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Audio
Sound travels in the form of sound waves, and every sound that is transmitted begins and ends its life as a wave. The original sound recording and storage equipment was analog, meaning it stored audio information in a continuous wave-like manner. As technology progressed however, analog formats gave way to digital ones. The most visible example of this transformation was from records and tapes to CDs. Digital audio recordings store a representation of a sound wave in a form that can read and processed by a computer. Digital audio can be easily manipulated in a myriad of ways, it can be duplicated flawlessly, it can be compressed efficiently, it can be encrypted, and it can be easily transported over a network. For these and many other reasons, digital audio has dramatically changed the audio landscape.
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Digital Audio Properties
To work effectively with digital audio, it is important to understand some basic terminology and properties in making informed decisions about how to compress digital audio for streaming on the Web. 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
- Frequency Response
- Bit-depth
- Compression
- Common Audio Formats
Channels
A stereo audio signal has two discrete 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.
Frequency Response
The range of human hearing is generally regarded as being from 20 Hz, very low bass tones, through 20 kHz (20,000 Hz), the very highest treble. To reproduce audio digitally, the sample rate must be at least double the highest frequency, a relationship posited by the Nyquist Theorom. That’s why the sample rate for CD Audio is 44.1 kHz, twice the rate of the highest range of human hearing, 20 kHz.
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 sample rate for CD-quality audio is 44,100 samples per second.
A sine wave represents one cycle of a sound wave.
Bit Depth
Bit depth defines how many bits are used to describe each of the samples taken from the sample rate. It describes the potential accuracy of a digital audio file. Higher bit-depth audio will sound better than smaller bit-depth audio. Currently, 8- and 16-bit audio are the most common sample sizes; 8-bit audio takes up less hard drive space but is inherently noisier than 16- or 24-bit 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
The storage space required for one minute of uncompressed stereo audio at 16-bit 44.1 kHz is 10 MB and for one-minute of mono at 16-bit 44.1 kHz is 5 MB.
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, files can be 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 a streaming audio file for the Web, as well.
*One important exception is DV (Digital Video) where 16-bit 48 kHz is a standard sample rate.
Common Audio File Formats
Examples of uncompressed file 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, Audio Interchange File Format: an audio file format developed by Apple Computer
Examples of compressed file formats
- 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 of AAC for online music sales.
Additional materials provided by
University of Wisconsin—Madison, Division of Information Technology
