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Do not accept 'it is old/out of date' or 'it cannot store numbers'.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 one mark for any of the following up to a maximum of two marks:

• Only a limited/fixed number of amplitude values can be stored (1)
• The amplitude of each sample is rounded/approximated to the nearest value that can be stored (1)
• The recording is less accurate than the original analogue sound/does not match the analogue wave exactly (1)
• Sounds louder than the highest amplitude that can be stored are clipped/distorted (1)
• Sounds quieter than the lowest amplitude that can be stored cannot be represented/are lost (1)

Allow quantisation error/noise for the amplitude being rounded to the nearest value that can be stored, but do not award a second mark for the same idea

Allow a stated consequence as a separate mark only when it is a different point from the list, e.g. fewer amplitude values can be stored (1) so very quiet sounds are lost (1)

Do not accept 'lower quality'/'poorer sound' on its own; the answer must say the recording is less accurate than the original/does not match the analogue wave

Do not accept sample rate/sample interval/number of channels (stereo/mono), as these are not limited by the number of bits available for each sample

Do not accept remedies such as 'a larger bit depth would make it more accurate', as these do not describe a limitation

Do not accept answers about file size, compression or playback method

Do not accept a statement that repeats the stem, e.g. 'the sound is stored in binary using a fixed number of bits'

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

• The computer needs to tell/distinguish the uppercase A from the lowercase a (1) because each different character has its own unique 7-bit code/pattern (1)
• A and a are different characters (1) so each needs a different 7-bit pattern to be stored and displayed correctly (1)
• If A and a had the same 7-bit pattern (1) the computer could not tell which character to display (1)

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

Do not accept 'because they are different letters/cases' without saying they need different/unique codes or that the computer must tell them apart.

Do not accept 'because A is a capital letter' on its own.

Do not accept 'because the codes are different numbers' or '65 is not 97' without linking to the need to tell the characters apart or to unique codes.

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.