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Computer Science IGCSEPDF 7
142 pagesrevision 46930
PDF 7
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ASCII
7-bit standard (128 characters) includes letters, digits, punctuation, control codes. Extended
ASCII uses 8 bits (256 chars).
Unicode
Supports characters from all global languages. UTF-8, UTF-16, and UTF-32 are common forms.
File Size Calculation
File size (bits) = number of characters × bits per character.
Example: 2000 × 8 = 16000 bits = 2000 bytes = 1.95 KB.
Sound Representation
Sound is analogue and must be digitised by sampling.
1.4 Images, Sound & Data Storage
Images are grids of pixels. Each pixel stores its colour in binary according to colour depth (bits
per pixel). Resolution = width × height.
-Sample rate: samples per second (Hz)
-Sample resolution / Bit depth: bits per sample
-Amplitude: height of wave, determines loudness
-Channels: mono (1), stereo (2)
File Size = Sample Rate × Bit Depth × Channels × Duration
Example: 44.1kHz, 16-bit, stereo, 10s = 14,112,000 bits ≈ 1.68 MB
File Size = (Resolution × Colour Depth) ÷ 8
Example: 800×600×24 = 11,520,000 bits = 1.37 MB
-UTF-8 uses 1–4 bytes per character.
-UTF-16 uses mostly 2 bytes.
-UTF-32 uses 4 bytes.
ASCII is a subset of Unicode. Both assign numeric codes to characters, but Unicode is global.