Digital audio
Digital audio is a representation of sound
recorded in, or converted into, digital
form. In digital audio, the sound wave of
the audio signal is encoded as numerical
samples in a continuous sequence. For
example, in CD audio, samples are taken
44100 times per second each with 16 bit
sample depth. Digital audio is also the
name for the entire technology of sound
recording and reproduction using audio
signals that have been encoded in digital
form. Following significant advances in
digital audio technology during the
1970s, it gradually replaced analog audio
technology in many areas of audio
engineering and telecommunications in
the 1990s and 2000s.
Audio levels display on a digital audio recorder
(Zoom H4n)
In a digital audio system, an analog
electrical signal representing the sound
is converted with an analog-to-digital
converter (ADC) into a digital signal,
typically using pulse-code modulation.
This digital signal can then be recorded,
edited, modified, and copied using
computers, audio playback machines,
and other digital tools. When the sound
engineer wishes to listen to the recording
on headphones or loudspeakers (or when
a consumer wishes to listen to a digital
sound file), a digital-to-analog converter
(DAC) performs the reverse process,
converting a digital signal back into an
analog signal, which is then sent through
an audio power amplifier and ultimately
to a loudspeaker.
Digital audio systems may include
compression, storage, processing, and
transmission components. Conversion to
a digital format allows convenient
manipulation, storage, transmission, and
retrieval of an audio signal. Unlike analog
audio, in which making copies of a
recording results in generation loss and
degradation of signal quality, digital
audio allows an infinite number of copies
to be made without any degradation of
signal quality.
Overview
A sound wave, in red, represented digitally, in blue
(after sampling and 4-bit quantization).
Digital audio technologies are used in the
recording, manipulation, massproduction, 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.[1]
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.
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 the disturbance is so large as to
result in a symbol being misinterpreted
as another symbol or disturb 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 optionally 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 a channel
code used in the audio compact disc
(CD).
Conversion process
…
The lifecycle of sound from its source, through an
ADC, digital processing, a DAC, and finally as sound
again.
A digital audio system starts with an ADC
that converts an analog signal to a digital
signal.[note 1] 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. Samplerate conversion including upsampling
and downsampling may be used to
conform 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, 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
bandlimited version of the original analog
signal can be accurately reconstructed
from the digital signal.
History
Coding
…
Pulse-code modulation (PCM) was
invented by British scientist Alec Reeves
in 1937.[2] In 1950, C. Chapin Cutler of
Bell Labs filed the patent on differential
pulse-code modulation (DPCM),[3] a data
compression algorithm. Adaptive DPCM
(ADPCM) was introduced by P.
Cummiskey, Nikil S. Jayant and James L.
Flanagan at Bell Labs in 1973.[4][5]
Perceptual coding was first used for
speech coding compression, with linear
predictive coding (LPC).[6] Initial
concepts for LPC date back to the work
of Fumitada Itakura (Nagoya University)
and Shuzo Saito (Nippon Telegraph and
Telephone) in 1966.[7] During the 1970s,
Bishnu S. Atal and Manfred R. Schroeder
at Bell Labs developed a form of LPC
called adaptive predictive coding (APC),
a perceptual coding algorithm that
exploited the masking properties of the
human ear, followed in the early 1980s
with the code-excited linear prediction
(CELP) algorithm.[6]
Discrete cosine transform (DCT) coding,
a lossy compression method first
proposed by Nasir Ahmed in 1972,[8][9]
provided the basis for the modified
discrete cosine transform (MDCT), which
was developed by J. P. Princen, A. W.
Johnson and A. B. Bradley in 1987.[10]
The MDCT is the basis for most audio
coding standards, such as Dolby Digital
(AC-3),[11] MP3 (MPEG Layer III),[12][6]
Advanced Audio Coding (AAC), Windows
Media Audio (WMA), and Vorbis
(Ogg).[11]
Recording
…
PCM was used in telecommunications
applications long before its first use in
commercial broadcast and recording.
