S4 ICT | Data Representation and File Size Lesson Prep Guide (for tutors)

S4 ICT | Data Representation and File Size Lesson Prep Guide (for tutors) | Sampling, resolution and compression

2026-11-09

Tutor lesson-prep guides · Secondary ICT (DSE)

This chapter has only three formulas, yet the marks are lost by reading kHz as Hz and 24-bit as 24 Byte.

Objectives — Students can: (1) state the bit, Byte, KB and MB relationships and convert correctly; (2) compute file size with width times height times colour depth divided by eight, and sampling rate times bit depth divided by eight times channels times seconds; (3) explain Nyquist sampling and the difference between lossy and lossless compression, scoring at least 9 of 12 in the diagnostic.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit relations	How many bits in a Byte? How many KiB in a MiB?	8; 1024
Multiplication	What is 1920 times 1080?	2073600
Percentages	What percentage of 1000 is 90?	9 percent

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	The unit ladder	bit, Byte, KiB, MiB, GiB, marking times eight or times 1024 at each step	Write the ladder and convert two values
6-16 min	Image size	1920 by 1080 at 24-bit is 6,220,800 Byte, about 5.93 MiB	Repeat for 1280 by 720
16-30 min	Audio size	44.1 kHz, 16-bit, stereo, 180 seconds gives 31,752,000 Byte, about 30.28 MiB	Repeat in mono
30-42 min	Sampling and compression	Nyquist needs twice the top frequency (20 kHz gives 40 kHz); 128 kbps MP3 for 180 seconds is only 2.75 MiB	Explain lossy versus lossless
42-56 min	12-question diagnostic	Four question types, marked on the spot: 9 right, 3 wrong	Independent work
56-60 min	Exit check	Three oral questions; any miss points to the stage to reteach	Oral answers

03 Teaching script (read it out)

Scene 1: Units are the foundation

One Byte is eight bits, and every step up multiplies by 1024 (KiB, MiB, GiB). Many students mix MB and MiB; exam questions usually mean 1024, but follow the paper when it states 1000.
Teaching move: require every answer to begin with one written line stating whether the given value is in bit or Byte, which protects the whole calculation.

Students do: express 6,220,800 Byte in both MiB and MB and explain the difference.

Scene 2: Two formulas, one skeleton

Images: width times height times colour depth divided by eight gives 6,220,800 Byte, which is 5.93 MiB after two divisions by 1024. Audio: sampling rate times bit depth divided by eight, times channels, times seconds gives 31,752,000 Byte, or 30.28 MiB.
Both are the same skeleton: data per unit times the number of units, divided by eight to turn bit into Byte. Name the skeleton and nothing needs memorising separately.

Students explain: compute a 1280 by 720 image at 24-bit (answer 2,764,800 Byte, about 2.64 MiB).

Scene 3: Compression: three trade-offs

Lossy compression discards detail the eye or ear barely notices. The 180-second stereo original is 30.28 MiB while a 128 kbps MP3 is 2.75 MiB, about 9 percent of the original, saving 91 percent. Lossless formats such as FLAC and PNG restore exactly but typically only halve the size. Sampling theorem: to keep 20 kHz you need at least 40 kHz, and 8-bit colour gives 256 levels.

Students do: compress the same clip at 64 kbps and 256 kbps, and say which file is larger and why.

04 Question types and worked steps

Type	Example	Steps	Answer
Image size	1920 by 1080 at 24-bit	Width times height times depth divided by 8	6,220,800 Byte (5.93 MiB)
Audio size	44.1 kHz, 16-bit, stereo, 180 seconds	Rate times depth divided by 8, times channels and seconds	31,752,000 Byte (30.28 MiB)
Compression ratio	Original 30.28 MiB, MP3 2.75 MiB	Compressed over original times 100	9 percent (saves 91 percent)
Sampling rate	Top frequency 20 kHz	Nyquist: multiply by 2	40 kHz

Sanity check (teach this): Sanity checks: compressed size must be smaller than the original; sampling rate must exceed twice the top frequency; doubling colour depth doubles the file; doubling each side quadruples the area; 9 of 12 is 75 percent, not 9 percent.

05 Independent practice (3 questions)

Question	Answer	What to watch
Two image items	800 by 600 at 8-bit; 1920 by 1080 at 24-bit	Did they divide by eight
Two audio items	44.1 kHz mono for 60 seconds; 22.05 kHz stereo for 120 seconds	Did they multiply channels and seconds
Three mixed items	Compression ratio, Nyquist, how many photos fit in 4 GB	Are units consistent

06 Common mistakes and what to say

Mistake	Student writes	Say this
kHz read as Hz	Substituting 44.1 for 44.1 kHz	44.1 kHz is 44100 Hz, and the formula needs Hz.
bit read as Byte	Treating 24-bit depth as 24 Byte per pixel	The 24 refers to bits; after dividing by eight a pixel is three Byte.
Channels ignored	Computing stereo as mono	Stereo carries two channels, so double the data before dividing by eight.
Lossy called lossless	Claiming MP3 restores perfectly	MP3 is lossy and cannot restore discarded detail; use FLAC or WAV for exact restoration.

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — All chapters in one place
Master prep guide index — Compare related chapters
Redo this chapter — Redo diagnostic questions 1 to 6, writing units first

08 Exit check (last 5 minutes)

Question	Answer	Decision
Bytes for 1920 by 1080 at 24-bit	6,220,800	Missing the divide by eight means reteach stage two
Which Hz value for 44.1 kHz	44100	Answering 44.1 means reteach stage one
Is MP3 lossy or lossless	Lossy, cannot restore exactly	Answering lossless means reteach stage three

09 Tutor FAQ

Should we use 1000 or 1024?

Follow the paper when it states one; otherwise storage conventionally uses 1024 and network bandwidth uses 1000, and one sentence stating the assumption is enough.

How much encoding must be memorised?

ASCII in 7 bits and commonly 8, plus variable-length Unicode (1 Byte for English, usually 3 Byte for Chinese) is sufficient.

Why study Nyquist?

It explains why CDs use 44.1 kHz: hearing tops out near 20 kHz, and sampling must exceed twice that to avoid distortion.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.