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S6 ICT: Multimedia and Compression – Practice Paper 1 (Foundation, 10 Questions)
S6 ICT: Multimedia and Compression – Practice Paper 1 (Foundation, 10 Questions)
Paper one starts from the two basic formulas: image size is pixel count times colour depth over eight, and sound size is sample rate times bit depth times channels times time.
Quick answer An 800 by 600 image at 24 bits takes 1 440 000 bytes, or 1.44 MB; 8-bit colour gives 256 colours and 16-bit gives 65 536; 44.1 kHz stereo for ten seconds takes 1 764 000 bytes; a ratio of eight to one saves 87.5 per cent.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | about 20 minutes |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 2 · Error redo 2 |
| Chapter | Chapter: S6 ICT | Multimedia and Compression: File Size, Sampling and Bitrate |
| Level | Foundation |
02 Questions (10 items)
- FoundationAn uncompressed 800 by 600 bitmap at 24-bit colour depth: how many bytes, and roughly how many MB at 1 MB = 1 000 000 bytes?
- FoundationHow many colours can 8-bit colour depth represent, and how many can 16-bit represent?
- FoundationWhat is the uncompressed size in bytes of a 1 024 by 768 image at 24 bits per pixel?
- IntermediateA ten-second stereo clip at 44.1 kHz and 16 bits per sample, uncompressed: how many bytes?
- IntermediateA thirty-second mono recording at 8 kHz and 8 bits, uncompressed: how many bytes, and about how many MB?
- IntermediateA ninety-second video at twenty-five frames per second: how many frames, and what total size in MB if each uncompressed frame is 60 000 bytes?
- ChallengeStoring the same image at 8 bits per pixel: what fraction of the 24-bit size is it, and what is lost?
- ChallengeAn image of 1 440 000 bytes compresses to 180 000 bytes: what is the ratio and the percentage saved?
- Error redoError retry: a student worked out a 640 by 480 image at 24 bits as 7 372 800 bytes. Which step is wrong, and what is correct?
- Error redoError retry: a student treated a 44.1 kHz, 16-bit, sixty-second stereo clip as mono and got 5 292 000 bytes. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 1 440 000 bytes, about 1.44 MB | The 480 000 pixels times 24 bits give 11 520 000 bits, and dividing by eight gives 1 440 000 bytes. |
| 2 | 256 colours at 8 bits and 65 536 at 16 bits | The number of colours is two raised to the bit depth, so 2 to the 8 is 256 and 2 to the 16 is 65 536. |
| 3 | 2 359 296 bytes | 1 024 times 768 gives 786 432 pixels, and three bytes per pixel multiply to 2 359 296 bytes. |
| 4 | 1 764 000 bytes | Each second needs 44 100 samples times two bytes times two channels, that is 176 400 bytes, so ten seconds give 1 764 000. |
| 5 | 240 000 bytes, about 0.24 MB | 8 000 samples a second times one byte times thirty seconds give 240 000 bytes, which is 0.24 MB. |
| 6 | 2 250 frames, 135 MB in total | Twenty-five frames times ninety seconds give 2 250 frames, and 60 000 bytes each make 135 000 000 bytes, or 135 MB. |
| 7 | One third of the size, at the cost of colour depth | Dropping from 24 bits to 8 bits cuts each pixel from three bytes to one, so the file is one third as large but only 256 colours remain. |
| 8 | A ratio of eight to one, saving 87.5 per cent | 1 440 000 over 180 000 is eight, so the file is one eighth; the 1 260 000 bytes saved are 87.5 per cent of the original. |
| 9 | An extra division by eight; the correct size is 921 600 bytes | 24 bits already means three bytes per pixel, so multiply the pixel count once; dividing by eight again inflated the answer eight times. |
| 10 | 10 584 000 bytes | 44 100 times two bytes times sixty seconds covers one channel only, so stereo doubles it to 10 584 000 bytes. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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