Digital audio

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A sound wave, in gray, represented digitally, in red (after a zero-order hold but before filtering)

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.

Contents

[edit] Overview of digital audio

Sampling and 4-bit quantization of an analogue signal (red) using Pulse Code Modulation.

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.

[edit] 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 increase overall cost.
Quantization Noise
An example of audio with progressively worsening quantization noise.
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[edit] 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, an MP3 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.

[edit] Subjective evaluation

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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.[dubious – discuss] 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.

[edit] 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. Others, such as former Beatles producer George Martin, felt that the multitrack digital recording technology of the early 1980s had not reached the sophistication of analogue systems. Martin used digital mixing,[citation needed] however, to reduce the distortion and noise that an analogue master tape would introduce (thus ADD). An early example of an analogue recording that was digitally mixed is Fleetwood Mac's 1979 release Tusk.

[edit] Digital audio technologies

Storage technologies:

[edit] Digital audio interfaces

Audio-specific interfaces include:

Naturally, any digital bus (e.g., USB, FireWire, and PCI) can carry digital audio.

[edit] 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)

[edit] See also

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Digital systems
Components
Theory
Applications
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