Edexcel GCSE Computer Science (1CP2) Exam Questions

Award one mark for:

• 64 (1)
• 2^6 (1)
• 2 × 2 × 2 × 2 × 2 × 2 (1)

Allow 64 characters / sixty-four.

Allow 2 to the power of 6 for 2^6.

Do not accept 128, which is the number of characters that can be represented using 7 bits.

Do not accept 63, the largest value that can be represented using 6 bits, rather than the number of different characters.

Do not accept 12 (2 × 6), ie the number of bits multiplied by 2.

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 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.

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.