Edexcel GCSE Computer Science (1CP2) Exam Questions

Award one mark for any of the following:

• The concentration/density of pixels in an area (1)
• The number of pixels per inch (PPI) / pixels per unit area (1)
• The width and height/dimensions of the image in pixels (1)

Do not accept the number of pixels in the image on its own, without dimensions or an area, e.g. 'the total number of pixels'

Do not accept colour depth / the number of bits used per pixel / the number of colours

Do not accept file size in bytes / the amount of storage the image uses / screen size / image quality

Award one mark for any of the following:

• The number of bits per pixel (1)
• The number of bits used to represent/store the colour of each pixel (1)

Allow bit depth

Do not accept the number of colours a pixel can have on its own

Do not accept the number of pixels in the image (this is the resolution/image size)

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

• Fewer samples are taken every second (1) so less detail of the sound wave is captured (1)
• Fewer samples are taken every second (1) so the file is smaller/uses less storage space (1)
• The wave is measured less often (1) so parts/changes in the wave between samples are missed (1)

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

The 'fewer samples are taken/measured less often' point can only be credited once; the second mark is for a valid expansion linked to it.

Do not accept restating the question, e.g. 'the time between measurements is longer' or 'the sample interval is longer', as either the point or the expansion.

Do not accept 'the sample rate is lower' as the expansion, as this says the same thing as a longer sample interval.

Do not accept 'the sound is lower quality/less accurate' on its own without saying what is lost, e.g. less detail of the wave captured/parts of the wave missed.

Do not accept answers about using fewer bits per sample or about compression, as these do not follow from the sample interval.

Indicative content

The number of characters that 7-bit ASCII can represent

• Seven-bit ASCII gives 2 to the power of 7, which is 128 different codes, so it can represent a maximum of 128 different characters.
• This is enough for the English alphabet, digits and common punctuation, but it is a fixed and limited set.
• The website may need more than 128 characters once customer names from different countries are included.

Characters that cannot be represented

• Accented letters such as é do not have a 7-bit ASCII code, so they cannot be stored correctly.
• Characters from any alphabet other than the Latin one, such as Greek or Cyrillic letters, cannot be represented in 7-bit ASCII.
• If a customer enters a character that is not in the character set, the website may store the wrong character or show a replacement symbol.
• The company may have to misspell or simplify a customer's name because the required character cannot be stored.

The effect of using more bits for each character

• Each extra bit doubles the number of available codes, so for n bits there are 2 to the power of n codes; 8 bits give 256 codes and 16 bits give 65 536 codes.
• Using more bits allows a larger character set to represent accented letters and other alphabets.
• More bits per character means each character takes more storage, so the website's text files and pages are larger.
• Larger pages take longer to transmit over the internet to a customer.

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 the limit of 128 codes and at least one type of character that cannot be represented; a point about using more bits on its own is not sufficient for Level 3, and not every bullet needs to be covered for Level 3.

Do not accept the idea that 7-bit ASCII can represent all characters, that extended or 8-bit ASCII can represent every character in every alphabet, or that accented letters are stored as plain letters without the accent.