Edexcel GCSE Computer Science (1CP2) Exam Questions

• C Pixel (1)
• The only correct answer is C. The correct answer is pixel.
• A is not correct because colour depth is the number of bits used to store the colour of each pixel
• B is not correct because file size is the amount of storage an image takes up, not an area of the image
• D is not correct because resolution is a measure of the number of pixels in an image (such as its width × height), not the smallest identifiable area

• C Image A has more pixels per inch (1)
• The only correct answer is C. The correct answer is more pixels per inch.
• A is not correct because bits per pixel is the colour depth
• B is not correct because more colours is caused by a higher colour depth, not a higher resolution
• D is not correct because a higher resolution usually needs more storage, not less

Award one mark for:

• 64 (1)
• 2^6 (1)
• 2 × 2 × 2 × 2 × 2 × 2 (1)

Allow 64 characters / sixty-four.

Allow 2 to the power of 6 for 2^6.

Do not accept 128, which is the number of characters that can be represented using 7 bits.

Do not accept 63, the largest value that can be represented using 6 bits, rather than the number of different characters.

Do not accept 12 (2 × 6), ie the number of bits multiplied by 2.

Award one mark for any of the following up to a maximum of two marks:

• The smallest element/area of a bitmap image (1)
• The smallest identifiable part of an image (1)
• Stores/represents a single colour as a binary value (1)
• Stores/represents a single colour using a number of bits/colour depth (1)

Allow 'picture element' for element/part in the first two bullets, but not on its own

Allow 'bit depth' for colour depth

Do not accept 'picture element'/'pixel' on its own

Do not accept 'a group of pixels' or 'a dot' on its own; allow 'dot' only when it is described as the smallest element/area of the image

Do not accept 'single colour' on its own for the second mark; it must be linked to a binary value/number of bits

Do not accept 'lots of coloured dots' (it contradicts a single colour)

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

• 6 bits give only 64 different codes (1) which is fewer than the number of letters, digits and symbols that must be represented (1)
• 7 bits give 128 different codes (1) which is enough to represent every letter, digit and symbol that is needed (1)
• Adding a bit doubles the number of different codes from 64 to 128 (1) so that every character needed can have its own code (1)
• There are more than 64 characters that must be represented (letters, digits and symbols) (1) so 6 bits cannot give each one a different code (1)

A statement of the number of codes without a development can earn one mark only; the development must be linked to the statement for the second mark.

Do not accept responses that only say computers use binary (1s and 0s).

Do not accept that 7 bits are needed because 7 is a bigger number than 6, with no reference to the number of codes or characters.

Do not accept responses about 8 bits, extended ASCII or Unicode.

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

• The codes run in alphabetical order/consecutively (1) so each letter has a code one more than the letter before it (1)
• A has the code 65 and Z has the code 90 (1) so the codes for the letters in between increase by 1 each time (1)
• The letters are stored in sequence (1) so B is A + 1, C is B + 1 and so on (1)

Accept the codes given in denary (65, 90) or as 7-bit binary (1000001, 1011010).

Do not accept 'the codes go up' or 'the codes are in order' on its own, as neither gives the step of 1 or links one letter to the next.

Do not accept 'each uppercase letter has a different code' on its own, as this does not describe the arrangement.

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

• A higher sample rate means more samples are taken each second (1) so more binary values are stored to represent the sound (1)
• A higher sample rate records the wave more frequently (1) so the binary representation follows the shape of the analogue wave more closely/is more accurate (1)
• A lower sample rate records fewer samples (1) so the binary representation is less detailed/less accurate (1)

A statement about the effect of sample rate on sampling (e.g. more/fewer samples) without a link to the binary representation scores a maximum of one mark.

A definition of sample rate on its own, with no effect on sampling, scores no marks.

Accept 'more accurate' for the second mark only when linked to more samples or binary values being stored, for example by 'so', 'which means' or 'because'.

Do not accept responses that describe sample interval or bit depth instead of sample rate.

Do not accept 'a higher sample rate means more bits per sample' or 'higher resolution', as these describe bit depth.

Do not accept a larger file size unless it is linked to more samples/binary values being stored.

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

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

• More binary data is stored for each pixel (1) because a higher colour depth uses more bits per pixel (1)
• Each pixel has a longer binary code (1) so more storage is needed to hold the image (1)
• More colours can be represented (1) so more bits are needed for each pixel (1)
• The file contains more bits in total (1) because colour depth is the number of bits per pixel (1)

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

Allow 'more bits per pixel' for one mark.

Do not accept 'more pixels' or 'higher resolution' as the reason, because changing colour depth does not change the number of pixels.

Indicative content

Number of colours

• 4 bits give 2^4 = 16 different binary values, so at most 16 colours can be stored for each pixel.
• The console cannot store every shade that the designers want; colours outside the 16 must be approximated or left out.
• Adding more bits per pixel would allow more colours, but this may not be possible on the console's hardware.

Effect on appearance

• The sprite may show banding where a smooth change from green to blue should be.
• A sprite that needs many shades, such as a detailed landscape, will look unrealistic or have blocks of colour.
• Simple sprites with flat colours, such as a retro character, can still look acceptable.
• The limitation is in the number of colours, not the number of pixels; resolution is a separate issue.

Managing the limitation

• Designers can choose a palette of 16 colours that includes the most important greens and blues for the game.
• They can design sprites with flat colours rather than smooth gradients.
• They can use more bits per pixel for important sprites if the console allows it.
• They can accept that some shades will be approximated and check that the sprite still looks clear on the console's screen.

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 cover the number of colours and the effect on appearance.

Do not accept 4 bits give 4 colours or 8 colours.

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.