IGCSE1.1.2Paper 1 — Computer Systems

Text, Sound and Images

Everything on a computer — letters, music, photos — is stored as numbers. This is how.

YOU SHOULD BE ABLE TO

Explain how characters are stored using a character set (ASCII, Unicode).
Explain how sound is sampled and stored.
Describe sample rate and sample resolution, and their effect on quality and file size.
Explain how an image is stored as pixels.
Describe resolution and colour depth, and their effect on quality and file size.
Calculate the file size of an image or a sound file.

FILE SIZE0/0 correct

Width: 800 pixels
Height: 600 pixels
Colour depth: 8 bits per pixel

Size in kibibytes (KiB):

//ASCII and Unicode

ASCII uses 7 bits, so it can represent 128 characters — enough for English but not for Mongolian, Chinese or emoji. Unicode uses more bits per character so it covers every writing system, but the files are larger.

//How sound is captured

A microphone produces a smooth (analogue) wave. The computer measures its height many times a second and stores each measurement as a binary number. More measurements per second, and more bits per measurement, means a recording closer to the original — and a bigger file.

//File size calculations

Learn both formulas and always state the unit. Divide bits by 8 for bytes, then by 1024 for KB.

IMAGE
  size (bits) = width × height × colour depth

  100 × 100 pixels, 24-bit colour
  = 100 × 100 × 24 = 240 000 bits
  = 30 000 bytes ≈ 29.3 KB

SOUND
  size (bits) = sample rate × resolution × seconds

  44 100 Hz, 16 bit, 30 seconds
  = 44 100 × 16 × 30 = 21 168 000 bits
  = 2 646 000 bytes ≈ 2.52 MB

KEY TERMS

FLASHCARDS1 / 6

IN THE EXAM

If asked for the EFFECT of increasing sample rate or colour depth, give BOTH sides: better quality AND larger file size.
Write the formula down before substituting numbers — the formula itself often earns a mark.

CHECK YOURSELF

1.An image is 200 × 100 pixels with a colour depth of 8 bits. What is its size in bytes?

2.What is the effect of INCREASING the sample rate of a sound recording?

3.Why can Unicode represent Mongolian text when ASCII cannot?