Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• The denary code 77 is converted to binary (1) to give the 7-bit pattern 1001101 (1)
• The character M is represented by a unique ASCII code/number (1) that is stored as a 7-bit binary pattern (1)
• 7 bits are used to store the code for M (1) so 128 different characters can be represented (1)
• The binary pattern 1001101 is stored in memory (1) using one 7-bit code for each character (1)

Do not accept answers that only state the ASCII code 77, as this is given in the question.

Do not accept 01001101, as this is an 8-bit pattern.

Do not accept ASCII described as a programming language or a method of encryption.

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.

Indicative content

Range and number of values

• With a bit depth of 3, the keyboard can store only 2^3 = 8 different amplitude values.
• The 3-bit codes run from 000 to 111, so the loudest and quietest amplitudes the keyboard can record are fixed by those codes.
• Because only eight levels of loudness are available, the keyboard cannot store the full variation in amplitude of the original recording.
• Any sound in between two of the eight values cannot be stored at all, so much of the detail of the original sound has no code to represent it.

Accuracy of the stored sample

• Each true amplitude must be rounded to the nearest of the eight available values, so the stored sample is not the same as the amplitude that was measured.
• The difference between the true amplitude and the rounded value is the quantisation error, which makes every stored sample less accurate.
• The eight values are far apart, so there is a large step between one stored amplitude and the next and the stored sound rises and falls in big jumps rather than following the original wave.
• A quiet sound whose amplitude is smaller than one step may be stored with the same code as silence, so that part of the recording is lost.

Effect on the user

• When the keyboard plays back the recording, the large steps between stored amplitudes are heard as distortion or noise rather than a smooth copy of the original sound.
• Quiet parts of the recording may not be heard at all, because they are stored as the same value as silence.
• The reproduced sound may not be recognisable as the instrument or voice that was recorded.

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 explain that 3 bits gives only 8 amplitude values and link the large steps between those values to the distortion or the loss of quiet sounds that the user hears; an explicit binary range of 000 to 111 is not required for Level 3.

Level 2 requires one point to be developed with a reason, e.g. eight values means each amplitude is rounded to the nearest and the quantisation error is large enough to be heard; stating 8 values, reduced accuracy and distortion as separate basic points without a reason is Level 1. Do not accept bit depth described as the number of samples taken per second, or a bare statement that the quality is worse with no explanation.