Digital audio uses
digital signals for
sound reproduction. This
includes
analog-to-digital
conversion,
digital-to-analog conversion,
storage, and transmission. In effect, the system commonly referred
to as digital is in fact a discrete-time, discrete-level analog of
a previous electrical analog. While modern systems can be quite
subtle in their methods, the primary usefulness of a digital system
is that, due to its discrete (in both time and amplitude) nature,
signals can be corrected, once they are digital, without loss, and
the digital signal can be reconstituted. The discreteness in both
time and amplitude is key to this reconstitution, which is
unavailable for a signal in which at least one of time or amplitude
is continuous. While the hybrid systems (part discrete, part
continuous) exist, they are no longer used for new modern
systems.
Digital audio has emerged because of its usefulness in the
recording, manipulation, mass-production, and distribution of
sound. Modern distribution of music across the internet through
on-line stores depends on digital recording and digital
compression algorithms. Distribution of audio as data files
rather than as physical objects has significantly reduced costs of
distribution.
From the
wax cylinder to the
compact cassette, analogue audio
music storage and reproduction have been based on the same
principles upon which human hearing are based. In an analogue audio
system, sounds begin as physical waveforms in the air, are
transformed into an electrical representation of the waveform, via
a transducer (for example, a microphone), and are stored or
transmitted. To be re-created into sound, the process is reversed,
through amplification and then conversion back into physical
waveforms via a loudspeaker. Although its nature may change, its
fundamental wave-like characteristics remain unchanged during its
storage, transformation, duplication, and amplification. All
analogue audio signals are susceptible to noise and distortion, due
to the inherent noise present in electronic circuits. In other
words, all distortion and noise in a digital signal are added at
capture or processing, and no more is added in repeated copies,
unless the entire signal is lost, while analog systems degrade at
each step, with each copy, and in some media, with time,
temperature, and magnetic or chemical issues.
The digital audio chain begins when an
analogue audio signal is first sampled, and
then (for
PCM, the usual form
of digital audio) converted into
binary signals — ‘on/off’ pulses —
which are stored as binary electronic, magnetic, or optical
signals, rather than as continuous time, continuous level
electronic or electromechanical signals. This signal may then
further encoded to combat any errors that might occur in the
storage or transmission of the signal, however this encoding is for
the purpose of error correction, and is not strictly part of the
digital audio process. This "channel coding" is essential to the
ability of broadcast or recorded digital system to avoid loss of
bit accuracy. The discrete time and level of the binary signal
allow a decoder to recreate the analogue signal upon replay. An
example of a channel code is
Eight to Fourteen Bit
Modulation as used in the
audio Compact Disc.
Overview of digital audio
Digital audio is the method of representing
audio in
digital form.
An
analog signal is converted to a
digital signal at a given
sampling rate and
bit resolution; it may contain multiple
channels (2 channels for
stereo
or more for
surround sound).Generally
speaking: the higher the sampling rate and bit resolution the more
fidelity, as well as increase the amount of
digital data.
Sound quality
While the goal of both analogue and digital systems is to reproduce
audio perfectly, there are several obstacles to achieving this,
including:
In order to achieve better fidelity, higher quality components are
required, which increases overall cost.
Conversion process
A digital audio signal starts with an
analog-to-digital converter
(ADC) that converts an analog signal to a digital signal. The ADC
runs at a sampling rate and converts at a known bit resolution. For
example,
CD audio has a
sampling rate of 44.1
kHz (44,100 samples per second) and 16-bit
resolution for each channel (stereo). If the analog signal is not
already
bandlimited then an
anti-aliasing filter is necessary
before conversion, to prevent
aliasing in
the digital signal. (Aliasing occurs when frequencies above the
Nyquist frequency have not been
band limited, and instead appear as audible artifacts in the lower
frequencies).
An overview of the digital <->
analogue conversion process.
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.
After being sampled with the ADC, the digital signal may then be
altered in a process which is called
digital signal processing where it
may be
filter or have
effect applied.
The digital audio signal may then be stored or transmitted.Digital
audio storage can be on a
CD, a
digital audio player, a
hard drive,
USB flash drive,
CompactFlash, or
any other digital
data storage device.
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
streamed to other devices.
The last step for digital audio is to be converted back to an
analog signal with a
digital-to-analog converter
(DAC).Like ADCs, DACs run at a specific sampling rate and bit
resolution but through the processes of
oversampling,
upsampling, and
downsampling, this sampling rate may not be the
same as the initial sampling rate.
Subjective evaluation
Fidelity evaluation is a long-standing issue with audio systems in
general and introduction of
lossy
compression algorithms and
psychoacoustic models has only increased
debate.
Audio can be measured and analyzed more precisely than can be done
manually by listening to the content, but what this technical
measurement and analysis lacks is the ability to determine if it
sounds "good" or "bad" to any given listener.Like any other human
opinion, there are numerous parameters that widely vary between
people that affect their subjective evaluation of what is good or
bad.Such things that pertain to audio include hearing capabilities,
personal preferences, location with respect to the speakers, and
the room's physical properties.
This is not to say that subjective evaluation is unique to digital
audio; digital audio can add to the fervor of discussion because it
does introduce more things (e.g., lossy compression, psychoacoustic
models) that can be debated.
History of digital audio use in commercial recording
Commercial digital recording of classical and jazz music began in
the early 1970s, pioneered by Japanese companies such as
Denon, the
BBC, and British record
label
Decca (who in the mid-70s
developed digital audio recorders of their own design for mastering
of their albums), although experimental recordings exist from the
1960s.
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.
In most cases there was no mixing stage involved; a stereo digital
recording was made and used unaltered as the master tape for
subsequent commercial release. These unmixed digital recordings are
still described as
DDD since the
technology involved is purely digital. (Unmixed analogue recordings
are likewise usually described as
ADD to
denote a single generation of analogue recording.)
Although
the first-ever digital recording of a non-classical music piece,
Morrissey-Mullen's cover of the
Rose Royce hit "Love Don't Live Here Anymore"
(released 1979 as a vinyl EP) was recorded in 1978 at EMI's Abbey
Road
recording studios, the first entirely digitally
recorded (DDD) popular music album was Ry
Cooder's Bop Till You
Drop, recorded in late 1978. It was unmixed, being
recorded straight to a two-track
3M digital
recorder in the studio. Many other top recording artists were early
adherents of digital recording.
Digital audio technologies
Digital audio broadcasting
Storage technologies:
Digital audio interfaces
Audio-specific interfaces include:
Naturally, any digital bus (e.g.,
USB,
FireWire, and
PCI) can carry digital audio. Also, several
interfaces are engineered to carry digital video and audio
together, including
HDMI and
DisplayPort.
References
- Borwick, John, ed., 1994: Sound Recording Practice
(Oxford: Oxford University Press)
- 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)
- Bosi, Marina, and Goldberg, Richard E., 2003: Introduction
to Digital Audio Coding and Standards (Springer)
See also
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