Digital audio

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This article is about the technology used to record sound. For the defunct magazine originally named "Digital Audio", see CD Review.
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Digital audio is a representation of

record production and telecommunications
in the 1990s and 2000s.

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 (PCM). This digital signal can then be recorded, edited, modified, and copied using computers, audio playback machines, and other digital tools. For playback, 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

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, mass-production, and distribution of sound, including recordings of

data compression. The availability of music as data files, rather than as physical objects, has significantly reduced the costs of distribution as well as made it easier to share copies.[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. Popular streaming services such as Apple Music, Spotify, or Youtube, offer temporary access to the digital file, and are now the most common form of music consumption.[2]

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.

digital system do not result in error unless they are 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]

A digital audio signal may 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 the channel code used for the audio compact disc
(CD).

Conversion process

Analog to Digital to Analog conversion
The lifecycle of sound from its source, through an ADC, digital processing, a DAC, and finally as sound again.

If an audio signal is analog, a digital audio system starts with an ADC that converts an analog signal to a digital signal.

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 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 band-limited version of the original analog signal can be accurately reconstructed from the digital signal.

During conversion, audio data can be embedded with a digital watermark to prevent piracy and unauthorized use. Watermarking is done using a direct-sequence spread-spectrum (DSSS) method. The audio information is then modulated by a pseudo-noise (PN) sequence, then shaped within the frequency domain and put back in the original signal. The strength of the embedding determines the strength of the watermark on the audio data.[4]

History

Coding

Main articles:
Audio data compression

Adaptive DPCM (ADPCM) was introduced by P. Cummiskey, Nikil S. Jayant and James L. Flanagan at Bell Labs in 1973.[7][8]

Perceptual coding was first used for speech coding compression, with linear predictive coding (LPC).[9] Initial concepts for LPC date back to the work of Fumitada Itakura (Nagoya University) and Shuzo Saito (Nippon Telegraph and Telephone) in 1966.[10] 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.[9]

MPEG Layer III),[15][9] Advanced Audio Coding (AAC), Windows Media Audio (WMA), and Vorbis (Ogg).[14]

Recording

Main article: Digital 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.[16]

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.[16]

Reel-to-Reel Tape Recorder

The first 16-bit PCM recording in the

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.[16]

Popular professional digital multitrack recorders produced by Sony/Studer (

reel-to-reel tape in either 1/4", 1/2", or 1" widths, with the audio data being recorded to the tape using a multi-track stationary tape head. PCM adaptors allowed for stereo digital audio recording on a conventional NTCS or PAL video tape recorder
.

The 1982 introduction of the CD popularized digital audio with consumers.[16]

DTRS
performed a similar function with Hi8 tapes.

Formats like ProDigi and DASH were referred to as SDAT (Stationary-head Digital Audio Tape) formats, as opposed to formats like the PCM adaptor-based systems and DAT, which were referred to as RDAT (Rotating-head Digital Audio Tape) formats, due to their helical-scan process of recording.

Like the DAT cassette, ProDigi and DASH machines also accommodated the obligatory 44.1 kHz sampling rate, but also 48 kHz on all machines, and eventually a 96 kHz sampling rate. They overcame the problems that made typical analog recorders unable to meet the bandwidth (frequency range) demands of digital recording by a combination of higher tape speeds, narrower head gaps used in combination with metal-formulation tapes, and the spreading of data across multiple parallel tracks.

Unlike analog systems, modern digital audio workstations and audio interfaces allow as many channels in as many different sampling rates as the computer can effectively run at a single time. Avid Audio and Steinberg released the first digital audio workstation software programs in 1989.[17] Digital audio workstations make multitrack recording and mixing much easier for large projects which would otherwise be difficult with analog equipment.

Telephony

Main article:
Digital telephony

The rapid development and wide adoption of PCM

digital audio tape
(DAT) recorder PCM-7030

Technologies

Digital audio is used in

Digital audio broadcasting (DAB), Digital Radio Mondiale (DRM), HD Radio and In-band on-channel
(IBOC).

Digital audio in recording applications is stored on audio-specific technologies including CD,

Digital Audio Workstation

Interfaces


For personal computers, USB and IEEE 1394 have provisions to deliver real-time digital audio. USB interfaces have become increasingly popular among independent audio engineers and producers due to their small size and ease of use. 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.

Several interfaces are engineered to carry digital video and audio together, including HDMI and DisplayPort. Some interfaces offer MIDI support as well as XLR and TRS analog ports.

Focusrite USB Interfaces

Digital-audio-specific interfaces include:

See also

Notes

  1. ^ Anti-alias filtering and optional digital signal processing may degrade the audio signal via passband ripple, non-linear phase shift, numeric precision quantization noise or time distortion of transients. However, these potential degradations can be limited by careful digital design.[3]
  2. ^ 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. hdl:1854/LU-608677
    .
  2. ISSN 0953-5438
    .
  3. ^ Story, Mike (September 1997). "A Suggested Explanation For (Some Of) The Audible Differences Between High Sample Rate And Conventional Sample Rate Audio Material" (PDF). dCS Ltd. Archived (PDF) from the original on 28 November 2009.
  4. S2CID 3008374
    .
  5. ^ Genius Unrecognised, BBC, 2011-03-27, retrieved 2011-03-30
  6. ^ US patent 2605361, C. Chapin Cutler, "Differential Quantization of Communication Signals", issued 1952-07-29 
  7. ^ 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
  8. ISSN 0005-8580
    .
  9. ^
    ISBN 9783319056609
    .
  10. ISSN 1932-8346
    .
  11. doi:10.1016/1051-2004(91)90086-Z
    .
  12. S2CID 149806273
    .
  13. ^ 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.
  14. ^
    ISBN 9780387782638
    .
  15. ^ Guckert, John (Spring 2012). "The Use of FFT and MDCT in MP3 Audio Compression" (PDF). University of Utah. Retrieved 14 July 2019.
  16. ^ a b c d Fine, Thomas (2008). Barry R. Ashpole (ed.). "The Dawn of Commercial Digital Recording" (PDF). ARSC Journal. Retrieved 2010-05-02.
  17. S2CID 242779244
    .
  18. ^
    ISBN 9788793609860. Archived from the original
    (PDF) on 2021-09-30. Retrieved 2019-11-29.
  19. ^
    ISBN 9781420041163
    .

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

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