Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• (Only) 128 different characters can be represented / 7 bits give 2^7 = 128 different codes (1)
• It cannot represent characters from languages other than English / non-English alphabets, e.g. Greek, Chinese (1)
• It cannot represent accented characters, e.g. é, ñ (1)
• It cannot represent symbols or emoji that are not in the ASCII character set (1)

Award a maximum of one mark for the last three bullets, as they are all examples of characters that fall outside the 128 ASCII codes.

Allow 'it can only represent 128 characters, so some characters are missing' for the first bullet.

Do not accept 'it uses 7 bits' / 'it only uses 7 bits' on its own, without saying what this limits.

Do not accept 'it can only store a limited amount of text' / 'it can only store short messages', as 7 bits limits the number of characters available, not the length of a message.

Do not accept 'it can only represent the characters in the ASCII character set', as this repeats the question.

Do not accept 'it is old/out of date' or 'it cannot store numbers'.

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

• Only a fixed number of amplitude values/levels can be represented (1) so the amplitude is rounded to the nearest available value/level (1)
• Fewer amplitude values/levels are available to choose from (1) so similar/close amplitudes cannot be stored as different values (1)
• The exact amplitude often falls between available values/levels (1) so the stored amplitude is not exact/less accurate (1)

Answers must relate to the number of bits per sample (bit depth) and the amplitude values/levels it can represent.

Do not accept answers about the sample rate, sample interval or the number of samples taken, or that confuse bit depth with the number of samples.

Do not award 'lower quality'/'more detail'/'less detail' unless it is linked to the number of amplitude values/levels.

Do not accept 'the sound cannot be played/stored' without a linked limitation.

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.

Indicative content

Pixels

• A bitmap image is made up of picture elements, called pixels.
• Each pixel is the smallest identifiable area of the image and has a single colour.
• The logo is stored as a grid of pixels, and its width and height in pixels give its dimensions.
• Each pixel is stored separately, so a larger grid needs more pixel values to be stored.

Binary values

• Each pixel's colour is represented by a binary value.
• The number of bits used for each pixel is the colour depth.
• With n bits per pixel there are 2^n different binary values, so 2^n possible colours.
• A black-and-white image can use one bit per pixel, with 0 for white and 1 for black.
• More bits per pixel allow more colours to be represented in the logo.

Grid

• The binary values are stored in order, row by row, so the computer can rebuild the grid.
• The resolution is the number of pixels in the image, for example width × height.
• A higher-resolution grid stores more pixels, so the logo can show more detail.
• The image file stores the pixel values and data such as the image dimensions.

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 cover both pixels and binary values.

Do not accept resolution as the number of colours.