4.5.6 Representing images, sound and other data

Question 1

What is a Bitmap Image made up of?

Answer 1

Bitmap images are made up of pixels.

Question 2

Define Pixel.

Answer 2

The smallest unit of an image, representing a single point with a specific colour and brightness.

Question 3

What is the equation for image file size?

Answer 3

image file size = colour depth x image height(px) x image width(px)

Question 4

Define Analogue Data?

Answer 4

Data whose values can vary continuously and take on any value between two extremes.

Question 5

Define Digital Data?

Answer 5

Data whose values can vary discretely and can only take on one of a finite number of values between two extremes.

Question 6

Define Analogue Signals.

Answer 6

A transmission of a set of analogue data structures, that varies with time, between computational processes.

Question 7

Define Digital Signals.

Answer 7

A transmission of a set of digital data structures, that varies with time, between computational processes.

Question 8

Define Analogue to Digital Converter (ADC).

Answer 8

An integrated circuit capable of converting continuous analogue data to discrete digital data for a computer.

Question 9

Define Digital to Analogue Converter (DAC).

Answer 9

An integrated circuit capable of converting discrete digital data from a computer to continuous analogue data.

Question 10

Define Bitmapped Graphics.

Answer 10

An image composed of an array of pixels each with an allocated number of bits, arranged to form an image. Also known as raster graphics.

Question 11

What is the Bitmap storage requirements?

Answer 11

The amount of storage required for a bitmapped image is at least its image size x colour depth.

Question 12

Define Colour Depth.

Answer 12

A measure of the amount of colour used in an image, expressed in terms of the number of bits per pixel, used to represent the colour of each pixel in an image.

Question 13

What is the number of colours that can be represented in a colour depth of 4? And how many bits are used?

Answer 13

2^4 = 16 colours
2^2 = 4, so 2 bits.

Question 14

Define Image size.

Answer 14

The total number of pixels in an image expressed in terms of its dimensions: width in pixels x height in pixels.

Question 15

Define Metadata.

Answer 15

Data related to the image file data itself. e.g width, height, colour depth

Question 16

Define Resolution.

Answer 16

A measure of the total number of pixels in an image, typically expressed in terms of the number of dots/pixels per inch.

Question 17

How do you add more colours to an image?

Answer 17

Add more bits per pixel.

Question 18

Define Vector Graphics.

Answer 18

An image composed from mathematical coordinates and functions (lines and curves).

Question 19

Define Sample Resolution.

Answer 19

The number of bits used to represent a single sample.

Question 20

Define Sampling Rate.

Answer 20

The numbers of samples taken per second/over a given period of time.
The frequency at which samples are taken.
On the x axis.

Question 21

Define Sound Sampling.

Answer 21

The process of converting analogue sound waves to a digital waveform, by storing a finite number of readings in binary.

Question 22

Define Audio Bit Depth.

Answer 22

The number of bits used to represent the amplitude of each audio sample, affecting the sound quality and dynamic range.
On the y axis.

Question 23

Define Amplitude.

Answer 23

The height of a sound wave, determining the loudness of the sound.

Question 24

Define Frequency.

Answer 24

The number of cycles of a sound wave per second, determining the pitch of the sound.

Question 25

Define Event Messages.

Answer 25

Binary data transmitted between the MIDI device and computer processor that carries properties controlling when and how sounds are produced.

Question 26

Define MIDI.

Answer 26

Musical Instrument Digital Interface is a protocol for ADC audio transmission to a digital interface used for the majority of electronic musical instruments and computers.

Question 27

What is an Analogue to Digital Convertor used for?

Answer 27

Used to convert the sound waves to a digital format.

Question 28

What does the quality of the sound conversion depend on?

Answer 28

- sample interval - sample resolution

Question 29

How does the sample rate affect quality?

Answer 29

The sample rate allows the measurement of the frequency of a wave. The more samples the more accurate the tone is.

Question 30

How does sample resolution affect quality?

Answer 30

The sample resolution allows the measurement of the amplitude of a wave. The higher the bit depth the more accurate it is.

Question 31

Describe the process of analogue to digital conversion.

Answer 31

1. Microphone (sensor) detects/ measures the analogue wave. 2. The sound wave is sampled at regular intervals. 3. For each sample the amplitude is measured. 4. This is converted into discrete binary values. 5. The values are stored in a sound file.

Question 32

Define Sampling Error.

Answer 32

When the sample of the amplitude is incorrect.

Question 33

Define Lossy Compression.

Answer 33

A compression algorithm that removes non-essential data from a file leading to a noticeable decrease in accuracy of the data. Data lost is non-recoverable.

Question 34

Define Lossless Compression.

Answer 34

A compression algorithm that retains all the data in the file by only storing the instructions needed to reconstruct the original file. No data is lost.

Question 35

Define Dictionary - Based Coding.

Answer 35

A type of lossless compression where text is searched for entries that match the entries in a dictionary. Entries are substituted by a unique code which can then be translated.

Question 36

Define Run-Length Encoding.

Answer 36

A type of lossless compression where repeated occurrences of the same data (like several pixels of the same colour in an image) are stored as single data values with their counts.

Question 37

Define Encryption.

Answer 37

The process of converting the original data into a form which cannot be understood by unauthorised users using an encryption algorithm.

Question 38

Define Decryption.

Answer 38

The process of reversing encryption. Converting the encrypted data back into the original data using the same encryption algorithm and key.

Question 39

Define Caesar Cipher.

Answer 39

A substitution cipher where each letter of plain text is substituted for another that is a fixed number of letters ahead in the alphabet, which becomes the cipher text.

Question 40

Define Vernam Cipher.

Answer 40

A cipher that uses a one-time pad (a secret random key) to convert each character to cipher text by modularly adding it with the corresponding character of the key. This is impossible to decrypt without a key.

Question 41

Define Symmetric encryption scheme.

Answer 41

Where the same key is used to encrypt and decrypt so the key must be shared between the sender and the receiver.

Question 42

Define Asymmetric encryption scheme.

Answer 42

Where different keys (that are mathematically linked) are used to encrypt and decrypt. This means that the keys do not need to be shared. Uses public/private key. One is to encrypt and one is to decrypt.

Question 43

Give reasons as to why the Caesar Cipher is not secure.

Answer 43

- it is a shift cipher, so there are only 25 possible keys - can crack by listing all the keys and solutions to find plain text - the frequency at which each character occurs can provide a clue as to which letter has been replaced with which - once you discover just one character, a shift cipher can be completely cracked as the key is found

Question 44

Give reasons as to why the Vernam Cipher is said to be perfectly secure.

Answer 44

- each character is encrypted using its own key - key is chosen at random, so cipher text is also random - if the key only contains letters (english letters), there are 26 options for each character of the plain text, where n = len(word), there can be 26 ^ n no.of possible representations - if a random bitstream is used for the key, it becomes 256 ^ n possible representations - mathematically proven to be completely secure.

Question 45

What logic gate does the Vernam Cipher use?

Answer 45

XOR.

Question 46

What are the requirement for a Vernam Cipher?

Answer 46

- characters in the key must be truly random - key must only be used once - there must only be 2 copies of the key (sender and receiver) and must be secret - key must be destroyed after use

Question 47

What is meant by computational security?

Answer 47

Ciphers that are crackable in theory but not within reasonable timeframe and current computing power use this form of security.