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Edexcel GCSE Computer Science (1CP2) Exam Questions

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3 questions · 10 marks

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1Explain2 marksCharacter Sets Swap Remove

Characters typed on a keyboard are stored using ASCII.

Explain how the computer encodes the letter A using ASCII.

[2 marks]
Mark scheme

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

  • The computer looks up the character in the ASCII character set/table (1) to find the code that represents it (1)
  • Each character has its own unique ASCII code (1) so the letter A is converted to and stored as a 7-bit binary pattern (1)
  • ASCII uses 7 bits for each character (1) so A is stored as its ASCII code 1000001 (binary) / 65 (denary) (1)

The second mark requires a linked expansion, so a point on its own (e.g. 'A has the code 65') can only earn one mark.

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

Accept an 8-bit pattern with a leading 0 (e.g. 01000001) as the binary code for A.

Do not accept 'the character is stored as text' without saying it is changed into a code/binary.

Do not accept a binary value on its own (e.g. 1000001) without saying that it is the ASCII code/that the character is converted to it.

Example answers and how they were marked
Full marks2/2

The computer finds the unique ASCII code for A, which is 65 in denary, and stores it as the 7-bit binary pattern 1000001.

Why: Linked point that A is given its unique ASCII code (1) and reason/expansion that it is converted to and stored as a 7-bit binary pattern (1).

Some marks1/2

The computer changes the letter A into a number.

Why: Gives the point that the character is given a code/number (1) but no linked expansion saying it is converted to binary or stored, so stays on one mark.

No marks0/2

The letter A is stored as the number 1 because it is the first letter of the alphabet.

Why: Wrong reason and no creditworthy point: it does not say that the character is given its ASCII code and converted to binary.

2Explain2 marksImages Swap Remove

A bitmap image uses a colour depth of 8 bits. Each pixel is stored as a binary value.

Explain how the colour of a pixel is represented in a bitmap image.

[2 marks]
Mark scheme

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

  • Each pixel is stored as a binary value/code (1) which represents/maps to a colour (1)
  • The binary value is used to look up a colour in a palette (1) so the pixel can be displayed in that colour (1)
  • A bit pattern is assigned to each colour (1) and each pixel stores the bit pattern for its colour (1)
  • The number of bits per pixel sets how many different colour codes exist (1) so each code can represent a different colour (1)

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

Do not accept 'pixels are binary' or 'the image is made of 1s and 0s' without saying a pixel's colour is represented by a binary value.

Do not accept RGB values stored as denary or hexadecimal.

Example answers and how they were marked
Full marks2/2

Each pixel stores a binary value; this value is used to look up the pixel's colour in a palette.

Why: Linked point (1) and reason (1): a binary value is stored for each pixel and maps to a colour.

Some marks1/2

A bit pattern is assigned to each colour.

Why: One mark for stating that colours have bit patterns; it does not say that each pixel stores the pattern for its colour.

No marks0/2

Each pixel stores the RGB colour as denary numbers between 0 and 255.

Why: Bitmap pixels are stored in binary, not as denary values; this common misconception earns no mark.

3Discuss6 marksSound Swap Remove

A toy company is designing an electronic keyboard. The keyboard can store and play back short recorded sounds. The memory chip in the keyboard has very little space, so each sample is stored using only 3 bits.

Discuss the limitations of storing the recorded sounds using only 3 bits per sample.

Your answer should include:

  • the range and number of amplitude values that can be stored
  • the accuracy of each sample that is stored
  • how the limitation could affect what the user hears.
[6 marks]
Mark scheme

Indicative content

Range and number of values

  • With a bit depth of 3, the keyboard can store only 2^3 = 8 different amplitude values.
  • The 3-bit codes run from 000 to 111, so the loudest and quietest amplitudes the keyboard can record are fixed by those codes.
  • Because only eight levels of loudness are available, the keyboard cannot store the full variation in amplitude of the original recording.
  • Any sound in between two of the eight values cannot be stored at all, so much of the detail of the original sound has no code to represent it.

Accuracy of the stored sample

  • Each true amplitude must be rounded to the nearest of the eight available values, so the stored sample is not the same as the amplitude that was measured.
  • The difference between the true amplitude and the rounded value is the quantisation error, which makes every stored sample less accurate.
  • The eight values are far apart, so there is a large step between one stored amplitude and the next and the stored sound rises and falls in big jumps rather than following the original wave.
  • A quiet sound whose amplitude is smaller than one step may be stored with the same code as silence, so that part of the recording is lost.

