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:

• Version B has more pixels in the same area (1) so each pixel represents a smaller part of the image (1)
• Version B has a higher pixel density (1) so finer detail can be represented (1)
• The pixels in Version B are smaller (1) so more detail can be captured in the same space (1)
• Version B has a higher resolution (1) because there are more pixels per unit of area (1)

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

Do not accept 'more pixels' on its own without saying same area/pixel density/smaller pixels.

Do not accept higher colour depth or more colours.

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

• Each sample is stored using more bits (1) so the amplitude can be recorded more precisely (1)
• More different amplitude values can be represented (1) because each extra bit doubles the number of possible values (1)
• The height of the wave at each sample point is measured more accurately (1) so the digital wave is closer to the analogue wave (1)
• The gap between the true amplitude and the stored value is smaller (1) because there are more values for the sample to be rounded to (1)

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

Do not accept 'the sound is sampled more often' or 'more samples are taken per second', as these describe the sample rate, not the bit depth.

Do not accept 'the file size increases' on its own as an effect on the representation of the sound.

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.