Digital Audio

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.

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. However, it has brought about a 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 severely harms the profitability of their business.

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, amplification. All analogue audio signals are susceptible to noise and distortion, due to the inherent noise present in electronic circuits.

On the other hand, the digital audio chain begins when an analogue audio signal is converted into electrical signals — ‘on/off’ pulses — rather than electro-mechanical signals. This signal is then re-encoded (rather like a spy might use a code book), in order to combat any errors that might occur in the storage or transmission of the signal. It is this "channel coding" that is essential to the ability of the digital system 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 [hide]
1 Overview of digital audio
2 Subjective evaluation
3 History of digital audio use in commercial recording
4 Digital audio technologies
5 Digital audio interfaces
6 References
7 See also



[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. Both systems introduce noise at the capturing stage, in analogue recording this is due to the noise floor of the circuit, and in digital recording due to quantization noise.

Quantization Noise (file info) — play in browser (beta)
An example of audio with progressively worsening quantization noise.
Problems listening to the file? See media help.

Much like an analog audio system, a digital audio system strives to reproduce the audio perfectly but neither can ultimately prevail. Analog systems have inherent 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 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).

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 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 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
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.[dubious — see talk page] 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.)

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, 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
DAB (Digital Audio Broadcasting)
Digital audio workstation
Digital audio player
Storage technologies:

Digital Audio Tape (DAT)
Compact disc (CD)
DVD DVD-A
MiniDisc
Super Audio CD
various audio file formats

[edit] Digital audio interfaces
Audio-specific interfaces include:

AC97 (Audio Codec 1997) interface between Integrated circuits on PC motherboards
ADAT interface
AES/EBU interface with XLR connectors
AES47, Professional AES3 digital audio over Asynchronous Transfer Mode networks
I²S (Inter-IC sound) interface between Integrated circuits in consumer electronics
MIDI low-bandwidth interconnect for carrying instrument data; cannot carry sound
S/PDIF, either over coaxial cable or TOSLINK
TDIF, Tascam proprietary format with D-sub cable
Naturally, any digital bus (e.g., USB, FireWire, and PCI) can carry digital audio.