Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• 6 bits give only 64 different codes (1) which is fewer than the number of letters, digits and symbols that must be represented (1)
• 7 bits give 128 different codes (1) which is enough to represent every letter, digit and symbol that is needed (1)
• Adding a bit doubles the number of different codes from 64 to 128 (1) so that every character needed can have its own code (1)
• There are more than 64 characters that must be represented (letters, digits and symbols) (1) so 6 bits cannot give each one a different code (1)

A statement of the number of codes without a development can earn one mark only; the development must be linked to the statement for the second mark.

Do not accept responses that only say computers use binary (1s and 0s).

Do not accept that 7 bits are needed because 7 is a bigger number than 6, with no reference to the number of codes or characters.

Do not accept responses about 8 bits, extended ASCII or Unicode.

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

• More samples are taken every second / the sample rate is higher (1) so the digital wave follows the shape of the analogue wave more closely (1)
• The amplitude is measured more often / at shorter intervals (1) so fewer changes in the sound are missed between measurements (1)
• The sample interval is shorter (1) so less of the sound wave is lost/goes unrecorded between samples (1)
• More data points are recorded for the same length of sound (1) so the digital representation is a closer copy of the original sound (1)

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

Do not accept 'the quality is better/higher' or 'the sound is clearer' as a point on its own without a reason such as more samples or more detail.

Do not accept 'the samples are more accurate' or 'more data is collected' without saying what becomes more accurate or what the extra data records.

Do not accept 'the file size is larger' as a reason for improved quality.

Do not accept answers about using more bits per sample, as these describe bit depth, not the sample rate.

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

• Only a limited number of different amplitude values can be stored (1) so each sample is rounded to the nearest available value and the digital wave does not match the analogue wave exactly (1)
• The wave is continuous but only a limited set of values can be represented (1) so amplitude detail/accuracy is lost (1)
• Quiet and loud parts of the sound may be stored as the same value (1) because there are too few values to tell them apart (1)
• The range of amplitudes that can be represented is limited (1) so the loudest/quietest parts of the sound cannot be stored accurately (1)

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

Do not award marks for answers that say the sound cannot be stored or heard at all.

Do not award answers about the number of samples taken/sample rate, as the number of bits limits the amplitude values, not how often the wave is sampled.

Do not accept 'lower quality'/'less accurate'/'less detail' on its own without a link to the amplitude values that can be stored/the number of bit combinations.

Do not accept 'the file size is smaller' without a link to the accuracy or the range of values that can be stored.

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.