and 4-bit
quantization). Digital audio technologies are used in the recording, manipulation, mass-production, and distribution of sound, including recordings of
songs, instrumental pieces,
podcasts, sound effects, and other sounds. Modern
online music distribution depends on digital recording and
data compression. The availability of music as data files, rather than as physical objects, has significantly reduced the costs of distribution as well as making it easier to share copies. Before digital audio, the music industry distributed and sold music by selling physical copies in the form of
records and
cassette tapes. With digital audio and online distribution systems such as
iTunes, companies sell digital sound files to consumers, which the consumer receives over the Internet. Popular streaming services such as
Apple Music,
Spotify, or
YouTube, offer temporary access to the digital file, and are now the most common form of music consumption. An analog audio system converts physical waveforms of sound into electrical representations of those waveforms by use of a
transducer, such as a
microphone. The sounds are then stored on an analog medium such as
magnetic tape, or transmitted through an analog medium such as a
telephone line or
radio. The process is reversed for reproduction: the electrical audio signal is
amplified and then converted back into physical waveforms via a
loudspeaker. Analog audio retains its fundamental wave-like characteristics throughout its storage, transformation, duplication, and amplification.
Analog audio signals are susceptible to noise and distortion, due to the innate characteristics of electronic circuits and associated devices. Disturbances in a
digital system do not result in error unless they are so large as to result in a symbol being misinterpreted as another symbol or disturbing the sequence of symbols. It is, therefore, generally possible to have an entirely error-free digital audio system in which no noise or distortion is introduced between conversion to digital format and conversion back to analog. A digital audio signal may be encoded for correction of any errors that might occur in the storage or transmission of the signal. This technique, known as
channel coding, is essential for broadcast or recorded digital systems to maintain bit accuracy.
Eight-to-fourteen modulation is the channel code used for the audio
compact disc (CD).
Conversion process If an audio signal is analog, a digital audio system starts with an ADC that converts an analog signal to a digital signal. The ADC runs at a specified
sampling rate and converts at a known bit resolution.
CD audio, for example, has a sampling rate of 44.1
kHz (44,100 samples per second), and has 16-bit
resolution for each
stereo channel. Analog signals that have not already been
bandlimited must be passed through an
anti-aliasing filter before conversion, to prevent the
aliasing distortion that is caused by audio signals with frequencies higher than the
Nyquist frequency (half the sampling rate). A digital audio signal may be stored or transmitted. Digital audio can be stored on a CD, a
digital audio player, a
hard drive, a
USB flash drive, or any other digital
data storage device. The digital signal may be altered through
digital signal processing, where it may be
filtered or have
effects applied.
Sample-rate conversion, including
upsampling and
downsampling, may be used to change signals that have been encoded with a different sampling rate to a common sampling rate prior to processing. Audio data compression techniques, such as
MP3,
Advanced Audio Coding (AAC),
Opus,
Ogg Vorbis, or
FLAC, are commonly employed to reduce the file size. Digital audio can be carried over
digital audio interfaces such as
AES3 or
MADI. Digital audio can be carried over a network using
audio over Ethernet,
audio over IP or other
streaming media standards and systems. For playback, digital audio must be converted back to an analog signal with a DAC. According to the
Nyquist–Shannon sampling theorem, with some practical and theoretical restrictions, a band-limited version of the original analog signal can be accurately reconstructed from the digital signal. During conversion, audio data can be embedded with a
digital watermark to prevent piracy and unauthorized use. Watermarking is done using a
direct-sequence spread-spectrum (DSSS) method. The audio information is then modulated by a
pseudo-noise (PN) sequence, then shaped within the frequency domain and put back in the original signal. The strength of the embedding determines the strength of the watermark on the audio data. ==History==