GCSE AQA Computer Science - Data Representation

Images
1. Bitmap images are made up of picture elements, or pixels
2. A pixel is a single point in a graphical image
3. Resolution - concentration of pixels in a certain area
4. Colour depth - how many colours can be represented in a single pixel
5. File size = total pixels x colour depth
6. Higher resolution / colour depth = higher quality & higher file size
Sound
1. Microphones record analogue and convert to digital
2. Speakers convert from digital to analogue
3. Sample resolution - the amplitude used to record each sample / measurement
4. Sample rate - number of samples taken a second
5. File size = sample rate x resolution x time
Compression
1. Lossy
  1. JPG, GIF, MP3
  2. Permanently removes some data
  3. Uncompressed data is not the same as the original
2. Lossless
  1. PNG, TIFF
  2. Uncompressed data is same as the original
  3. File quality is not lost
  4. Run length encoding
  5. Huffman tree
    1. Analyses the frequency of characters in a body of text
    2. Characters are placed in a binary tree structure
    3. Most common characters appear near the top of the tree
Number systems
1. Denary
  1. Base 10
  2. The number system we use
2. Binary
  1. Base 2
  2. It is the only thing a computer understands
  3. Made up of 1's and 0's
  4. Binary addition
    1. 0 + 0 = 0
    2. 0 + 1 = 1
    3. 1 + 1 = 10
    4. 1 + 1 + 1 = 11
  5. Logical shifts
    1. Each left shift multiplies the value by 2 (unless there is an overflow)
    2. Each right shift divides the value by 2 (unless there is an underflow)
3. Hexadecimal
  1. Base 16
  2. Made up of the numbers 0-9 and the letters A-F
  3. Used as a shorter way of representing binary
  4. Can be used to represent colours
  5. Computers do not understand hexadecimal
Storage
1. Bit - a single 1 or 0
2. Nibble - 4 bits
3. Byte - 8 bits
4. Kilobyte - 1000 bytes
5. Megabyte - 1000 kilobytes
6. Gigabyte - 1000 megabytes
7. Terabyte - 1000 gigabytes
Text
1. ASCII
  1. 7 bits per character
  2. Encodes 128 characters
  3. Only represents the English alphabet
2. Extended ASCII
  1. 8 bits per character
  2. Encodes 256 characters
  3. Used to represent more symbols / special characters
3. Unicode
  1. Made so that different languages could be represented
  2. 16 bits per character
  3. 65536 possible characters
  4. First 128 characters use the same codes as ASCII - backwards compatability

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2019 08 15+16 42 00 Window (binary/octet-stream)

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GCSE AQA Computer Science - Data Representation

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