IGCSE Computer Science 0478 — 1.2 Text, Sound and Images Hangman

The 20 key terms

1binary

A number system using only 0s and 1s (bits). Digital systems use binary, as on/off signals, to represent all data: text, images, sound and video. 8 bits make 1 byte.

3character code

A number the computer uses to stand for one letter or symbol. It stores the number, then turns it back into the character on screen. In one A=1 code, A is 1 and B is 2.

4character set

The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.

5ASCII

A character set that gives each character a 7-bit code (128 characters), usually stored in 8 bits. Extended ASCII uses 8 bits for 256 characters.

78-bit ASCII encoding

Look up each letter's ASCII code and write it as an 8-bit binary group, then join the groups in order. Example: A = 65 = 01000001.

8Unicode

A system that gives every character a unique number, including letters from many languages and emoji, so it covers far more than the small ASCII table.

9Unicode vs ASCII

Unicode supports far more characters than 8-bit ASCII but uses 16 or 32 bits per character, so the same text takes up more storage space.

10File size

How much storage a file needs, based on how much data (text, image, audio) it contains.

11analogue to digital

A sound wave is analogue: its amplitude changes smoothly. A computer stores only separate binary numbers, so the wave a microphone captures must be converted into discrete digital values.

12Sampling

Measuring a sound wave thousands of times per second and storing each measurement as a binary number.

13sample rate

The number of sound measurements taken each second, in hertz (Hz). CDs use 44,100 Hz. A higher rate gives better quality but makes a bigger file.

14sample resolution

The number of bits used to store each sample of a sound. More bits give more possible volume levels and a more accurate recording. 16 bits per sample is common.

15higher sample rate or resolution

Raising the sample rate or sample resolution records the sound more accurately, but it also makes the audio file bigger. Lowering either does the opposite: less accurate, smaller file.

16pixel

A pixel is one tiny square in a grid that makes up a digital image. Each pixel is stored as a number, such as 0 for white or 1 for black.

17resolution

The number of pixels in an image, given as width x height (like 1920 x 1080). More pixels give more detail but make a bigger file.

18colour depth

The number of bits used to store the colour of each pixel. More bits per pixel means more possible colours, such as 1 bit for 2 colours or 24 bits for true colour.

19colours = 2^n

A colour depth of n bits gives 2^n different colours, so each extra bit doubles the number. For example, 2 bits give 4 colours and 8 bits give 256 colours.

20colour code table

A table giving each colour a binary code. Write each pixel's code row by row to encode an image; read the codes back to redraw it. 4 colours need 2 bits.

21higher resolution and colour depth

Raising either one improves image quality (sharper detail, smoother colours) but stores more bits, so the file size gets larger.

22trade-off

Balancing quality against file size: pick the lowest sample rate, resolution or colour depth that still looks or sounds good enough, so the file stays small enough to store or send.