Digital audio is a representation of sound recorded in, or converted into,digital form. In digital audio, thesound wave of theaudio signal is typically encoded as numericalsamples in a continuous sequence. For example, inCD audio,samples are taken 44,100times per second, each with 16-bitresolution. Digital audio is also the name for the entire technology ofsound recording and reproduction using audio signals that have been encoded in digital form. Following significant advances in digital audio technology during the 1970s and 1980s, it gradually replacedanalog audio technology in many areas ofaudio engineering,record production andtelecommunications in the 1990s and 2000s.
In a digital audio system, ananalog electrical signal representing the sound is converted with ananalog-to-digital converter (ADC) into a digital signal, typically usingpulse-code modulation (PCM). This digital signal can then be recorded, edited, modified, and copied usingcomputers, audio playback machines, and other digital tools. For playback, adigital-to-analog converter (DAC) performs the reverse process, converting a digital signal back into an analog signal, which is then sent through anaudio power amplifier and ultimately to aloudspeaker.
Digital audio systems may includecompression,storage,processing, andtransmission 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 ingeneration loss and degradation of signal quality, digital audio allows an infinite number of copies to be made without any degradation of signal quality.
Digital audio technologies are used in the recording, manipulation, mass-production, and distribution of sound, including recordings ofsongs, instrumental pieces,podcasts, sound effects, and other sounds. Modernonline music distribution depends on digital recording anddata compression. The availability of music as data files, rather than as physical objects, has significantly reduced the costs of distribution as well as making it easier to share copies.[1] Before digital audio, the music industry distributed and sold music by selling physical copies in the form ofrecords andcassette tapes. With digital audio and online distribution systems such asiTunes, companies sell digital sound files to consumers, which the consumer receives over the Internet. Popular streaming services such asApple Music,Spotify, orYouTube, 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 atransducer, such as amicrophone. The sounds are then stored on an analog medium such asmagnetic tape, or transmitted through an analog medium such as atelephone line orradio. The process is reversed for reproduction: the electrical audio signal isamplified and then converted back into physical waveforms via aloudspeaker. Analog audio retains its fundamental wave-like characteristics throughout its storage, transformation, duplication, and amplification.
Analog audio signals are susceptible to noise and distortion, due to the innate characteristics of electronic circuits and associated devices. Disturbances in adigital system do not result in error unless they are so large as to result in a symbol being misinterpreted as another symbol or disturbing 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 aschannel coding, is essential for broadcast or recorded digital systems to maintain bit accuracy.Eight-to-fourteen modulation is the channel code used for the audiocompact disc (CD).
If an audio signal is analog, a digital audio system starts with an ADC that converts an analog signal to a digital signal.[b] The ADC runs at a specifiedsampling rate and converts at a known bit resolution.CD audio, for example, has a sampling rate of 44.1 kHz (44,100 samples per second), and has 16-bitresolution for eachstereo channel. Analog signals that have not already beenbandlimited must be passed through ananti-aliasing filter before conversion, to prevent thealiasing distortion that is caused by audio signals with frequencies higher than theNyquist frequency (half the sampling rate).
A digital audio signal may be stored or transmitted. Digital audio can be stored on a CD, adigital audio player, ahard drive, aUSB flash drive, or any other digitaldata storage device. The digital signal may be altered throughdigital signal processing, where it may befiltered or haveeffects applied.Sample-rate conversion, includingupsampling anddownsampling, may be used to change signals that have been encoded with a different sampling rate to a common sampling rate prior to processing. Audio data compression techniques, such asMP3,Advanced Audio Coding (AAC),Opus,Ogg Vorbis, orFLAC, are commonly employed to reduce the file size. Digital audio can be carried overdigital audio interfaces such asAES3 orMADI. Digital audio can be carried over a network usingaudio over Ethernet,audio over IP or otherstreaming media standards and systems.
