored on physically small devices
5. Manipulation of data
   - Can be quickly and cheaply manipulated

Question 15

What are the moral and ethical implications for society for increasing data storage?

Answer 15

1. Issues concerning the privacy and anonymity of personal data
2. Issues related to access and ownership of personal data

Question 16

What are the social implications for increasing data storage?

Answer 16

1. Social networking sites have both positive and negative features

Question 17

What are the economic implications for increasing data storage?

Answer 17

1. Many financial scandals involve the secret modification of accounting and financial data
2. The integrity of currencies may be undermined by electronic cash
3. Increased access to training and education via computer-assisted teaching and learning
4. Economic decision-making by consumers will be faster and more accurate

Question 18

What are the environmental implications for increasing data storage?

Answer 18

1. Could replace traditional techniques, however data centres consume electricity
2. The resources needed for the maintenance of electric data will be in higher demand (fossil fuels and silicon)

Question 19

What is capacitance?

Answer 19

The charge per unit potential difference that can acculumate on a conductor

Unit is the farad (F) (CV^–1)

Question 20

What is a capacitor?

Answer 20

A device that can store charge (electrons)

Question 21

What is a charge-coupled device (CCD)?

Answer 21

• A small silicon chip with a surface covered with a very large number of light sensitive elements called pixels
• Each pixel behaves like a small capacitor

Question 22

How does incident light cause charge to build up within a pixel?

Answer 22

When light is exposed:

• Each pixel releases electrons (similar to the photoelectric effect)
• The greater the intensity of the light, the higher the number of photons incident, the higher the number of electrons released

When exposure to light stops:

• The charge that has built up in each pixel is stored

Question 23

How is an image on a CCD digitised?

Answer 23

• The charges stored across the pixel array represent the variation in intensity of light incident on the surface
• By measuring the voltage developed across each pixel and recording the position of each pixel, a digital intensity map of the surface is built
• The larger the charge, the greater the intensity of the incident light

Question 24

How is the information retrieved from a CCD?

Answer 24

• Pixels are arranged in columns
  1. By applying potential difference, the charges in each row are pushed down to the row below called the serial register
  2. The packets of charge on the serial register are moved sideways
  3. One by one they pass through and amplifier and then to an analogue-to-digital converter (ADC)
  4. The process continues until the entire row has been read and converted (called clocking the CCD)
  5. The process ir repeated for the next row until the whole array has been processed
  6. The pixels can be reused after the data has been read

Question 25

How is the producing of black and white images different from producing colour image?

Answer 25

- To produce a black and white image, only the overall light intensity on each pixel and the pixel's position are needed Colour images: - One type of colour CCD has pixels arranged into groups of four, two green, one red, and one blue filter - The intensity of the light in each of the four pixels is measured - The group acts as a sensor unit giving information for all colours for that area - Computer combines the information to produce digital data and an image

Question 26

Why does a colour CCD have two green filters?

Answer 26

Because the human eye is more sensitive to green light than to red or blue light

Question 27

What is the quantum efficiency of a pixel?

Answer 27

The ratio of the number of photoelectrons emitted to the number of photons incident on the pixel

Question 28

What does the quantum efficiency give information about?

Answer 28

The capability of the CCD to capture images at low light intensity. The higher the quantum efficiency means that a clear image is formed even at low light intensity

Question 29

What is magnification?

Answer 29

The ratio of the length of the image as its formed on the CCD to the actual length of the object.

Question 30

When are two points on an object just resolved?

Answer 30

When the images of the points on a CCD are at least two pixels apart Objects are not resolved in and just resolved in b

Question 31

What are the effects of quantum efficiency, magnification, and resolution on the quality of the processed image?

Answer 31

Quantum efficiency: - A higher efficiency means that the image will require less time to form if the incident light intensity is very low Resolution: - An image of high resolution is of high quality because it includes more details Magnification: - A greater magnification will produce a more detailed picture of higher resolution

Question 32

What are the practical uses of CCDs?

Answer 32

1. Endoscopes 2. Medical X-ray imaging 3. Digital cameras and video cameras 4. Telescopes

Question 33

How are CCDs used in endoscopy?

Answer 33

- Used to look inside the body - CCDs produce high-quality colour images in real time

Question 34

How are CCDs used in X-ray imaging?

Answer 34

- Special CCDs that can detect X-rays - Exposure times are shorter - Can be immediately shown - Relatively expensive

Question 35

How are digital cameras better than photographic films?

Answer 35

- Image produced can be enhanced and edited electronically - Storing, archiving, and sorting of images is easy and cheap - Images can be viewed immediately

Question 36

How are CCDs used in telescopes?

Answer 36

- Respond to a wide range of electromagnetic radiation in electrical form - Very sensitive to low intensities of light - Specialised CCD arrays