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S6 ICT: Multimedia and Compression – Practice Paper 2 (Intermediate, 10 Questions)
S6 ICT: Multimedia and Compression – Practice Paper 2 (Intermediate, 10 Questions)
Paper two applies the formulas to real cases: full HD images, a three-minute track as MP3, lower resolution, higher sample rate, and working back from storage capacity.
Quick answer A full HD 24-bit image takes 6 220 800 bytes; three minutes of stereo takes 31 752 000 bytes but only 2.88 MB as a 128 kbps MP3, about eleven times smaller; 1 280 by 720 keeps 44.4 per cent of the pixels.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | about 25 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 | Intermediate |
02 Questions (10 items)
- FoundationA 1 920 by 1 080 full HD image at 24 bits: how many bytes uncompressed, and about how many MB?
- Foundation16-bit colour depth is usually split across the three channels: how many bits each, and how many colours in total?
- FoundationThree minutes of 44.1 kHz, 16-bit stereo audio uncompressed: how many bytes?
- IntermediateThe same three-minute track as a 128 kbps MP3: roughly how many MB, and how many times smaller than the raw audio?
- IntermediateChanging a 1 920 by 1 080 frame to 1 280 by 720: what fraction of the pixels remain, and roughly what percentage of the file?
- IntermediateRaising the sample rate of a recording from 22.05 kHz to 44.1 kHz with everything else fixed: how does the file size change and why?
- ChallengeRaw video at 25 fps, 1 920 by 1 080, 24 bits: how many MB per second, and roughly what bitrate in Gbps?
- ChallengeA 4 GB disc, taken as 4 000 MB, holds 128 kbps audio: how many seconds, and about how many hours?
- Error redoError retry: a student divided 31 752 000 bytes by 2 880 000 bytes and claimed a factor of 288. What is the real factor?
- Error redoError retry: a student read 24 bits as 24 bytes per pixel and got 49 766 400 bytes for a 1 920 by 1 080 image. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 6 220 800 bytes, about 6.22 MB | 1 920 times 1 080 gives 2 073 600 pixels, and three bytes each make 6 220 800 bytes. |
| 2 | Five or six bits per channel, 65 536 colours in total | The sixteen bits are shared by red, green and blue, and the total is still two to the sixteen, that is 65 536 colours. |
| 3 | 31 752 000 bytes | Each second is 44 100 times two times two, that is 176 400 bytes, and 180 seconds give 31 752 000 bytes. |
| 4 | About 2.88 MB, roughly eleven times smaller | 128 000 bits times 180 seconds give 23 040 000 bits, or 2 880 000 bytes, and 31 752 000 over 2 880 000 is about eleven. |
| 5 | The pixel count falls by a factor of 2.25, so about 44.4 per cent remains | 1 920 by 1 080 is 2.25 times 1 280 by 720, so the smaller frame keeps only 44.4 per cent of the pixels. |
| 6 | It doubles, because twice as many samples are taken each second | File size is proportional to sample rate, so doubling the rate doubles the samples per second and the byte count. |
| 7 | 155.52 MB per second, about 1.24 Gbps | 6 220 800 bytes per frame times twenty-five gives 155 520 000 bytes a second, and eight bits a byte make about 1.24 Gbps. |
| 8 | 250 000 seconds, about 69.4 hours | 4 000 MB is 32 000 Mb, and dividing by 0.128 Mbps gives 250 000 seconds, or about 69.4 hours. |
| 9 | About eleven times | 31 752 000 over 2 880 000 is about 11.03; the answer was out by one order of magnitude because MB and bytes were mixed. |
| 10 | 6 220 800 bytes | 24 bits is three bytes, not twenty-four, and that eight times slip explains the wrong answer. |
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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