Data Representation

Question 1

What do computers use ASCII and Unicode to do?

Answer 1

Encode characters

Question 2

What does ‘ASCII’ stand for?

Answer 2

American Standard Code for Information Interchange

Question 3

(Exam-style question)

Define what is meant by the ‘character set’ of a computer. (2)

Answer 3

The list of binary codes that can be recognised by a computer’s hardware and software

Question 4

(Exam-style question)

Describe how ASCII is used to represent characters in a computer system. (2)

Answer 4

ASCII is a 7-bit code, therefore making use of 128 code sequences to represent different English characters

Question 5

How many bits are used to represent characters in the ASCII character set?

Answer 5

7

Question 6

(Exam-style question)

The 7-bit ASCII code for the letter ‘Q’ is 81.

Give the ASCII code for the letter ‘H’. (1)

Answer 6

72

Question 7

(Exam-style question)

The ASCII code for the character ‘C’ is 67.

Give the ASCII code for the character ‘M’. (1)

Answer 7

77

Question 8

(Exam-style question)

Explain why Unicode was developed. (2)

Answer 8

Any from:

• Uses a minimum of 16 bits, so it can represent at least 2¹⁶ characters
• Before it existed, there were many different encoding systems but none could contain enough characters to represent all major languages
• Standard ASCII only provides 128 different patterns, it can’t represent all major languages/symbols/characters
• Unicode can represent all languages, ASCII was developed for English

Question 9

(Exam-style question)

State what is meant by the size of an image. (1)

Answer 9

The product of the no. of pixels for the image’s width (W) and the no. of pixels for its height (H)

Question 10

(Exam-style question)

State what is meant by the resolution of an image. (1)

Answer 10

The number of pixels per unit area of the display

Question 11

(Exam-style question)

State what is meant by the term ‘pixel’. (1)

Answer 11

The smallest block of colour in an image

Question 12

What is meant by colour depth?

Answer 12

The number of bits used to encode the colour of each pixel

Question 13

What is the equation to calculate the file size of an image?

Answer 13

Width × Height × Colour Depth

(file size measured in bits)

Question 14

(Exam-style question)

Describe the factors that affect the quality of a digital image. (4)

Answer 14

• The quality of the image is affected by the number of pixels that make it up, therefore, the more pixels the better the image resolution
• The quality of the image is affected by the number of bits used to encode the colour depth, therefore, if more bits are used, more colours can be displayed

Question 15

How is file size affected if an image has good quality?

Answer 15

The file size will be large

Question 16

What are analogue recordings?

Answer 16

Original sound recordings

Question 17

What are digital recordings?

Answer 17

Recordings that use samples of the sound at regular fixed intervals

Question 18

(Exam-style question)

Sound can be represented digitally by taking samples of the original sound.

State what is meant by sample rate. (1)

(also known as ‘sampling frequency’)

Answer 18

Sample rate describes the number of sound samples that are taken each second

Question 19

(Exam-style question)

Describe the effect of increasing the sample rate. (2)

Answer 19

Increasing the sample rate gives a more accurate reproduction of the analogue wave, as more samples are taken with less time between them

Question 20

(Exam-style question)

Explain what is meant by the bit depth of a recording. (2)

Answer 20

Bit depth describes the number of bits used to encode the data taken in each sample

Question 21

(Exam-style question)

State the effect of increasing the bit depth of a recording. (1)

Answer 21

Allows more data to be stored, meaning the range of sound can be represented more accurately

Question 22

(Exam-style question)

The sample rate and bit depth affect the size of the file produced.

Name two other factors which will affect the size of the file. (2)

Answer 22

• Length of recording
• Number of channels

Question 23

(Exam-style question)

An analogue signal is never fully reproducible in a digital format.

Explain one reason why this statement is true. (2)

Answer 23

• A digital recording samples the analogue signal at fixed intervals
• Therefore, the entire analogue signal is not represented in the digital recording

Question 24

(Exam-style question)

Describe the steps taken to convert the analogue sound to a digital sound file. (3)

Answer 24

• Sample the analogue sound at equal intervals
• Measure the sound amplitude
• Give a binary value for each measurement
• Store data as digital signals

Question 25

What is the equation to calculate the file size of an audio file?

Answer 25

Sample rate (Hz) × Bit depth × Recording length (seconds) | *(file size measured in bits)*

Question 26

What is 1 hertz (Hz) equal to, in terms of samples?

Answer 26

1 sample per second

Question 27

Why is the total file size for CDs doubled?

Answer 27

They are recorded in stereo and so have two channels

Question 28

Answer 28

The digital signal

Question 29

Answer 29

The analogue signal

Question 30

Answer 30

2Hz

Question 31

Answer 31

101

Question 32

# **(Exam-style question)** State the formula used to determine the number of distinct binary patterns for a known number of bits, 'n'. (1)

Answer 32

2ⁿ

Question 33

# **(Exam-style question)** Describe the reason 4 bits may be the preferred way to store colour depth in an image rather than 8 bits. (2)

Answer 33

Any one from: * The increase in colours that cause a slight improvement in quality might not matter to the person viewing the image * The image may be on a web page that is transferred over the internet, which needs to load very quickly * The higher colour depth file would mean the image takes longer to transfer and loads more slowly

Question 34

# **(Exam-style question)** Explain **one** reason for reducing the bit depth of an audio recording. (2)

Answer 34

Any one from: * Reducing the bit depth reduces the file size, so the audio file will transmit faster * Reducing the bit depth reduces the number of distinct values and these may not be noticed by the human ear

Question 35

# **(Exam-style question)** Explain **one** reason that a 5-bit colour depth is needed to store 24 colours in an image. (2)

Answer 35

**Five bits** are needed to store 24 colours because 2⁴ = 16, which is less than 24, and 2⁵ = 32, which is the next largest power of 2 greater than 24