Audio signal processing is a subfield of
signal processing
Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
that is concerned with the electronic manipulation of
audio signals. Audio signals are electronic representations of
sound waves—
longitudinal waves which travel through air, consisting of compressions and rarefactions. The energy contained in audio signals is typically measured in
decibels. As audio signals may be represented in either
digital or
analog format, processing may occur in either domain. Analog processors operate directly on the electrical signal, while digital processors operate mathematically on its digital representation.
History
The motivation for audio signal processing began at the beginning of the 20th century with inventions like the
telephone,
phonograph, and
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a tr ...
that allowed for the transmission and storage of audio signals. Audio processing was necessary for early
radio broadcasting, as there were many problems with
studio-to-transmitter links. The theory of signal processing and its application to audio was largely developed at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mul ...
in the mid 20th century.
Claude Shannon
Claude Elwood Shannon (April 30, 1916 – February 24, 2001) was an American mathematician, electrical engineer, and cryptographer known as a "father of information theory".
As a 21-year-old master's degree student at the Massachusetts I ...
and
Harry Nyquist
Harry Nyquist (, ; February 7, 1889 – April 4, 1976) was a Swedish-American physicist and electronic engineer who made important contributions to communication theory.
Personal life
Nyquist was born in the village Nilsby of the parish Stora ...
's early work on
communication theory,
sampling theory and
pulse-code modulation
Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the ...
(PCM) laid the foundations for the field. In 1957,
Max Mathews became the first person to
synthesize audio from a
computer
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
, giving birth to
computer music
Computer music is the application of computing technology in music composition, to help human composers create new music or to have computers independently create music, such as with algorithmic composition programs. It includes the theory and ...
.
Major developments in
digital audio coding and
audio data compression include
differential pulse-code modulation (DPCM) by
C. Chapin Cutler at Bell Labs in 1950,
linear predictive coding (LPC) by
Fumitada Itakura (
Nagoya University) and Shuzo Saito (
Nippon Telegraph and Telephone) in 1966,
adaptive DPCM (ADPCM) by P. Cummiskey,
Nikil S. Jayant and
James L. Flanagan at Bell Labs in 1973,
discrete cosine transform (DCT) coding by
Nasir Ahmed, T. Natarajan and
K. R. Rao in 1974,
and
modified discrete cosine transform (MDCT) coding by J. P. Princen, A. W. Johnson and A. B. Bradley at the
University of Surrey in 1987. LPC is the basis for
perceptual coding and is widely used in
speech coding,
while MDCT coding is widely used in modern
audio coding formats
An audio coding format (or sometimes audio compression format) is a content representation format for storage or transmission of digital audio (such as in digital television, digital radio and in audio and video files). Examples of audio coding f ...
such as
MP3 and
Advanced Audio Coding
Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves higher sound quality than MP3 encoders at the same bit rate.
AAC has been stan ...
(AAC).
Analog signals
An analog audio signal is a continuous signal represented by an electrical voltage or current that is ''analogous'' to the sound waves in the air. Analog signal processing then involves physically altering the continuous signal by changing the voltage or current or charge via
electrical circuits.
Historically, before the advent of widespread
digital technology, analog was the only method by which to manipulate a signal. Since that time, as computers and software have become more capable and affordable, digital signal processing has become the method of choice. However, in music applications, analog technology is often still desirable as it often produces
nonlinear
In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many oth ...
responses that are difficult to replicate with digital filters.
Digital signals
A digital representation expresses the audio waveform as a sequence of symbols, usually
binary numbers
A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" (zero) and "1" (one).
The base-2 numeral system is a positional notation ...
. This permits signal processing using
digital circuits
Digital electronics is a field of electronics involving the study of digital signals and the engineering of devices that use or produce them. This is in contrast to analog electronics and analog signals.
Digital electronic circuits are usual ...
such as
digital signal processors,
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
s and general-purpose
computer
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
s. Most modern audio systems use a digital approach as the techniques of digital signal processing are much more powerful and efficient than analog domain signal processing.
Applications
Processing methods and application areas include
storage,
data compression,
music information retrieval,
speech processing,
localization,
acoustic detection,
transmission,
noise cancellation,
acoustic fingerprinting,
sound recognition
Sound recognition is a technology, which is based on both traditional pattern recognition theories and audio signal analysis methods. Sound recognition technologies contain preliminary data processing, feature extraction and classification algori ...
,
synthesis
Synthesis or synthesize may refer to:
Science Chemistry and biochemistry
* Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors
**Organic synthesis, the chemical synthesis of organ ...
, and enhancement (e.g.
equalization,
filtering,
level compression,
echo and
reverb
Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abs ...
removal or addition, etc.).
Audio broadcasting
Audio signal processing is used when broadcasting audio signals in order to enhance their fidelity or optimize for bandwidth or latency. In this domain, the most important audio processing takes place just before the transmitter. The audio processor here must prevent or minimize
overmodulation, compensate for non-linear transmitters (a potential issue with
medium wave and
shortwave broadcasting), and adjust overall
loudness
In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of ph ...
to the desired level.
Active noise control
Active noise control is a technique designed to reduce unwanted sound. By creating a signal that is identical to the unwanted noise but with the opposite polarity, the two signals cancel out due to
destructive interference.
Audio synthesis
Audio synthesis is the electronic generation of audio signals. A musical instrument that accomplishes this is called a synthesizer. Synthesizers can either
imitate sounds or generate new ones. Audio synthesis is also used to generate human
speech using
speech synthesis.
Audio effects
Audio effects alter the sound of a
musical instrument or other audio source. Common effects include
distortion, often used with electric guitar in
electric blues and
rock music
Rock music is a broad genre of popular music that originated as " rock and roll" in the United States in the late 1940s and early 1950s, developing into a range of different styles in the mid-1960s and later, particularly in the United States a ...
;
dynamic
Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to:
Physics and engineering
* Dynamics (mechanics)
** Aerodynamics, the study of the motion of air
** Analytical dyn ...
effects such as
volume pedals and
compressors, which affect loudness;
filters
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component that ...
such as
wah-wah pedals and
graphic equalizers, which modify frequency ranges;
modulation effects, such as
chorus
Chorus may refer to:
Music
* Chorus (song) or refrain, line or lines that are repeated in music or in verse
* Chorus effect, the perception of similar sounds from multiple sources as a single, richer sound
* Chorus form, song in which all verse ...
,
flangers and
phasers;
pitch effects such as
pitch shifters; and time effects, such as
reverb
Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abs ...
and
delay
Delay (from Latin: dilatio) may refer to:
Arts, entertainment, and media
* ''Delay 1968'', a 1981 album by German experimental rock band Can
* '' The Delay'', a 2012 Uruguayan film
People
* B. H. DeLay (1891–1923), American aviator and ac ...
, which create echoing sounds and emulate the sound of different spaces.
Musicians,
audio engineers and record producers use effects units during live performances or in the studio, typically with electric guitar, bass guitar,
electronic keyboard
An electronic keyboard, portable keyboard, or digital keyboard is an electronic musical instrument, an electronic derivative of keyboard instruments. Electronic keyboards include synthesizers, digital pianos, stage pianos, electronic organs ...
or
electric piano. While effects are most frequently used with
electric or
electronic instruments, they can be used with any audio source, such as
acoustic instruments, drums, and vocals.
Computer audition
See also
*
Sound card
*
Sound effect
References
Further reading
*
*
{{DEFAULTSORT:Audio Signal Processing
Audio electronics
Signal processing