Representing text, images and sound

GCSE Computer Science revision notes, key terms and practice questions.

Characters

  • A character set gives each character a binary code. ASCII uses 7 bits, giving 128 characters (extended ASCII uses 8 bits for 256). Unicode uses up to 32 bits, so it can represent characters from every language, and emoji.
  • The codes are in order: if 'A' is 65, then 'C' is 67. The lower-case letters come later ('a' is 97).

Images

  • A bitmap image is made of pixels. Each pixel's colour is stored as a binary code.
  • Colour depth is the number of bits per pixel. n bits give 2ⁿ colours: 1 bit gives 2 colours, and 8 bits give 256.
  • Resolution is the number of pixels in the image (width × height). More pixels or a greater colour depth give better quality, but a bigger file.
  • File size in bits = width × height × colour depth. A 100 × 50 image with 8-bit colour is 100 × 50 × 8 = 40 000 bits = 5000 bytes. The file also stores metadata, such as the width, height and colour depth.

Sound

  • Sound is sampled: the amplitude of the sound wave is measured at regular intervals and each measurement is stored in binary.
  • The sample rate is the number of samples per second, in hertz (Hz). The bit depth (sample resolution) is the number of bits per sample.
  • File size in bits = sample rate × bit depth × length in seconds. 10 seconds at 44 100 Hz and 16 bits is 7 056 000 bits = 882 000 bytes.
  • A higher sample rate or bit depth makes the recording closer to the original sound, but the file is bigger.

Key terms

Character set
A list of characters and the binary code that represents each one.
ASCII
A character set that uses 7 bits to represent 128 characters.
Unicode
A character set that uses more bits, so it can represent characters from every language.
Pixel
A single dot of colour in a bitmap image.
Colour depth
The number of bits used to store the colour of each pixel.
Resolution
The number of pixels in an image.
Metadata
Data about a file, such as an image's width, height and colour depth.
Sample rate
The number of sound samples taken per second.
Bit depth
The number of bits used to store each sound sample.

Practise Representing text, images and sound: 13 questions