Digital audio

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Digital audio is a digital signal representing audio. Broadly speaking, digital audio encompasses audio whenever it is in digital form. This include analog-to-digital conversion, digital-to-analog conversion, storage, and transmission.

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[edit] The digital shift

Digital audio has emerged because of its supreme usefulness to sound recording, manipulation, mass-production and distribution. The modern day distribution of music across the internet through on-line stores depends on digital recording, and digital compression algorithms. "Dematerialization" of the music software into computer files has significantly reduced costs of distribution. However, it has brought about the concomitant rise in music sharing through peer to peer networks, which is illegal in many countries as copyright infringement. The Recording Industry Association of America and other organizations claim that music sharing is severely harming the profitability of their business.

From the Long-play gramophone record and compact cassette, the 78 rpm vinyl records and wax cylinders before them, analogue audio music storage and reproduction have been based on the same principles upon which human hearing are based. Sounds begin and end as mechanical energy wave forms in air, are captured in said wave form, and transformed into an electrical energy by a microphone transducer. Although its nature may change, its fundamental wave-like characteristics remain unchanged during its storage, transformation, duplication, amplification. Up until very recently, analogue audio has been susceptible to significant information loss, as noise and distortions tend to creep in at each stage.

On the other hand, the digital audio chain begins when sound is converted into electrical signals — ‘on/off’ pulses — rather than electro-mechanical signals. The advantage of digital audio is the ability to be copied or transmitted more conveniently, and with arguably lower loss. This ability to control signal losses is important in a professional studio environment, where signals could pass many times through cables, mixing desks and processing equipment before the recording is finally mixed down onto a two-track master for manufacturing.

[edit] Overview of digital audio

Digital audio, simply put, is audio in digital form. Specifically, digital audio is a digital signal encoded as bits of information.

An analog signal is converted to a digital signal at a given sampling rate and bit resolution; it very likely will also 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.

Much like an analog audio system, a digital audio system strives to reproduce the audio perfectly but neither can ultimately prevail. Analog systems have inhereit capacitance and inductance which limit the bandwidth of the system and resistance limits the amplitude. Digital systems' sampling rate limits the bandwidth and bit resolution limits the dynamic range (resolution of amplitude creation). Both systems require increased cost and attention to achieve higher fidelity.

A digital audio signal starts with an analog-to-digital converter (ADC) that converts, as the name implies, 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 (44,1000 Hz) and 16-bit resolution for both channels (stereo). If the analog signal is not bandlimited then an anti-aliasing filter is necessary to prevent aliasing in the digital signal. (Aliasing essentially means that higher frequencies — those above half the sampling rate — are aliased to other frequencies.)

One caveat to this is that audio can originate in the digital world such as with software synthesizers or digitally created sheet music. Creation in this manner does not rely upon a predetermined sampling rate or bit resolution.

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 iPod, a hard drive, USB flash drive, CompactFlash, or any other digital data storage device. Audio data compression techniques — such as MP3, Ogg Vorbis, or AAC — are commonly employed to reduce the size. Digital transmission 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

Fidelity evaluation is a long-standing issue with audio systems in general and introduction of lossy compression algorithms and psychoacoustic models have only increased debate.

Audio can be measured and analyzed to more exacting measures than can be done by ear, 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.

[edit] History of digital audio

Commercial digital recording of classical and jazz music began in the early 1970s, pioneered by Japanese companies such as Denon 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.)

The first 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. Stevie Wonder adopted the technology in early 1979 for Journey through the Secret Life of Plants and used it on all later recordings. 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, however, to eliminate 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

Stroage 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

Digital systems
Logic gate | digital circuit | integrated circuit (IC)
Theory — boolean algebra/logic | digital signal processing | computer architecture
Applications — digital photography | digital audio | digital video
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