Edexcel GCSE Computer Science (1CP2) Exam Questions

• C 128 (1)
• The only correct answer is C. The correct answer is 128.
• A is not correct because 6 bits give only 64 different combinations
• B is not correct because 127 is the largest 7-bit value, not the number of characters that 7 bits can represent
• D is not correct because 256 characters need 8 bits, as used by 8-bit extended ASCII

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

• The computer looks up the character in the ASCII character set/table (1) to find the code that represents it (1)
• Each character has its own unique ASCII code (1) so the letter A is converted to and stored as a 7-bit binary pattern (1)
• ASCII uses 7 bits for each character (1) so A is stored as its ASCII code 1000001 (binary) / 65 (denary) (1)

The second mark requires a linked expansion, so a point on its own (e.g. 'A has the code 65') can only earn one mark.

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

Accept an 8-bit pattern with a leading 0 (e.g. 01000001) as the binary code for A.

Do not accept 'the character is stored as text' without saying it is changed into a code/binary.

Do not accept a binary value on its own (e.g. 1000001) without saying that it is the ASCII code/that the character is converted to it.

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

• Each pixel is stored as a binary value/code (1) which represents/maps to a colour (1)
• The binary value is used to look up a colour in a palette (1) so the pixel can be displayed in that colour (1)
• A bit pattern is assigned to each colour (1) and each pixel stores the bit pattern for its colour (1)
• The number of bits per pixel sets how many different colour codes exist (1) so each code can represent a different colour (1)

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

Do not accept 'pixels are binary' or 'the image is made of 1s and 0s' without saying a pixel's colour is represented by a binary value.

Do not accept RGB values stored as denary or hexadecimal.

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

• Only 4 colours can be represented (1) because 2 bits give 2^2 = 4 possible values (1)
• Some of the 12 colours cannot be stored (1) so they are lost/replaced with a colour that can be represented (1)
• The image will not match the original exactly (1) because there are fewer colour codes than the number of colours needed (1)
• Many colours will share the same binary value (1) so the image will lose colour detail (1)

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

Allow 'four colours' or '2^2'.

Do not accept 'only two colours' (confusing the number of bits with the number of values) or 'resolution is too low'.

Indicative content

Sample rate and sample interval

• The sample rate is the number of samples taken each second, measured in hertz (Hz).
• The sample interval is the time between one sample and the next.
• The sample interval is the reciprocal of the sample rate, so doubling the sample rate halves the sample interval.
• A higher sample rate therefore gives a shorter sample interval, so the concert is measured more often.
• A lower sample rate gives a longer sample interval, so fewer points on the sound wave are recorded.
• The school could sample at 44,100 Hz, taking 44,100 measurements of the concert every second.

Accuracy of the recording

• A higher sample rate, and so a shorter sample interval, records more points on the sound wave, so its shape is followed more closely and the recording is more like the live performance.
• A higher sample rate can capture high-frequency sounds in the music, such as a violin, more accurately.
• With a low sample rate the gap between samples is long, so fast changes such as a drum beat can happen between samples and be missed, making the recording less accurate.
• If the sample rate is too low, a high-frequency sound changes several times between samples, so the digital file does not represent that sound properly.

Effect on file size

• Each sample is stored using a number of bits, so more samples each second means more data to store for each second of the concert.
• The file size is the sample rate multiplied by the bit depth multiplied by the duration, so doubling the sample rate doubles the size of the file for the same concert.
• A lower sample rate means fewer samples are stored, so the file is smaller and quicker to download from the school website.
• A higher sample rate gives a more accurate recording but a larger file, so the school must trade off quality against file size.

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)

Additional guidance

• Level 3 responses must cover both sample rate and sample interval, including how they are related, and link them to the accuracy of the recording and to the file size. For Level 2, the response must link a change in sample rate/sample interval to the accuracy of the recording and link a change to the size of the file, with some application to the concert; definitions alone, or points that are not linked, are Level 1.
• Accept reasoning that uses sample rate only, or sample interval only, if the relationship between them is stated or implied. Do not accept sample rate described as the number of bits per sample (that is bit depth), and do not accept the claim that a higher sample rate makes the file smaller.