Edexcel GCSE Computer Science (1CP2) exam questions

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9 questions · 20 marks

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1Multiple choice1 markCharacter Sets Swap Remove

A new character set must be able to represent 70 different characters.

Identify the minimum number of bits needed to represent each character.

[1 mark]

Mark scheme

B 7 (1)
The only correct answer is B. The correct answer is 7.
A is not correct because 6 bits give only 64 combinations, which is not enough for 70 characters
C is not correct because 7 bits already give 128 combinations, so 8 bits is not the minimum needed
D is not correct because 70 is the number of characters needed, not a number of bits

Example answers and how they were marked

Full marks1/1

B 7

Why: The only correct answer: 7 bits give 128 combinations, enough for 70 characters, and it is the smallest number of bits that does (1).

No marks0/1

C 8

Why: Distractor, no credit: a single-tick item has no partial credit, and 8 bits is not the minimum because 7 bits already give 128 combinations, so the scheme's C line gives no mark.

No marks0/1

A 6

Why: Misconception, no credit: 6 bits give only 64 combinations, which cannot represent 70 different characters, so the scheme's A line gives no mark.

2Short recall1 markSound Swap Remove

To store sound in binary, an analogue sound wave is sampled at regular intervals.

Define the term 'sample rate'.

[1 mark]

Mark scheme

Award one mark for:

The number of samples taken per second (1)

Allow captured/recorded for taken.

Allow 'samples per second'.

Allow 'each second' for 'per second'.

Allow Hz/hertz as the unit.

Do not accept the time between samples/sample interval.

Do not accept 'the number of samples' without a per second/time reference.

Do not accept the number of bits used per sample/bit depth.

Example answers and how they were marked

Full marks1/1

The number of samples taken per second.

Why: States samples per second (1); 'per second' is accepted.

No marks0/1

The time between each sample.

Why: Defines sample interval, not sample rate; do not accept.

No marks0/1

The number of bits used for each sample.

Why: Describes bit depth, not sample rate; do not accept.

3Short recall2 marksSound Swap Remove

A sound wave is sampled and the amplitude of each sample is stored using a small number of bits.

Give two limitations of storing the sound in binary when only a small number of bits is available for each sample.

[2 marks]

Mark scheme

Award one mark for any of the following up to a maximum of two marks:

Only a limited/fixed number of amplitude values can be stored (1)
The amplitude of each sample is rounded/approximated to the nearest value that can be stored (1)
The recording is less accurate than the original analogue sound/does not match the analogue wave exactly (1)
Sounds louder than the highest amplitude that can be stored are clipped/distorted (1)
Sounds quieter than the lowest amplitude that can be stored cannot be represented/are lost (1)

Allow quantisation error/noise for the amplitude being rounded to the nearest value that can be stored, but do not award a second mark for the same idea

Allow a stated consequence as a separate mark only when it is a different point from the list, e.g. fewer amplitude values can be stored (1) so very quiet sounds are lost (1)

Do not accept 'lower quality'/'poorer sound' on its own; the answer must say the recording is less accurate than the original/does not match the analogue wave

Do not accept sample rate/sample interval/number of channels (stereo/mono), as these are not limited by the number of bits available for each sample

Do not accept remedies such as 'a larger bit depth would make it more accurate', as these do not describe a limitation

Do not accept answers about file size, compression or playback method

Do not accept a statement that repeats the stem, e.g. 'the sound is stored in binary using a fixed number of bits'

Example answers and how they were marked

Full marks2/2

The amplitude of each sample has to be rounded to the nearest value that can be stored, and very quiet sounds cannot be represented.

Why: Amplitude rounded to the nearest stored value (1) and quiet sounds cannot be represented (1): two different points from the list.

Some marks1/2

The recording is less accurate than the original sound.

Why: The recording is less accurate than the original analogue sound (1); a second, different limitation is not given.

No marks0/2

The sample rate is too low, so parts of the wave are missed.

Why: Sample rate is not limited by the number of bits per sample, so it is not accepted.

4Short recall2 marksImages Swap Remove

Bitmap images are stored as a grid of pixels.

Define the term pixel.

[2 marks]

Mark scheme

Award one mark for any of the following up to a maximum of two marks:

The smallest element/area of a bitmap image (1)
The smallest identifiable part of an image (1)
Stores/represents a single colour as a binary value (1)
Stores/represents a single colour using a number of bits/colour depth (1)

Allow 'picture element' for element/part in the first two bullets, but not on its own

Allow 'bit depth' for colour depth

Do not accept 'picture element'/'pixel' on its own

Do not accept 'a group of pixels' or 'a dot' on its own; allow 'dot' only when it is described as the smallest element/area of the image

Do not accept 'single colour' on its own for the second mark; it must be linked to a binary value/number of bits

Do not accept 'lots of coloured dots' (it contradicts a single colour)

Example answers and how they were marked

Full marks2/2

The smallest identifiable area of a bitmap image, which has one colour stored as a binary value.

