Digital Sound

Digital Sound

1. Recording Sound

   • When we record sound the analogue wave is picked up by the recording device as changes in air pressure. These changes in pressure are registered by the device, which outputs varying voltages depending on the amplitude and frequencies of the sound wave.
   • In its raw form sound is in analogue form; it is a continuous wave with effectively infinite points. To store and play audio using a computer we need to store it in a finite digital form.
   • The output voltage is picked up by an ADC (Analogue-Digital Converter) which samples the electrical signal representing the sound waveform at a regular frequency. The waveform is plotted as a graph of displacement against time and stored as a series of binary co-ordinates.
   • Nyquist's Theorem states that to accurately approximate an analogue waveform digitally the sampling rate must be at least twice the highest frequency in the recording.

2. Features of Digital Sound

   • Sample Rate - The number of samples of the analogue signal made per second. This is usually around 40kHz, twice the human hearing range. Higher rates allow for more accurate sampling but require more processing.
   • Bit Depth - The range of amplitude values available per sample. This allows for a sample closer to the actual sound waveform but also requires more processing.
   • Bit Rate - The product of sample rate and bit depth, denoting the number of bits required per second of audio.

3. MIDI Files

   • Stands for Musical Instrumental Digital Interface
   • MIDI files are sets of instructions that cause a device to produce a certain sound. Essentially, they simulate actual sound without recording.
   • MIDI is not a digital file format in itself; it is simply a set of instructions.
   • Instructions
     • Note On - Start to play a certain note. This instruction includes the pitch and velocity (initial volume) of the note.
     • Note Off - Stop playing a certain note.
     • Control Change - Indicates that some control element of the virtual instrument has been pressed or released (eg: a keyboard's sustain pedal)
     • Polyphonic Key Pressure - Indicates how hard a key has been pressed after bottoming out.
   • MIDI files can be played by any device with the necessary software (eg: a sound card)
   • MIDI files can be created using MIDI instruments or MIDI controllers.
     • MIDI instruments (eg: a synthesiser keyboard) both produce sound and output MIDI data to devices.
     • MIDI controllers resemble synthesisers but only output MIDI data and may be used to control several virtual instruments on one peripheral.