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:
- Digital quantization noise in the capturing circuitry, and sampling rate limits the bandwidth and its bit resolution limits the dynamic range (resolution of amplitude creation).
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
filtered or have
effects 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:
- MADI Multichannel Audio Digital Interface
- MIDI low-bandwidth interconnect for carrying instrument data; cannot carry sound
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.