Commercial digital recording was
pioneered in Japan by NHK and Nippon
Columbia and their Denon brand, in the
1960s. The first commercial digital
recordings were released in 1971.[13]
The BBC also began to experiment with
digital audio in the 1960s. By the early
1970s, it had developed a 2-channel
recorder, and in 1972 it deployed a digital
audio transmission system that linked
their broadcast center to their remote
transmitters.[13]
The first 16-bit PCM recording in the
United States was made by Thomas
Stockham at the Santa Fe Opera in 1976,
on a Soundstream recorder. An improved
version of the Soundstream system was
used to produce several classical
recordings by Telarc in 1978. The 3M
digital multitrack recorder in
development at the time was based on
BBC technology. The first all-digital
album recorded on this machine was Ry
Cooder's Bop till You Drop in 1979. British
record label Decca began development
of its own 2-track digital audio recorders
in 1978 and released the first European
digital recording in 1979.[13]
Popular professional digital multitrack
recorders produced by Sony/Studer
(DASH) and Mitsubishi (ProDigi) in the
early 1980s helped to bring about digital
recording's acceptance by the major
record companies. The 1982 introduction
of the CD popularized digital audio with
consumers.[13]
Telephony
The rapid development and wide
adoption of pulse-code modulation
(PCM) digital telephony was enabled by
metal–oxide–semiconductor (MOS)
switched capacitor (SC) circuit
technology, developed in the early
…
1970s.[14] This led to the development of
PCM codec-filter chips in the late
1970s.[14][15] The silicon-gate CMOS
(complementary MOS) PCM codec-filter
chip, developed by David A. Hodges and
W.C. Black in 1980,[14] has since been the
industry standard for digital
telephony.[14][15] By the 1990s,
telecommunication networks such as the
public switched telephone network
(PSTN) had been largely digitized with
VLSI (very large-scale integration) CMOS
PCM codec-filters, widely used in
electronic switching systems for
telephone exchanges, user-end modems
and a range of digital transmission
applications such as the integrated
services digital network (ISDN), cordless
telephones and cell phones.[15]
Technologies
Sony digital audio tape recorder PCM-7030
Digital audio is used in broadcasting of
audio. Standard technologies include
Digital audio broadcasting (DAB), Digital
Radio Mondiale (DRM), HD Radio and Inband on-channel (IBOC).
Digital audio in recording applications is
stored on audio-specific technologies
including Compact disc (CD), Digital
Audio Tape (DAT), Digital Compact
Cassette (DCC) and MiniDisc. Digital
audio may be stored in a standard audio
file formats and stored on a Hard disk
recorder, Blu-ray or DVD-Audio. Files may
be played back on smartphones,
computers or MP3 player.
Interfaces
This section is in list format, but may read
better as prose.
Learn more
Digital-audio-specific interfaces include:
A2DP via Bluetooth
AC'97 (Audio Codec 1997) interface
between integrated circuits on PC
motherboards
ADAT Lightpipe interface
AES3 interface with XLR connectors,
common in professional audio
equipment
AES47 - professional AES3-style digital
audio over Asynchronous Transfer
Mode networks
Intel High Definition Audio - modern
replacement for AC'97
I²S (Inter-IC sound) interface between
integrated circuits in consumer
electronics
MADI (Multichannel Audio Digital
Interface)
MIDI - low-bandwidth interconnect for
carrying instrument data; cannot carry
sound but can carry digital sample
data in non-realtime
S/PDIF - either over coaxial cable or
TOSLINK, common in consumer audio
equipment and derived from AES3
TDIF, TASCAM proprietary format with
D-sub cable
Several interfaces are engineered to
carry digital video and audio together,
including HDMI and DisplayPort.
For personal computers, USB and IEEE
1394 have provisions to deliver real-time
digital audio. In professional architectural
or installation applications, many audio
over Ethernet protocols and interfaces
exist. In broadcasting, a more general
audio over IP network technology is
favored. In telephony voice over IP is
used as a network interface for digital
audio for voice communications.
See also
Digital audio editor
Notes
1. Some audio signals such as those
created by digital synthesis originate
entirely in the digital domain, in
which case analog to digital
conversion does not take place.
References
1. Janssens, Jelle; Stijn Vandaele; Tom
Vander Beken (2009). "The Music
Industry on (the) Line? Surviving
Music Piracy in a Digital Era".
European Journal of Crime, Criminal
Law and Criminal Justice. 77 (96):
77–96.
doi:10.1163/157181709X429105 .
hdl:1854/LU-608677 .
