Representing images and sound – GCSE Computer Science exam questions with mark scheme

1Grade 6–76 marks

A logo is stored as a bitmap image that is 640 pixels wide and 480 pixels high. It has a colour depth of 8 bits.

(a) Calculate the file size of the image in kilobytes. Ignore metadata. Show your working. [3]

(b) The designer increases the colour depth to 24 bits. Describe the effect of this on the quality of the image and on its file size. [2]

(c) The logo will be compressed before it is put on a website. State the type of compression that should be used, giving a reason. [1]

(a) 640 × 480 × 8 (= 2 457 600 bits); allow 640 × 480 = 307 200 pixels × 1 byte (8 bits = 1 byte) for this and the next mark(1)
(a) ÷ 8 (= 307 200 bytes)(1)
(a) ÷ 1000 = 307.2 KB; allow ÷ 1024 = 300 KB; correct answer alone scores all 3 marks(1)
(b) More colours can be represented / colours more accurate / better image quality(1)
(b) The file size increases; allow it triples / becomes 921.6 KB (900 KB using 1024)(1)
(c) Lossless AND a reason: no data is lost / the logo stays identical to the original / sharp edges and exact colours are kept; allow lossy only with a reason such as a smaller file that loads faster where a small loss of quality is acceptable; type with no reason scores 0(1)
Answer: (a) 307.2 KB (or 300 KB using 1024) (b) More colours / higher quality; file size increases (×3) (c) Lossless – no data lost so the logo is unchanged
Examiner tip: Remember to divide by 8 to turn bits into bytes before dividing by 1000 for kilobytes – missing this is the most common slip.

2Grade 6–76 marks

A 30-second mono sound clip is recorded with a sample rate of 44 100 Hz and a bit depth of 16 bits.

(a) Calculate the file size of the sound clip in megabytes. Show your working. [3]

(b) Explain how increasing the sample rate would affect the sound file. [2]

(a) 44100 × 16 × 30 (= 21 168 000 bits); allow 44 100 × 30 × 2 bytes (16 bits = 2 bytes) (= 2 646 000 bytes) for this and the next mark(1)
(a) ÷ 8 (= 2 646 000 bytes)(1)
(a) ÷ 1 000 000 = 2.646 MB; allow 2.65 MB / 2.6 MB; allow 2.52 MB / 2.5 MB (using 1024 × 1024) or 2.58 MB (using 1024 and 1000); correct answer alone scores all 3 marks(1)
(b) More samples are taken per second, so the recording is closer to the original sound / better quality(1)
(b) The file size increases (because more samples must be stored)(1)
(c) Lossy reduces the file size more (than lossless) so the message sends faster / uses less data / less storage; the file-size idea alone scores; allow 'some quality is lost but this does not matter / is not noticeable for a voice message' only if linked to the smaller file; do not accept 'quality is lost' on its own(1)
Answer: (a) 2.646 MB (b) More samples per second → better quality but larger file (c) Much smaller file; small quality loss acceptable
Examiner tip: Sample rate is samples per SECOND – multiply by the length in seconds, or the answer will be 30 times too small.

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