05 Data Representation

Question 1

Define encryption and its purpose.

Answer 1

Encryption is the process of converting a message from plain text into cipher text.
The purpose of encryption is to make sure that data cannot be understood if intercepted.

Question 2

Explain why Caesar cipher is easy to crack.

Answer 2

The Caesar cipher is easily cracked because there are only 25 possible keys.
You can easily carry out a brute force attack and try each key in turn until you decipher the data.
Can also carry out frequency analysis - E is the most common character

Question 3

Briefly outline Vernam ciphers and the one-time pad.

Answer 3

• Substitution cipher where each plain text character is encrypted using its own key. (prevents any useful information being revealed through a frequency analysis of the cipher text.)
• This key is randomly generated or is taken from a one-time pad, e.g. a page of a book, atmospheric noise, radioactive decay (truly random and generated from a physical, unpredictable formula)
• The key must be equal in length to the plain text message.

Question 4

Compare Vernam cipher with ciphers that depend on computational security.

Answer 4

Vernam cipher reveals no fingerprint as each character is encrypted with its own key and key isn’t derived from computer algorithm (keys from algorithms can always be unpicked)

Question 5

Define cipher, plaintext and ciphertext.

Answer 5

cipher = an algorithm for encrypting and decrypting data
plaintext = any readable data that can be understood without the use of a decryption key or device
cipher text = data that is unreadable because it has been encrypted with a cipher

Question 6

Define what MIDI is.

Answer 6

Musical Instrument Digital Interface
a technical standard that includes hardware specifications and a protocol for the communication of electronic musical instruments to produce and manipulate music.

Question 7

Outline how MIDI works.

Answer 7

A MIDI file consists of a list of instructions or event messages that explain what notes must be played, when they must played, and how long or loud each note should be. e.g. pitch, volume, tone

Question 8

Describe the advantages of using MIDI files for representing music.

Answer 8

• A MIDI file uses far less storage space than an equivalent sampled recording as files only consist of event messages, not a series of precise measurements of amplitude
• Also, the performance data can be easily manipulated, for example, the pitch or duration of a note can be changed, or the note can be set to be played by a different instrument.
• Synthesised sound often doesn’t sound realistic

Question 9

Outline ASCII

Answer 9

American Standard Code for Information Interchange
Uses 7 bits to represent characters, which allowed for a maximum of 128 characters to be represented
Only consisted of European characters

Question 10

Outline Unicode and the need for this character set.

Answer 10

There are three standards of Unicode: UTF-8 is a variable-width encoding as it can expand to 16 or 32 when needed; UTF-16 can expand to 32 bits; UTF-32 is fixed-width encoding.

The widespread use of the World Wide Web made it more important to have a universal international coding system.
The range of platforms and programs has increased dramatically, with more developers from around the world using a much wider range of characters.

Question 11

List the four error-detection methods.

Answer 11

• Parity bit
• Checksum
• Check digits
• Majority voting

Question 12

Explain how parity bits work.

Answer 12

• Uses either odd parity or even parity
• When one byte is transmitted, one bit reps the parity bit
• If odd parity is used, the parity bit will be set to either 1 or 0 to make the number of ones in the byte odd
• The receiving device checks the number of 1’s against the type of parity (stored in packet header) and if they don’t align, we know there has been an error
• Packet is sent again

Question 13

Explain how majority voting works.

Answer 13

• Each bit in the data being sent is replicated an odd number of times (usually three)
• The receiver assumes the correct bit is the majority out of the three that are sent
• The possibility of an error being missed can be reduced by choosing a large number of repetitions, but, this increases the amount of redundant information that needs to be transmitted across the channel.

Question 14

Describe check digits.

Answer 14

• An extra digit that is placed at the beginning or end of a number and is used to identify a product (ISBN or a barcode) or verify a user
• The main purpose of check digits is to recognise and prevent human errors when entering or assigning identification numbers
• digits produced by algorithms that often use modulo arithmetic
• For ISBN-10, it is calculated so that if you take the sum of the multiplication of each digit with its position and perform mod 11, the result is 0.