Why: Smallest identifiable area (1) and single colour stored as a binary value (1)

Some marks1/2

The smallest part of a bitmap image, made of one colour.

Why: Smallest part (1); no credit for the second mark because the single colour is not linked to a binary value or number of bits

No marks0/2

A picture element made from lots of coloured dots.

Why: 'Picture element' on its own is not accepted and 'lots of coloured dots' contradicts a single colour

5Describe2 marksSound Swap Remove

Each sample of a sound is stored as a binary number.

Describe how bit depth affects the stored value of a sound sample.

[2 marks]

Mark scheme

Award up to two marks for a linked description such as:

The bit depth is the number of bits used to store each sample/measurement (1) so a higher bit depth gives more possible/discrete values for the amplitude (1)
A higher bit depth records the amplitude more accurately (1) because the stored value can be closer to the actual height of the wave (1)
A low bit depth gives fewer possible amplitude values (1) so the amplitude is rounded to the nearest value, making the recording less accurate (1)

Do not accept sample rate/sample interval (number of samples per second) in place of bit depth.

Do not accept 'more bits gives better quality' or 'more accurate' on its own, without a development linked to the number of values or the amplitude stored.

Do not accept file size on its own, and a file size point cannot earn the second mark, as it does not describe the stored value.

Example answers and how they were marked

Full marks2/2

The bit depth is the number of bits used to store each sample, so a higher bit depth gives more possible values for the amplitude that can be stored.

Why: First point, the number of bits per sample (1), then the linked development, more possible values for the amplitude (1).

Some marks1/2

The bit depth is the number of bits used to store one sample.

Why: Gives the first point, the number of bits per sample (1), but does not develop how this affects the value that is stored.

No marks0/2

A higher bit depth makes the sound better quality and more accurate.

Why: Near-miss: 'better quality' and 'more accurate' on their own, with no development linked to the number of values or the amplitude, so no mark is awarded.

6Explain2 marksCharacter Sets Swap Remove

A computer system uses a 4-bit code to represent the characters typed on its keyboard.

Explain one limitation of using 4 bits to represent characters.

[2 marks]

Mark scheme

Award up to two marks for a linked explanation such as:

Only 16 different characters can be represented (1) so there are not enough characters codes for every letter of the alphabet (1)
4 bits give only 16 different combinations (1) which is not enough for the uppercase letters, lowercase letters, digits and symbols on a keyboard (1)
More than 16 characters are needed on a keyboard (1) so some letters, digits or symbols cannot be represented (1)
The number of character codes available is limited to 16 (1) because 4 bits give 2 to the power of 4 combinations (1)

For both marks, the expansion must follow/associate with the statement.

Calculating that 4 bits give 2 to the power of 4 = 16 combinations can earn either the point or the linked reason, provided the other half of the answer says what this means (too few characters/codes).

Allow an expansion that names only the letters of the alphabet, e.g. there are not enough character codes for the 26 letters (1).

Do not accept '4 bits can only store 4 characters' (the number of bits is not the number of characters).

Do not accept 'it can only store 16 bits' (16 is the number of character codes/combinations, not the number of bits).

Do not accept 'not enough memory/storage' (the limitation is the number of character codes, not the amount of storage).

Do not accept 'it is limited/smaller' without saying how many characters/codes are available or what cannot be represented.

Example answers and how they were marked

Full marks2/2

Only 16 different characters can be represented, so there are not enough character codes for all 26 letters.

Why: Gives the point that only 16 characters can be represented (1) and links it to the reason that this is fewer than the letters needed (1).

Some marks1/2

Only 16 different characters can be represented.

Why: Gives the point that only 16 characters can be represented (1) but does not explain what cannot be represented.

No marks0/2

It can only store 4 characters because there are 4 bits.

Why: Misconception that the number of bits equals the number of characters, which the mark scheme says not to accept; no creditworthy point.

7Explain2 marksImages Swap Remove

An image uses a colour depth of 2 bits per pixel. The original image contains 12 different colours.

Explain why the image cannot be represented accurately at this colour depth.

[2 marks]

Mark scheme

Award up to two marks for a linked explanation such as:

Only 4 colours can be represented (1) because 2 bits give 2^2 = 4 possible values (1)
Some of the 12 colours cannot be stored (1) so they are lost/replaced with a colour that can be represented (1)
The image will not match the original exactly (1) because there are fewer colour codes than the number of colours needed (1)
Many colours will share the same binary value (1) so the image will lose colour detail (1)

For both marks, the expansion must follow/associate with the statement.

Allow 'four colours' or '2^2'.

Do not accept 'only two colours' (confusing the number of bits with the number of values) or 'resolution is too low'.

Example answers and how they were marked

Full marks2/2

Only four colours can be represented (1) because two bits give four possible binary values (1).

Why: Linked point (1) and reason (1): 2 bits give 2^2 = 4 colour codes.

Some marks1/2

Only four colours can be represented.

