Edexcel GCSE Computer Science (1CP2) Exam Questions

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.

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

• 7 bits give 128 different codes/combinations (1) which is enough to represent all the characters on a keyboard (1)
• 7 bits allow 128 unique characters (1) so uppercase and lowercase letters, digits and punctuation can all be represented (1)
• 6 bits give only 64 combinations (1) which is not enough for the number of characters on a keyboard (1)
• A keyboard needs to represent more than 64 different characters (1) so 7 bits are needed to give the 128 combinations required (1)
• A fixed number of 7 bits gives 128 possible codes (1) which is enough for the characters ASCII represents, so more bits are not needed (1)

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

Do not accept 'because ASCII is 7-bit' or 'because 7 bits is enough' as these repeat the question without giving the number of combinations or saying what they allow.

Do not accept '7 bits give 256 characters'; 256 combinations come from 8 bits (extended ASCII).

Allow 'ASCII has 128 characters/codes' for the 128 point.

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.