Question 15

What are checksums?

Answer 15

• Performs an arithmetic algorithm on the bytes of data that are being sent
• Sends the sum with the data
• Receiver device calculates the checksum using the same algorithm on the data received
• Verifies checksum received against the value calculated
• If they don’t match the data is resent

Question 16

How does sample resolution and sampling rate affect a sound sample?

Answer 16

High sampling resolution = more bits used per sample = height of wave can be measured more accurately

High sampling rate = more samples taken per second = greater precision and accuracy to measure changes in amplitude

Question 17

What frequency can humans hear sound at? So what is the typical sampling rate for CDs?

Answer 17

Humans can hear between 20 kHz and 22 kHz
Standard sampling rate is 44.1kHz (double maximum frequency)

Question 18

Describe the steps to recording sound.

Answer 18

1. A microphone converts sound waves into analogue electronic signals
2. The signals are put into an analogue to digital converter (ADC) which takes measurements of the amplitude (samples) at regular intervals
3. Converts the value of these samples into binary and stores the string of binary digits in memory

Question 19

How are digital audio files played?

Answer 19

1. You convert the sound from digital values stored on the computer into analogue electrical signals, using a digital to analogue converter (DAC).
2. These analogue signals are then passed to a speaker, which vibrates the speaker cone to move the air to create analogue sound waves.

Question 20

Compare the fixed point and floating point forms

Answer 20

• Floating point allows for far greater range of numbers with the same number of bits
• Very large numbers and very small fractional numbers can be represented
• Fixed point is a simpler system and faster to process

Question 21

Compare the vector graphics approach with the bitmapped graphics approach and understand the advantages and disadvantages of each.

Answer 21

Vector Advantages

• Can be resized without distortion (bitmaps look pixelated when scaled
• Smaller file size than bitmaps
• Easier to edit than bitmaps: each object in a vector graphic can be adapted independently by using its set of attributes, and objects can be added and removed easily. (bitmaps require specialist photo-editin software)
• Good quality regardless of resolution or medium it is displayed on

Bitmap Advantages

• Vector graphics are produced by combining a limited number of shapes, therefore the variety of images that they can represent is restricted.
• Bitmapped graphics can depict almost any level of complexity and detail.

Question 22

How can you calculate the angle between two vectors with coordinates?

Answer 22

cos x = (u DOT PRODUCT v) / (||u|| DP ||v||)

Question 23

What are appropriate uses for bitmapped graphics and vector graphs?

Answer 23

Vectors: illustrations, logos, designs, cartoons etc.
Bitmaps: photos

Question 24

Explain how vectors can represent an image with a series of objects.

Answer 24

• Vector graphics are images that are made up of lists of objects and their properties.
• An object is a mathematically or geometrically defined construct, such as a rectangle, line, polygon, or circle.
• Each of these objects has properties that determine the dimensions (e.g. width, height), appearance (e.g. colour, fill, stroke thickness), and position (e.g. x, y coordinates) of every object in the image.
• These objects and properties are stored as a list, often called the drawing list.

Question 25

Why are image files and text files compressed?

Answer 25

Reducing the amount of data that needs to be transferred means:
- Data is sent more quickly
- Less bandwidth is used up
- Videos streams are less likely to buffer
- Less storage required/used up

Question 26

Compare lossy and lossless compression.

Answer 26

Lossy reduces a file size by permanently removing redundant or unnecessary data e.g. frequencies that cannot be heard by the human ear. (more effective at reducing size)

Lossless reduces a file size by recording patterns in the data rather than the data itself. (maintains integrity of the data)

Question 27

Define run length encoding

Answer 27

A lossless compression method that summarises consecutive patterns of the same data into the symbol being repeated followed by the number of occurences.

Question 28

Define dictionary compression.

Answer 28

A lossless compression technique that identifies regularly occurring data and stores them in a dictionary with a corresponding bit pattern