Text uses a character set: ASCII vs Unicode
A computer cannot store letters directly: each character is converted to binary using a character set, a table giving each character a unique binary code. ASCII uses 7 bits per character (extended ASCII 8 bits), giving (or ) characters — enough for English text. Unicode uses more bits per character, so it represents far more characters, including many languages, symbols and emojis; the trade-off is a larger file than ASCII.
Sound: sample rate vs sample resolution
Sound is an analogue wave. To store it digitally the computer samples the wave — measuring its amplitude at regular intervals. Sample rate is the number of samples taken per second (hertz, Hz). Sample resolution (bit depth) is the number of bits used to store each sample. Increasing either improves quality and file size; different quantities — one per second, the other bits per sample. Sound file size (bits) = sample rate × sample resolution × time; divide by 8 for bytes.
Images: resolution and colour depth
A bitmap image is made of pixels; each pixel's colour is stored as a binary number. Resolution is the number of pixels, given as width × height. Colour depth is the number of bits used per pixel: with bits, colours are possible — 8 bits gives colours, 24 bits gives about 16.7 million ('true colour'). Increasing either improves quality and file size. Image file size (bits) = width × height × colour depth; divide by 8 for bytes.
Drawn from real examiner reports.
Swapping sample rate and sample resolution
A common error is swapping the two sound quantities. Sample rate is the number of samples taken per second (Hz); sample resolution is the number of bits used to store each sample. Writing 'sample resolution is the number of samples per second' actually defines the sample rate. Fix it by the unit: rate goes with Hz (per second), resolution with bits (per sample).
November 2023 Paper 2 (sound question): the most common incorrect answers described the sample rate instead of the sample resolution.
Sample resolution is not image resolution
Some candidates describe sample resolution as 'the number of pixels', borrowing the idea from images. Sample resolution has nothing to do with pixels: it is the number of bits used to store each sound sample. Image resolution is the number of pixels in an image. They share the word 'resolution' but measure completely different things.
June 2023 Paper 2 Q2(a): some candidates tried to describe the sample resolution in relation to an image.
Vague ASCII vs Unicode comparison
Asked to compare ASCII and Unicode, most candidates state only that 'Unicode has more characters' and stop. For full marks, link the reasoning: Unicode uses more bits per character, so it represents more characters — covering different languages, symbols and emojis — so a Unicode file is larger than the same text in ASCII.
November 2023 Paper 2 (character-set question): most candidates referred only to Unicode having more characters; few expanded to cover more languages.
Not naming the character set
Asked how text is converted to binary, 'it just turns into 1s and 0s' earns nothing. The mark-scheme answer: each character is matched to its unique code in a character set (e.g. ASCII), and that binary code is what is stored. You must name the character set as the lookup table, not just say the text becomes binary.
June 2023 Paper 2: candidates would benefit from a greater understanding of how text is converted to binary using a character set.
Image resolution is width times height
Image resolution is width height, not a single number. Treating it as one value means you never multiply the two dimensions, so the pixel count — and every file size built on it — is wrong. Always multiply width by height to get the total number of pixels before multiplying by the colour depth.
Colour depth is bits, not the colour count
Colour depth is the number of bits used per pixel, not the number of colours. In a file-size calculation, using 256 (the number of colours an 8-bit depth gives) instead of 8 (the depth in bits) makes the answer far too large. Read the depth as a bit count: 8 bits, not 256; 24 bits, not 16.7 million.
Get the file size in bits first
For a file-size question, find the size in bits first: image = width × height × colour depth; sound = sample rate × sample resolution × time. Then convert: ÷8 for bytes, ÷1024 for KiB, ÷1024 again for MiB. Stop at the unit the question asks for.
Match the formula to the media type
Use the image formula for images (width height colour depth) and the sound formula for sound (sample rate sample resolution time). Mixing them up is a common slip — check the media type before you substitute any numbers.
Show the multiplication for method marks
Write the formula in words, then substitute the numbers as a single multiplication before working out the answer. Cambridge awards method marks for the correct setup even if the final arithmetic slips, so never jump straight to a bare number.
Character codes run in sequence
Character-set codes are assigned in sequence, so in ASCII 'B' is one more than 'A', and lower-case letters have higher codes than upper-case ones. This ordering is what lets text be sorted alphabetically by comparing the binary codes.
A computer can only store binary. This topic is about how three kinds of media — text, sound and images — are turned into binary, and how to calculate the resulting file size.
| Term | Mark-scheme definition (two elements) |
|---|---|
| Character set | An agreed table that gives each character a unique binary code so text can be stored/processed |
| ASCII | American standard code for information interchange — uses 7 bits (extended: 8) per character → characters |
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