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| Computers-Hardware-Audio | ||
| Net World Directory: Computer audio |
Basics of Digital Audio When a sound is made it causes vibrations in the air. A microphone contains a membrane that will vibrate with the same frequency as the original sound wave. This vibration on the microphone membrane is changed to electrical signals. So acoustical waves are changed into electrical waves by the microphone.
Analog-to-Digital Converter (ADC) is the main device used in digital recording. The ADC captures a copy of the electric voltage on an audio line and changes it in to a digital number that can be sent to a computer. By capturing the voltage several times per second, you can get a very good approximation to the original audio signal:
Higher sampling rates typically allow a digital recording to accurately record higher frequencies of sound. Higher sample sizes allow for more dynamic range of sound. The dynamic range on an audio CD is theoretically about 90 dB, but realistically signals that are -24 dB or more in volume are greatly reduced in quality. Audacity supports two additional sample sizes: 24-bit, which is commonly used in digital recording, and 32-bit float, which has almost infinite dynamic range, and only takes up twice as much storage as 16-bit samples.
Two main types of audio files can be present on your computer. PCM is a short form for Pulse Code Modulation. In this, each number in the digital audio file represents exactly one sample in the waveform. Common examples of PCM files are WAV files, AIFF files, and Sound Designer II files.
The other type of audio file that may be present in your computer is a compressed file. Earlier formats used logarithmic encodings to squeeze more dynamic range out of fewer bits for each sample, like the u-law or a-law encoding in the Sun AU format. In contrast to older forms the current version use sophisticated psychoacoustics algorithms to represent the essential frequencies of the audio signal in far less space. You may be familiar with the common compressed file formats like MP3 (MPEG I, layer 3), Ogg Vorbis, and WMA (Windows Media Audio). Audacity supports MP3 and Ogg Vorbis.