Knowledge Organiser: Images and Sound
Key Terms
- Pixel: Smallest unit of a digital image (picture element)
- Resolution: Number of pixels in an image (width × height)
- Colour depth: Number of bits used to represent each pixel's colour
- Sample rate: Number of audio samples taken per second (measured in Hz)
- Bit depth (audio): Number of bits used to store each audio sample
- Metadata (OCR): Additional data stored with an image (e.g. width, height, colour depth)
Must-Know Facts
- Image file size (bits) = Width × Height × Colour depth
- Divide by 8 to convert bits to bytes
- 24-bit colour depth = True Colour (16.7 million colours)
- Sound file size (bits) = Sample rate × Bit depth × Duration
- CD quality audio: 44,100 Hz sample rate, 16-bit depth, stereo (2 channels)
- Extra: stereo = 2 channels, so multiply by 2; mono = 1 channel
- Higher resolution/sample rate = better quality but larger file size
Key Concepts
- Image calculation example: 100×100 pixels, 24-bit = 100×100×24 ÷ 8 = 30,000 bytes
- Sound calculation example: 44,100 Hz × 16-bit × 10 sec ÷ 8 = 882,000 bytes (mono); 1,764,000 bytes for stereo
- Quality vs file size trade-off: Increasing any factor improves quality but increases file size
- Always divide by 8 to convert from bits to bytes in any file size calculation
Common Mistakes
- Forgetting to divide by 8 to get bytes: The formula gives the answer in bits — divide by 8 to convert to bytes, then by 1,000 for KB etc.; omitting this step loses marks
- Confusing resolution with colour depth: Resolution is the number of pixels (width × height); colour depth is the number of bits per pixel — both affect file size independently
- Saying higher sample rate = more channels: Sample rate (Hz) measures how many times per second sound is sampled; the number of channels (mono/stereo) is a separate thing
- Mixing up image and sound file size formulas: Images use width × height × colour depth; sound uses sample rate × bit depth × duration — they are different formulas