2. Genius Unrecognised , BBC, 2011-0327, retrieved 2011-03-30
3. US patent 2605361 , C. Chapin
Cutler, "Differential Quantization of
Communication Signals", issued
1952-07-29
4. P. Cummiskey, Nikil S. Jayant, and J.
L. Flanagan, "Adaptive quantization in
differential PCM coding of speech",
Bell Syst. Tech. J., vol. 52, pp. 1105—
1118, Sept. 1973
5. Cummiskey, P.; Jayant, Nikil S.;
Flanagan, J. L. (1973). "Adaptive
quantization in differential PCM
coding of speech". The Bell System
Technical Journal. 52 (7): 1105–
1118. doi:10.1002/j.15387305.1973.tb02007.x . ISSN 00058580 .
. Schroeder, Manfred R. (2014). "Bell
Laboratories" . Acoustics,
Information, and Communication:
Memorial Volume in Honor of
Manfred R. Schroeder. Springer.
p. 388. ISBN 9783319056609.
7. Gray, Robert M. (2010). "A History of
Realtime Digital Speech on Packet
Networks: Part II of Linear Predictive
Coding and the Internet Protocol"
(PDF). Found. Trends Signal Process.
3 (4): 203–303.
doi:10.1561/2000000036 .
ISSN 1932-8346 .
. Ahmed, Nasir (January 1991). "How I
Came Up With the Discrete Cosine
Transform" . Digital Signal
Processing. 1 (1): 4–5.
doi:10.1016/1051-2004(91)90086-Z .
9. Nasir Ahmed; T. Natarajan; Kamisetty
Ramamohan Rao (January 1974).
"Discrete Cosine Transform" (PDF).
IEEE Transactions on Computers. C23 (1): 90–93. doi:10.1109/TC.1974.223784 .
10. J. P. Princen, A. W. Johnson und A. B.
Bradley: Subband/transform coding
using filter bank designs based on
time domain aliasing cancellation,
IEEE Proc. Intl. Conference on
Acoustics, Speech, and Signal
Processing (ICASSP), 2161–2164,
1987.
11. Luo, Fa-Long (2008). Mobile
Multimedia Broadcasting Standards:
Technology and Practice . Springer
Science & Business Media. p. 590.
ISBN 9780387782638.
12. Guckert, John (Spring 2012). "The
Use of FFT and MDCT in MP3 Audio
Compression" (PDF). University of
Utah. Retrieved 14 July 2019.
13. Fine, Thomas (2008). Barry R.
Ashpole (ed.). "The Dawn of
Commercial Digital Recording"
(PDF). ARSC Journal. Retrieved
2010-05-02.
14. Allstot, David J. (2016). "Switched
Capacitor Filters" (PDF). In
Maloberti, Franco; Davies, Anthony C.
(eds.). A Short History of Circuits and
Systems: From Green, Mobile,
Pervasive Networking to Big Data
Computing. IEEE Circuits and
Systems Society. pp. 105–110.
ISBN 9788793609860.
15. Floyd, Michael D.; Hillman, Garth D. (8
October 2018) [1st pub. 2000].
"Pulse-Code Modulation CodecFilters" . The Communications
Handbook (2nd ed.). CRC Press.
pp. 26–1, 26–2, 26–3.
Further reading
Borwick, John, ed., 1994: Sound
Recording Practice (Oxford: Oxford
University Press)
Bosi, Marina, and Goldberg, Richard E.,
2003: Introduction to Digital Audio
Coding and Standards (Springer)
Ifeachor, Emmanuel C., and Jervis,
Barrie W., 2002: Digital Signal
Processing: A Practical Approach
(Harlow, England: Pearson Education
Limited)
Rabiner, Lawrence R., and Gold,
Bernard, 1975: Theory and Application
of Digital Signal Processing (Englewood
Cliffs, New Jersey: Prentice-Hall, Inc.)
Watkinson, John, 1994: The Art of
Digital Audio (Oxford: Focal Press)
External links
Wikimedia Commons has media
related to Digital audio.
Monty Montgomery (2012-10-24).
"Guest Opinion: Why 24/192 Music
Downloads Make No Sense" .
evolver.fm. Retrieved 2012-12-07.
J. ROBERT STUART. "Coding High
Quality Digital Audio" (PDF). Archived
from the original (PDF) on 2007-06-27.
Retrieved 2012-12-07.
Dan Lavry. "Sampling Theory For
Digital Audio" (PDF). Retrieved
2012-12-07.
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