For playback, digital audio must be converted back to an analog signal with a DAC. According to theNyquist–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 adigital watermark to prevent piracy and unauthorized use. Watermarking is done using adirect-sequence spread-spectrum (DSSS) method. The audio information is then modulated by apseudo-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]
Pulse-code modulation (PCM) was invented by British scientistAlec Reeves in 1937.[5] In 1950,C. Chapin Cutler ofBell Labs filed the patent ondifferential pulse-code modulation (DPCM),[6] adata compression algorithm.Adaptive DPCM (ADPCM) was introduced by P. Cummiskey,Nikil S. Jayant andJames L. Flanagan at Bell Labs in 1973.[7][8]
Perceptual coding was first used forspeech coding compression, withlinear predictive coding (LPC).[9] Initial concepts for LPC date back to the work ofFumitada Itakura (Nagoya University) and Shuzo Saito (Nippon Telegraph and Telephone) in 1966.[10] During the 1970s,Bishnu S. Atal andManfred R. Schroeder at Bell Labs developed a form of LPC calledadaptive predictive coding (APC), a perceptual coding algorithm that exploited the masking properties of the human ear, followed in the early 1980s with thecode-excited linear prediction (CELP) algorithm.[9]
Discrete cosine transform (DCT) coding, alossy compression method first proposed byNasir Ahmed in 1972,[11][12] provided the basis for themodified discrete cosine transform (MDCT), which was developed by J. P. Princen, A. W. Johnson and A. B. Bradley in 1987.[13] The MDCT is the basis for mostaudio coding standards, such asDolby Digital (AC-3),[14] MP3 (MPEG Layer III),[15][9] AAC,Windows Media Audio (WMA), Opus andVorbis (Ogg).[14]
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PCM was used intelecommunications applications long before its first use in commercial broadcast and recording. Commercial digital recording was pioneered in Japan byNHK andNippon Columbia and theirDenon brand, in the 1960s. The first commercial digital recordings were released in 1971.[16]
TheBBC 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]
The first 16-bit PCM recording in theUnited States was made byThomas Stockham at theSanta Fe Opera in 1976, on aSoundstream recorder. An improved version of the Soundstream system was used to produce several classical recordings byTelarc in 1978. The3M digitalmultitrack recorder in development at the time was based on BBC technology. The first all-digital album recorded on this machine wasRy Cooder'sBop till You Drop in 1979. British record labelDecca 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 (DASH) and Mitsubishi (ProDigi) in the early 1980s helped to bring about digital recording's acceptance by the major record companies. Machines for these formats had their own transports built in as well, usingreel-to-reel tape in either 1/4, 1/2, or 1-inch 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 NTSC or PALvideo tape recorder.
The 1982 introduction of the CD byPhilips andSony popularized digital audio with consumers.[16]
ADAT became available in the early 1990s, which allowed eight-track44.1 or48 kHz recording on S-VHS cassettes, andDTRS performed a similar function with Hi8 tapes.
Formats like ProDigi and DASH were referred to asSDAT (stationary-head digital audio tape) formats, as opposed to formats like the PCM adaptor-based systems andDigital Audio Tape (DAT), which were referred to asRDAT (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, moderndigital audio workstations andaudio interfaces allow as many channels in as many different sampling rates as the computer can effectively run at a single time.Avid Audio andSteinberg 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.
The rapid development and wide adoption of PCMdigital telephony was enabled bymetal–oxide–semiconductor (MOS)switched capacitor (SC) circuit technology, developed in the early 1970s.[18] This led to the development of PCM codec-filter chips in the late 1970s.[18][19] Thesilicon-gateCMOS (complementary MOS) PCM codec-filter chip, developed byDavid A. Hodges and W.C. Black in 1980,[18] has since been the industry standard for digital telephony.[18][19] By the 1990s,telecommunication networks such as thepublic switched telephone network (PSTN) had been largelydigitized withVLSI (verylarge-scale integration) CMOS PCM codec-filters, widely used inelectronic switching systems fortelephone exchanges, user-endmodems and a range ofdigital transmission applications such as theintegrated services digital network (ISDN),cordless telephones andcell phones.[19]
Digital audio is used inbroadcasting of audio. Standard technologies includeDigital audio broadcasting (DAB),Digital Radio Mondiale (DRM),HD Radio andIn-band on-channel (IBOC).
Digital audio in recording applications is stored on audio-specific technologies including CD, DAT,Digital Compact Cassette (DCC) andMiniDisc. Digital audio may be stored in a standardaudio file formats and stored on aHard disk recorder,Blu-ray orDVD-Audio. Files may be played back on smartphones, computers orMP3 player. Digital audio resolution is measured inaudio bit depth. Most digital audio formats use either 16-bit, 24-bit, and 32-bit resolution.
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