Effect on the user

  • When the keyboard plays back the recording, the large steps between stored amplitudes are heard as distortion or noise rather than a smooth copy of the original sound.
  • Quiet parts of the recording may not be heard at all, because they are stored as the same value as silence.
  • The reproduced sound may not be recognisable as the instrument or voice that was recorded.

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)

Level 3 responses must explain that 3 bits gives only 8 amplitude values and link the large steps between those values to the distortion or the loss of quiet sounds that the user hears; an explicit binary range of 000 to 111 is not required for Level 3.

Level 2 requires one point to be developed with a reason, e.g. eight values means each amplitude is rounded to the nearest and the quantisation error is large enough to be heard; stating 8 values, reduced accuracy and distortion as separate basic points without a reason is Level 1. Do not accept bit depth described as the number of samples taken per second, or a bare statement that the quality is worse with no explanation.

Example answers and how they were marked
Full marks6/6

With only 3 bits per sample, the keyboard can store just 2^3 = 8 different amplitude values. Each recorded amplitude has to be rounded to the nearest of these eight values, so the stored sample is not the same as the original and a quantisation error is created. The eight values are far apart, so the stored sound moves in large steps instead of following the original wave. A quiet sound smaller than one step is stored as the same code as silence, so it is lost. When the keyboard plays the recording back, the user hears these large steps as distortion and noise, and quiet parts of the music may not be heard at all, so the sound is not a faithful copy of the original.

Why: Level 3 (6 marks): explains the 8 values from 3 bits, rounding creating quantisation error, the large steps between values, lost quiet sounds and the distortion the user hears, all applied to the keyboard.

Some marks2/6

With 3 bits, there are only 8 values that can be stored, so the amplitude is rounded to the nearest value. This makes the sound less accurate than the original recording. The user might hear distortion when the keyboard plays it back.

Why: Level 1 (2 marks): gives basic points (8 values, rounding to the nearest value, less accurate, possible distortion) but states them separately, without explaining why the eight far-apart values cause a large quantisation error or why that is heard as distortion, so it does not reach Level 2.

No marks0/6

3 bits means 3 samples per second, so the keyboard cannot store much sound. The quality will be low.

Why: Level 0 (0 marks): confuses bit depth with sample rate ('3 samples per second'), which the scheme says not to accept, and 'the quality will be low' is a bare statement with no explanation, so there is no creditworthy point.

Questions by topic

2.1 Binary

Build a paper on this topic 58
Adding Binary Numbers 6 questions Binary and Hexadecimal Basics 10 questions Binary Shifts 6 questions Binary to Denary Conversion 6 questions Binary to Hexadecimal Conversion 6 questions Denary to Binary Conversion 6 questions Denary to Hexadecimal Conversion 6 questions Denary to Negative Binary Conversion 6 questions Hexadecimal to Binary Conversion 6 questions

2.2 Data representation

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Character Sets 18 questions Images 19 questions Sound 19 questions

2.3 Data storage and compression

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Compression 18 questions Units & Conversion 6 questions

3.1 Hardware

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Embedded Systems 17 questions Fetch Decode Execute Cycle 15 questions Primary Memory 15 questions Secondary Memory 15 questions Stored Program Computers 15 questions

3.2 Software

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File Management 18 questions Memory Management 16 questions Peripheral Management 11 questions Process Management 12 questions Purpose & Function of Operating Systems 17 questions Software Basics 13 questions User Management 18 questions Utility Software 19 questions

3.3 Programming languages

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Low and High Level Languages 19 questions Translators 18 questions

4.1 Networks

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Addressing 16 questions LANs & WANs 16 questions Network Speed 5 questions Packet Switching 16 questions Protocols & Layers 17 questions The Internet 16 questions Topologies 17 questions Wired Connections 17 questions Wireless Connections 18 questions

4.2 Network security

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Network Security Risks 18 questions Preventing Network Security Risks 18 questions

5.1 Environmental

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E-waste 17 questions Manufacturing Technology 19 questions

5.2 Ethical and legal

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Data Protection Act 17 questions Impact of AI 18 questions Intellectual Property 18 questions Software Licences 18 questions

5.3 Cybersecurity

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Malware 18 questions Social Engineering 18 questions

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