Chapter 12. Sound

Key Points

• Sound is a complex mixture of physical and psychological factors, which is difficult to model accurately.
• Sounds can be characterized by their waveforms, which plot amplitude against time.
• CD quality sound is sampled at 44.1 kHz, using a sample size of 16 bits. Multimedia productions may have to use lower sampling rates and smaller sample sizes.
• The quality of digitized sound can be improved by dithering — adding a small quantity of noise to randomize the quantization error.
• Software can provide the functions of a recording studio, including multi-track recording, mixing and effects, on a desktop computer.
• The most vexatious aspect of recording is getting the levels right.
• Audio filters are used to remove noise and unwanted frequency components.
• Digital versions of established effects, such as reverb and envelope shaping are used to alter the quality of sounds. Digital technology permits new kinds of alteration, including time stretching and pitch alteration.
• Speech data can be compressed using established technology, including µ-law and A-law companding and ADPCM.
• MPEG-1 Layer 3 audio (MP3) is a lossy method of audio compression that uses a psycho-acoustical model to determine which information to discard.
• Each of the three major platforms has its own sound file format: AIFF for MacOS, WAV for Windows, and AU for Unix. RealAudio is used for streaming audio.
• MIDI (The Musical Instruments Digital Interface) provides a standard for controlling digital instruments and communicating between them and computers running sequencer programs.
• When sound is combined with video, synchronization must be established and maintained.