ASCII, Compression, Sound, Algorithms

• Characters are represented using the ASCII code
• ASCII stands for American Standard Code for Information Interchange
• Seven bits are enough to represent all the characters and symbols on an English keyboard
• The extended 8-bit ASCII code allows for 128 extra special characters
• You can’t do arithmetic on ASCII characters representing numbers - they must first be converted to pure binary numbers
• Unicode provides a unique binary code for every character in every language using 16 bits
• Compression is the name given to algorithms which reduce file sizes
• Decompression is the process where compressed data is restored to its original format
• Compression is heavily used with sound, image, and video files

  • Lossy (JPG, GIF, MP3)
  • Lossless (PNG, TIFF)
• Compression is often used for files and data that are sent via the internet
• Lossy compression permanently loses some data
• Lossy compression results in small mistakes known as ‘digital artifacts’ appearing in images and videos

  • Noise can often be seen where there are contrasting colors
  • Blocks can often result from lossy compression
• Lossless image compression will not lose any of the data
• A lossless compression algorithm finds groups of repeating data and records the data once, along with the number of repetitions
• When data is decompressed it is restored exactly as it was in the original
• RLE stands for Run Length Encoding
• RLE uses frequency/ data pairs to encode each run length of the same colored pixel
• Characteristics of compression:

  • Transmission times are reduced meaning downloads happen faster
  • Data allowances go further
  • Less bandwidth is required
  • Less storage space is required to save files
• Benefits of compression:

  • Reduces traffic over the internet
  • Images inside web pages appear faster
  • Reduces space on disk/ servers
• Video or music streaming causes buffering if the download speed is slower than the playback speed
• sound file size = sample rate * resolution * duration
• Analogue sound signals are continuous
• Digital sound signals are discrete
• Sound is digitized by repeatedly measuring and recording the sound wave, and converting the samples to binary
• Sounds must be converted into a digital form in order to be stored and processed by a computer
• An ADC (Analogue to Digital Converter) or DAC (Digital to Analogue Converter) is used to process the input or output
• Recording quality improves:

  • the more frequently we sample the sound
  • the more accurately we record the wave height
• The sample rate is the number of samples taken per second
• Increasing sampling rate and resolution means recording more data points
• Our hearing range is between 20-20,000 Hz, and varies depending on age
• 1 Hz = 1 sample per second
• Lossy compression removes the sounds in the frequency ranges that we can’t easily hear or that least affect the perceived playback quality
• Lossy compression leaves out some data, which can affect quality
• Lossless compression leaves out repeated data and instead makes a note of how many times it is repeated
• Sound file formats

  Lossless:

  • FLAC (Free Lossless Audio Codec)
  • ALAC (Apple Lossless Audio Codec)

  Lossy:

  • MP3
  • AAC (Advanced Audio Coding)

  Uncompressed:

  • WAV (Waveform Audio File Format)
  • AIFF (Audio Interchange File Format)
• MP4 is a digital multimedia format most commonly used to store video and audio

  • Can also be used to store subtitles and still images
  • Allows different multimedia streams (video, audio, text) to be combined into one file
• MIDI stands for Musical Instrument Digital Interface
• A MIDI file:

  • is not a recording of a live music source
  • is a set of instructions for digital instruments to play synthesized sounds
  • can be used to synchronize an orchestra of digital instruments to play simultaneously
  • uses up to 1,000 times less space than a conventional recording
  • is commonly used for mobile phone ringtones
• An algorithm is a series of steps to solve a problem or carry out a task.
• All programs are composed of three basic structures:

  • Sequence.
  • Selection.
  • Iteration (repetition).