1.2.3. Units and Data Storage

• What are some units for storage?

  • Bit - a single 1 or 0
  • Nibble - 4 bits
  • Byte - 8 bits
  • Kilobyte (KB) - 1000 bytes
  • Megabyte (MB) - 1000 kilobytes
  • Gigabyte (GB) - 1000 megabytes
  • Terabyte (TB) - 1000 gigabytes
  • Petabyte (PB) - 1000 terabytes
• Why do computers use binary?


  Because processors are made of transistors - which only have two states which are on and off - so computers use binary to represent same states as transistors.
• What are some number systems?

  • Binary - base 2, with digits 0 and 1
  • Denary - base 10, with digits 0 to 9
  • Hexadecimal - base 16, with digits 0 to F
• Numbers from 0 to 255 in binary can be represented in 8-bit numbers.
• Numbers from 0 to 255 in hexadecimal can be represented in 2-digit values.
• What are the basic rules for binary addition?


  • 0 + 0 = 0
  • 0 + 1 = 1
  • 1 + 1 = 0 carry 1
  • 1 + 1 + 1 = 1 carry 1
• What is an overflow?


  When the result of a binary calculation is too big, so the carry is lost.
• How are multiplications and divisions done in binary?


  • Moving numbers to the left multiplies them (1 shift left multiplies by 2, 2 shifts left multiplies by 4, and so on)
  • Moving numbers to the right divides them (1 shift left divides by 2, and so on)
• What is a problem with binary division?


  When any of the 1 values are lost, the result will not be accurate.
• What is a character set?


  A set of characters that a computer system can represent.
• How do character sets work?


  Each character is given a unique binary code - when character is typed, the binary code is stored and when character is displayed, the right character for the stored binary code is displayed.
• How do bits relate to character sets?


  More bits per character means more possible characters, meaning it is a larger character set.
• What is ASCII?


  A character set that uses 8 bits per character - it can display all keyboard symbols (the English alphabet, numbers and basic symbols)
• What is Unicode?


  A character set that uses 16 bits per character - it can display all characters from any language.
• How is sound stored in computers?

  By sampling.
• What is sampling?


  Sound waves sampled at regular intervals by measuring the amplitude - the measured value is stored in binary.
• What is sample interval?


  Gap between samples - smaller gap means more samples taken every second, meaning that the sound quality would better (and the file size would be bigger)
• What is bit rate?


  The amount of bits used to store each sample.
• How do sample intervals and bit rates affect file sizes?


  • Smaller sample intervals lead to bigger file sizes
  • Larger bit rates lead to bigger file sizes
• How do computers store images?


  By storing them in pixels.
• What is a pixel?


  A small square of a single colour - they make up pictures in computers.
• How is colour stored in computers?


  Each colour is given an unique binary value - when a pixel is stored, its binary value of the colour is saved.
• What is colour depth?


  Bits used to save each pixel.
• What is metadata?


  Data about the file (image) - e.g. where it was taken, date, colour depth, author, compression type, file type, and more.
• How does colour depth affect file size?


  More colour depth - more possible colours, therefore larger file sizes.
• What is compression?


  Reducing the size of a file.
• Why may compression be beneficial?


  The files would then be quicker to send in email or over the internet, would take up less storage space, and use less bandwidth.
• Describe lossy compression:


  Decreases the file size by removing parts of a file that aren't needed, such as reducing the number of colours in a picture - data is permanently lost and quality of file is reduced, but file size decreases significantly.
• Why is lossy compression (quality reduction) used?


  They often aren't noticeable in images or sounds.
• Describe lossless compression:


  Reduces size of file without removing any parts of a file, e.g. finds more efficient way to store colours - quality is not reduced, no data is permanently lost but file doesn't become smaller as much.
• Use of compression


  Lossy compression cannot be used for some things such as texts, as it is impossible to remove any data without ruining the file.