Representing Sound

• What do computers represent sound as?

  A sequence of samples, each sample takes a discrete digital value
• What is sampling rate?

  The number of samples per second (Hz)
• What is sample resolution?

  The number of bits allocated to each sample
• If the sample has a higher resolution, what will this lead to?

  Better audio quality and increased file size
• How do you calculate the size of a sound file?

  Duration (s) x sampling rate (Hz) x sample resolution (bits)
• What does the Nyquist Theorem state?

  The sampling rate of an audio file must be at least twice the frequency.
• If the sampling rate is too low (nyquist), what may happen?

  The sound may not be accurately represented
• What is the purpose of MIDI (musical instrument digital interface)?

  It is used with electronic instruments which can be connected to computers
• How does MIDI work?

  It stores sound as a series of event messages, each message represents 1event in the music
• What are some examples of event messages (MIDI)?

  Duration of a note

  The instrument which plays a note

  Volume of a note
• What are the main advantages of using MIDI?

  Easy manipulation of music

  Using programs and algorithms is simple

  No loss of data through sampling

  Elimination of human error
• What are the 2 main disadvantages of using MIDI?

  Can't be used for storing speech

  Less realistic sound
• Sound Capture

  Analogue to Digital:

  • A microphone is used to transform sound waves into analogue signals
  • The signals are processed by an ADC
  • The ADC uses sampling to take measurements at regular intervals
  • Each sample is recorded as a binary number
• Digital to Analogue:

  • Using a DAC (digital to analogue converter), the analogue signal is reconstructed from the binary numbers (samples)
  • The missing information (gaps between the samples) is created by the DAC
  • The analogue signal is sent to the speaker
  • A cone within the speaker vibrates and moves air particles to create the sound