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DIGITAL AUDIO
Digital audio comprises audio signals stored in a digital format. Specifically, the term encompasses the following:
- Audio conversion:
- Analogue to digital conversion (ADC) - the sampling and quantization of an analogue audio signal.
- Digital to analogue conversion (DAC) - the conversion of digital audio to a line level signal for playback or distribution.
- Audio signal processing - processing the digital signal in some way, such as to apply equalisation, reverberation, or to perform sample rate conversion.
- Storage, retrieval, and transmission of digital information in an audio format such as CD, MP3, Ogg Vorbis, etc.
The digital paradigm
Digital technology 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 thorough peer to peer networks
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 is 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.
Overview of digital audio
Sound inherently begins and ends as an analogue signal, and in order for the benefits of digital audio to be realised, the integrity of the signal during transmission must be preserved. The conversion process at both ends of the chain must also be of low loss in order to ensure sonic fidelity.
In an audio context, the digital ideal would be to reproduce signals sounding close to the original analogue signal. In other words, the theoretical limits of the human auditory system governs the technical scheme used during the conversion process, at least as of mid 2006. However, conversion is "lossy": conversion and compression algorithms deliberately discard the original sound information, mainly harmonics, outside the theoretical audio bandwidth. (This is discussed in the articles about CDs and MP3.)
Digital information is also lost in transfer through misreading, but can be "restored" by error correction and interpolation circuitry. Put another way, the information is only lost on the conversion from analogue to digital (and vice versa), and the amount of loss can be more predictable.
The restoration of the original music waveforms by decompression during playback should exactly mirror the compression process. However, upper harmonics which have been discarded can never be restored, with complete accuracy or otherwise. Upon its re-conversion into analogue via the amplifier/loudspeaker, the scheme relies heavily on the human brain to “fill in the gaps” – that is to say supply the missing sound during playback. This capability has been well discussed especially with respect to the brain supplying the fundamental frequency of a tone.
The generally accepted frequency response of human hearing is from 20Hz - 20kHz. According to Nyquist, the maximum bandwidth that can be represented by a digital signal less is half that of the sample rate. This leads to a required sample rate of at least 40 kHz. In practise, a slightly higher sample rate is needed to allow for a practical anti-aliasing filter. But, the Shannon equation for reconstructing the original data fully requires infinite samples.
In the early days of digital audio, the only practical storage device with sufficient bandwidth and storage space was a video recorder and these were adapted to store the digital signal, usually by interfacing said video recorder to a PCM adaptor. Some simple mathematics shows that it is possible to use either 525/60 NTSC or 625/50 PAL video with a sampling rate of 44.1 kHz, a sample rate which persisted with the introduction of CD.
16 bit digital audio was adopted as the broadcast standard because it offers 96 decibels (dB) of dynamic range, enough to match the quality of broadcast analogue. Modern systems do not suffer as much from the earlier constraints of bandwidth and storage space; 96 kHz and 192 kHz sample rates and 24-bit samples are now common. The sample rate timing can now be quite precise.
Pulse-code Modulation (PCM) is by far the most common way of representing a digital signal. It is simple and is compressed. A PCM representation of an analogue signal is generated by measuring (sampling) the instantaneous amplitude of the analogue signal, and then quantising the result to the nearest bit. However, such rounding contributes to the loss of the original information.
Subjective evaluation
Whether a sound is "good" or not is subjective -- it can not be easily or objectively measured. It will depend upon the listener's preferences and hearing capabilities, the listener's and speaker placement in a given room, and the room's physical properties. The idea is to reproduce the music in such a manner that the sonic and emotional message is faithfully communicated to the listener: for example where replay of a live recording captures the sensation of being at a "live" performance.
The arguments are valid for the evaluation of any audio system and not exclusively digital systems. Whilst "controlled "listening tests are difficult, a musician who has played the song, or one who attended several symphony concerts could be a good judge. Of interest are qualities like pitch, echo, spatial origins, tone, timing, phase, excitement/pace, body, timbre, detail, dynamic range and body.
History of digital audio
Commercial digital recording of classical and jazz music began in the early 1970s, pioneered by Japanese companies such as Denon, 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.
Digital audio technologies
Digital audio interfaces
Audio signals can also be carried losslessly over general-purpose buses such as USB or FireWire.
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)
See also
External links
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