From: http://academictech.doit.wisc.edu/audacity/audio-video-course/audio-editing-audacity/digital-audio-properties
To work effectively with digital audio, it is important to understand some basic terminology and properties of digital audio. Understanding these terms will help in making informed decisions about how to compress digital audio for the web and streaming. The three basic properties of digital audio are: stereo, bit-depth and sample rate. The definitions below are partly taken from whatis.techtarget.com.
- Channels
- Sample Rate
- Bit Depth
- Compression / CODECs
- Common Audio Formats
Channels
A stereo audio signal has two discreet channels of audio whereas a mono signal has only one. A stereo signal can be mixed down to a mono signal. An uncompressed stereo digital audio file will be twice the size of a mono file.
Sample Rate
Sample rate is the number of samples of a sound that are taken per second to represent the event digitally. The more samples taken per second, the more accurate the digital representation of the sound can be. For example, the current sample rate for CD-quality audio is 44,100 samples per second.
The more samples per second that are taken, the more accurate the digital representation is of the analog waveform. Each sample is really only a number that contains the amplitude value of a waveform measured over time. This brings us to bit-depth.
Bit Depth
Bit depth defines how many bits are used to describe each of the samples taken from the sample rate (above). It describes the potential accuracy of a digital audio file. Higher bit depth audio will sound better than smaller bit-depth audio. 8 and 16-bit audio are currently the most common sample sizes. 8-bit audio takes up less hard drive space but is inherently noisier than 16 or 24 bit-depth audio. CD quality is 16-bit.
Bit depth is frequently encountered in specifications for hardware and in the specifications for what kind of digital audio a piece of software can capture and process.
Compression and CODECs
Audio files should always be captured and edited at the highest bit-depth and sample rate available - usually 16-bit 44.1 kHz stereo audio on most consumer level hardware. However, once edited, it can compressed using a CODEC (compression / decompression algorithm) to decrease the size of the file. This is how an uncompressed digital audio file is converted into to a streaming audio file for the Web, as well.
Common Audio Formats
- WAV - A digital audio standard developed by Microsoft and IBM. One minute of uncompressed audio at CD quality (16-bit, 44.1 kHz stereo) requires about 10 MB of storage.
- AIFF - or "Audio Interchange File Format" - An audio file format developed by Apple Computer.
- MP3 - MPEG-1 Audio Layer-3 - A common compressed audio format often used for sharing music across the web (both illegally and legally).
- AAC - Advanced Audio Coding - Used both for commercial DVDs and Apple's iTunes Music Store for purchased songs.
- WMA - Microsoft's Windows Media file format. Version 9 of Windows Media is the competitor to AAC for online music sales.