representing images, sound and other data

• Sampling sound

  1. Taking a sample at regular intervals

  2. Intervals are known as the sample rate

  3. Sample rate, duration, and sample resolution (bit depth) impact the sound quality and size of a file
• Analogue data

  Continuous stream of data, must be converted to digital for a computer to be able to process it
• Converting analogue to digital data

  1. Use an analogue to digital converter (ADC)

  2. Example: microphone inputs sound in the form of changes in air pressure and converts them into electrical signals

  3. Analogue electrical signals are then converted by the ADC into digital signals that the computer can process
• Digital data

  Needs to be converted to analogue data for things such as telephone lines
• Analogue sounds purer than digital since it is constant and uses a higher sampling rate
• Pixel

  Picture Element
• Bitmap images

  • Made of pixels
  • Enlarging it leads to pixels being overstretched and poor quality
  • Pixels can be edited without changing the design of the image
  • Larger file size
  • More realistic image
• Colour depth

  Amount of memory allocated to the pixel
• Resolution

  Width x Height
• Storage

  Resolution x Colour depth
• File size

  Sample rate x Length x Sampling resolution
• An image with n bits can represent 2^n colours
• File size increases with amount of bits used
• Metadata

  Data about the data e.g. filename, dimensions, date, time, camera settings
• Sampling rate

  Higher sampling rate means higher sound detail, measured in Hertz (Hz), each sample represents the amplitude of the wave at a certain point in time, most common sample rate is 44.1kHz
• Bit depth

  Number of bits available per sample, higher bit depth means higher quality of audio
• Nyquist's Theorem: For accurate sampling, the sampling rate must be at least twice the frequency of the highest frequency in the original sound signal
• Musical Instrument Digital Interface (MIDI)

  Communication protocol that allows electronic musical instruments to interact with each other using information and control signals, uses a mini controller to send and receive event messages to each device
• MIDI files

  • List of instructions detailing information on the sound such as pitch etc
  • Developed when sound from an instrument is recorded and allows for music to be identical to the one recorded
  • Widely used in recording studios, orchestras, and musical scores in films
  • Requires less memory than digital recordings
  • Loads faster
  • Completely editable for individual instruments
  • Supports a very wide range of instruments providing more choices for music productions
  • High quality
• Lossless compression

  No data is lost during compression
• Lossy compression

  Some degradation of data occurs
• Types of compressed files

  • PNG (lossless, used for transferring over the internet)
  • JPEG (lossy, higher compression than PNG, used for digital cameras and webpages)
  • GIF (lossless, used for small images, or animated graphics)
• Compressing video files

  • Reduces resolution, dimensions and bit rate
  • Can lead to poor quality and artefacts (random coloured blocks on screen)
• Lossy video file formats

  • MP4, MOV
• MP3

  Used for audio compression, still high quality although not as high as a CD, no difference noticeable to humans as algorithm only removes sounds humans can't hear
• Run-length Encoding (RLE)

  Reduces size of repeated code, called a run, encoded with 2 bytes: number of characters, values, not efficient for a file with small number of runs, useful for simple images
• Dictionary Based Encoding

  Used for compressing text files, common strings used in texts, such as "tion", string is encoded to dictionary and used in combination with other prefixes
• Encryption

  Turning plaintext into ciphertext
• Caesar cipher


  A substitution cipher where each character of plaintext is substituted for another
• Polyalphabetic cipher


  A cipher that uses more than one alphabet
• Vernam cipher

  A one time pad cipher, the only cipher to be considered 100% mathematically accurate
• Key

  In cryptography, the data that is used to encrypt and decrypt the data
• Frequency analysis

  The study of how often different letters must decrypt the data
• Computational Security

  A concept of how secure data encryption is
• Computational hardware

  The degree of difficulty in cracking a cipher
• Encryption works by turning plaintext into ciphertext, used in wireless networks to ensure security and in https
• Disadvantages of Caesar cipher

  • Easy to decrypt
  • Longer messages are easy to decode
• Caesar cipher

  Substitutes each letter of the alphabet for another character by simply shifting the letters forwards or backwards
• Improving Caesar cipher

  • Can be made more secure by adding a keyword (delete repeated letters and then add the word to the alphabet)
  • Further complexity added by using more than one alphabet
  • Multiple versions can be used and the code passed through multiple times
  • Underlying principles of the Enigma machine
• Caesar cipher is one of the easiest to crack due to the use of nature of the language, due to having more common and easier to interpret letters