Why: One mark for identifying that four colours can be represented; the linked reason about two bits giving four values is missing.

No marks0/2

Only two colours can be represented because each pixel has two bits.

Why: A common misconception: two bits give four possible values, not two, so no creditworthy point is given.

8Explain2 marksSound Swap Remove

Each sample of a sound wave is stored as a binary number.

Explain how increasing the bit depth affects the representation of a sound wave.

[2 marks]

Mark scheme

Award up to two marks for a linked explanation such as:

Each sample is stored using more bits (1) so the amplitude can be recorded more precisely (1)
More different amplitude values can be represented (1) because each extra bit doubles the number of possible values (1)
The height of the wave at each sample point is measured more accurately (1) so the digital wave is closer to the analogue wave (1)
The gap between the true amplitude and the stored value is smaller (1) because there are more values for the sample to be rounded to (1)

For both marks, the expansion must follow/associate with the statement.

Do not accept 'the sound is sampled more often' or 'more samples are taken per second', as these describe the sample rate, not the bit depth.

Do not accept 'the file size increases' on its own as an effect on the representation of the sound.

Example answers and how they were marked

Full marks2/2

Each sample is stored using more bits, so the amplitude at each sample point can be recorded more precisely.

Why: Point (more bits per sample) (1) with linked reason (amplitude recorded more precisely) (1).

Some marks1/2

Each sample is stored using more bits.

Why: Creditworthy point (1) but no linked effect on the accuracy of the representation, so only one mark.

No marks0/2

It takes more samples every second, so the wave is stored in more detail.

Why: Confuses bit depth with sample rate, so no mark is awarded.

9Discuss6 marksCharacter Sets Swap Remove

A library is digitising its catalogue. The titles of books and the names of authors are stored as text.

Discuss how 7-bit ASCII is used to encode the text in the library catalogue.

Your answer should include:

how characters are mapped to codes
why 7 bits are used for each character
the benefits of using ASCII for the catalogue.

[6 marks]

Mark scheme

Indicative content

How characters are mapped to codes

Each character in a book title or author name is given a unique 7-bit binary code from the ASCII character set.
The letter A, for example, is stored as the code 1000001 and the letter B as 1000010, so the catalogue can store letters as binary.
Uppercase letters, lowercase letters, digits, punctuation and the space character each have their own ASCII code.
The same character always uses the same code, so the text can be stored and read back correctly by the library system.

Why 7 bits are used for each character

Seven bits give 2 to the power of 7, which is 128 different bit patterns, and this is enough to represent the letters, digits and punctuation needed in the catalogue.
A standard keyboard has around 98 characters, so 6 bits would give only 64 codes and would not be enough.
Using a fixed 7 bits per character means every character takes the same amount of storage, which makes the catalogue file simple to process.

Benefits of using ASCII for the catalogue

ASCII is a standard character set, so the catalogue data can be exchanged between different computers and software.
Author names and titles entered on one system will display correctly on another system that uses ASCII.
ASCII uses only 7 bits per character, so the catalogue takes less storage than a character set that uses more bits.
It is simple to encode and decode because each character has a single fixed code.

Levels

Level 0 (0 marks): No rewardable content.
Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Do not accept 8 bits or 256 characters as features of 7-bit ASCII; these are extended ASCII.

Example answers and how they were marked

Full marks6/6

Each character in a catalogue title or author name is given its own 7-bit ASCII code. For example, the letter A is 1000001 and the letter B is 1000010, so the library can store the text as binary. Seven bits give 128 different codes, which is enough for uppercase and lowercase letters, digits, punctuation and spaces. Six bits would give only 64 codes, which would not be enough for all the characters in the catalogue. ASCII is also a standard character set, so the catalogue data can be exchanged between different computers and will display correctly. Because each character uses only 7 bits, the catalogue also takes less storage than it would with a character set that uses more bits per character.

Why: Covers all three bullets with sustained application: mapping and example codes; 7 bits give 128 codes and why 6 bits is insufficient; standardisation and storage benefits. Level 3.

Some marks4/6

ASCII gives each character a unique code. For example, A is stored as 1000001 and B is stored as 1000010. Seven bits are used for each character because seven bits give 128 codes, which is enough for the letters and punctuation in the catalogue. The library can store titles and author names as binary.

Why: Covers mapping and the 7-bit reason with some application, but does not discuss the benefits of ASCII for the catalogue. Level 2.

Some marks1/6

ASCII is a programming language that gives each letter a binary code. It uses 8 bits for each character, so it can store 256 characters. This means it can store every book title in the library.

Why: A basic point that letters are given binary codes is credited, but the answer treats ASCII as a programming language and uses 8 bits/256 characters, which are misconceptions. Level 1.

Edexcel GCSE Computer Science (1CP2) Exam Questions

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Questions by topic

2.1 Binary

2.2 Data representation

2.3 Data storage and compression

3.1 Hardware

3.2 Software

3.3 Programming languages

4.1 Networks

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