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Short Answer Questions (1-3 Marks)

Review model answers and mark schemes for Cambridge IGCSE CS 0478.

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Key Takeaways

Essential concepts, definitions, and common exam pitfalls for Chapter 1.2: Text, Sound & Images.

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STATE / IDENTIFY

Brief, factual answer — no explanation needed

1State the number of characters standard ASCII represents
  • 128  (7 bits, 2⁷)
2State the minimum number of bits Unicode uses per character
  • 16 bits  (65 536+ characters)
3Identify the smallest identifiable component of a bitmap image
  • A pixel  (picture element)
4State the formula for the size of a bitmap image in bits
  • Width × Height × Colour Depth
5State the number of channels used when recording in stereo
  • Two channels

DESCRIBE

Say what happens or how it works (what / how)

1Describe what is stored in a bitmap file header
  • A signature at the start identifying the file type
  • The image resolution/dimensions and colour depth
  • The type of compression used
2Describe how sound is represented in a computer
  • The amplitude is measured a set number of times a second (sampling)
  • Each sample is given a corresponding binary number
  • The binary numbers are saved in sequence
3Describe how a vector graphic is encoded
  • Scalable shapes/lines defined by coordinates and geometry
  • Each drawing object has properties (attributes)
  • A drawing list stores commands, properties and relative positions
4Describe the difference between sampling rate and sampling resolution
  • Sampling rate = the number of samples taken per second
  • Sampling resolution = the number of bits used to store each sample
  • Rate is how often; resolution is how precisely
5Describe what happens when a bitmap is enlarged
  • The number of pixels does not change
  • Each pixel is stretched into a larger block
  • The image looks blocky and pixelated

EXPLAIN

Give reasons — say why something happens

1Explain why Unicode was created
  • ASCII cannot represent characters from languages other than English
  • ASCII lacks modern symbols such as emojis
  • Unicode uses at least 16 bits to cover all major world scripts
2Explain why a vector graphic scales with no loss of quality
  • Only the relative positions and properties of objects are stored
  • When the scale changes the file is read again and calculations are made
  • The objects are redrawn to the new scale, avoiding distortion
3Explain why increasing the colour depth increases the file size
  • Each pixel needs more bits to store its colour
  • More colour values can be represented (8 bits = 256 colours)
  • File size = width × height × colour depth, so it grows in proportion
4Explain why a band-limiting filter is used before digitising sound
  • It removes very high frequency components
  • The human ear cannot detect these frequencies
  • They would cause problems for the coding if not removed
5Explain the effect of increasing the sampling resolution
  • More values are available to store each sample
  • The accuracy of the digitised sound improves and distortion decreases
  • The quantisation error becomes smaller, but the file size increases

EVALUATE / DISCUSS

Weigh up pros and cons with a justified conclusion

1Discuss whether a logo should be stored as a bitmap or a vector
  • A vector is best — logos are resized frequently and vectors redraw with no quality loss
  • Vector files are smaller and easy to edit by changing shapes
  • A bitmap would become pixelated on a large poster
2Discuss the trade-offs of increasing the sampling rate of a recording
  • More samples per second improve accuracy/precision of the digitised sound
  • The file size increases proportionally
  • Nyquist requires at least twice the highest frequency — beyond that, extra rate adds size more than quality
3Discuss why photographs are stored as bitmaps while logos are stored as vectors
  • Photos need per-pixel colour for realism — vectors are limited to simple shapes
  • Logos need scaling and editing — vectors redraw perfectly at any size
  • Digital cameras automatically produce bitmaps; design tools produce vectors
4Evaluate the effect of increasing an image's resolution
  • Higher resolution gives more detail and sharpness
  • The file size grows (width × height × depth) — slower to download/transfer
  • Fewer images fit on storage; beyond the screen's resolution the extra pixels are wasted
5Discuss the impact of storing text in Unicode instead of ASCII
  • Unicode covers all languages and emojis — essential for global apps
  • Each character needs at least 16 bits vs 7, so files are larger
  • For multilingual communication the compatibility gain outweighs the storage cost