Computer science

• Not gate is the opposite(output is opposite the input) the symbol is the triangle
• and gate is both inputs have to be 1 for the output to be 1. the symbol looks like a capital d.
• Pixels

  Tiny dots that make up images
• Bitmap images

  • Mainly used for photos
  • Made up of lots of tiny dots, called pixels (picture elements)
• Colour of each pixel

  Represented by a binary code
• Number of colours in an image

  Related to the number of bits the code has
• Black-and-white images

  • Use two colours
  • Need 1-bit to represent each pixel (0 for white, 1 for black)
• 1. bit images

  • Can be made up of four colours
  • Each pixel can be one of four binary values (00, 01, 10, 11)
• You can make a greater range of shades and colours by increasing the number of bits for each pixel
• Increasing colour depth and image size


  Increases the file size
• Colour depth

  The number of bits used for each pixel
• Image size

  The number of pixels in the image, usually given as width x height
• More pixels in an image


  Better the quality of the image
• Using larger image size or colour depth means that there are more bits in the image, which can give a higher-quality image but also increases the file size
• Calculating file size of an 8-bit image that is 2000 pixels wide and 1000 pixels high

  • 2000 x 1000 x 8 = 16,000,000 bits

  16,000,000 / 8 = 2,000,000 bytes

  2,000,000 / 1000 / 1000 = 2 MB
• Bitmap images

  Made out of pixels
• Image

  Made up of bits and stored in computers
• Analogue to digital conversion

  Analogue signal converted into digits so computers can read and store sounds
• Sampling

  Process of converting analogue to digital
• Sample

  Each block of data that represents where the sound wave is at a particular point in time
• Factors affecting size and quality of sound files

  • Sample rate (sampling frequency)
  • Sample resolution
  • File size formula
• Sample rate

  How many samples you take in a second, measured in Hz or kHz
• Sample resolution

  Number of bits available for each sample
• Increasing sample rate means the analogue recording is sampled more often, resulting in better quality
• Increasing sample resolution means the digital file picks up quieter sounds even if they're happening at the same time as louder ones, resulting in better quality
• Increasing sample rate or sample resolution will increase the file size
• Compression


  Making file sizes smaller, while trying to make the compressed file as true to the original as possible
• Data compression


  1. Reduce file size

  2. Maintain original file quality as much as possible
• Uses of data compression


  • Smaller files take up less storage space on a device
  • Streaming and downloading files from the Internet is quicker as they take up less bandwidth
  • Allows web pages to load more quickly in web browsers
  • Allows sending the same content with a much smaller file size in email
• Lossy compression


  Works by permanently removing data from the file, limiting the number of bits the file needs and reducing its size
• Lossless compression


  Makes the file smaller by temporarily removing data to store the file and then restores it to its original state when it's opened
• Pros of lossy compression


  • Greatly reduced file size, meaning more files can be stored
  • Lossy files take up less bandwidth so can be downloaded and streamed more quickly
  • Commonly used-lots of software can read lossy files
• Pros of lossless compression


  • Data is only removed temporarily so there is no reduction in quality - the compressed file should look or sound like the original
  • Lossless files can be decompressed - turned back into the original
  • Lossless compression can be used on text and software files
• Cons of lossy compression


  • Lossy compression- the file can't be turned back into the original
  • Lossy compression can't be used on text or software files as these files need to retain all the information of the original
  • Lossy files are worse quality than the original.(lossless has better quality)
• Cons of lossless compression


  • Only a slight reduction in file size, so lossless files still take up quite a bit of space on your device.
• Run-length encoding is a form of lossless compression. It looks for consecutive repeating data. It store the number of times it repeats and makes one copy.
• Huffman coding is a form of lossless compression.
• if a binary number shifts left it multiplies and every shift left, the number is doubled.
• In binary, if it shifts right it divides the binary number. for every place shifted right the number is halved.
• 1 nibble is 4 bits.