User:Rlevan3/Digital audio

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Overview

Digital audio technologies are used in the recording, manipulation, mass-production, and distribution of sound, including recordings of songs, instrumental pieces, podcasts, sound effects, and other sounds. Modern online music distribution depends on digital recording and 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. 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.

Conversion process[edit]

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. The ADC runs at a specified sampling 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-bit resolution for each stereo channel. Analog signals that have not already been 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 player, a hard drive, a USB flash drive, or any other digital data storage device. The digital signal may be altered through digital signal processing, where it may be filtered or have effects applied. Sample-rate conversion including upsampling and downsampling 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 as MP3, Advanced Audio Coding, Ogg Vorbis, or FLAC, are commonly employed to reduce the file size. Digital audio can be carried over 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.

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.

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.

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.

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. An improved version of the Soundstream system was used to produce several classical recordings by Telarc in 1978. The 3M digital multitrack recorder in development at the time was based on BBC technology. The first all-digital album recorded on this machine was Ry Cooder's 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.

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, using 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.

ADAT became available in the early 1990s, which allowed eight-track 44.1 or 48 kHz recording on S-VHS cassettes, and 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.

Digital Audio Workstation

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. Soundstream created the first Digital audio workstation in 1977 which consisted of a minicomputer, disk drive, and video display. Digital audio workstations have become a standard in digital audio recording, as they have become the main hub to any recording studio. This makes multitrack recording and mixing much easier for large projects which would otherwise be difficult with analog gear.

Technologies

Digital audio is used in broadcasting of audio. Standard technologies include Digital audio broadcasting (DAB), Digital Radio Mondiale (DRM), HD Radio and In-band on-channel (IBOC).

Digital audio in Digital recording applications is stored on audio-specific technologies including CD, Digital Audio Tape (DAT), Digital Compact Cassette (DCC) and MiniDisc. Digital audio may be stored in a standard audio file formats and stored on a Hard disk recorder, Blu-ray or DVD-Audio. Files may be played back on smartphones, computers or MP3 player. Digital audio resolution is measured in sample depth. Most digital audio formats use a sample depth of either 16-bit, 24-bit, and 32-bit.

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

Focusrite USB Interfaces

Digital-audio-specific interfaces include:

• A2DP via Bluetooth AC'97 (Audio Codec 1997) interface between integrated circuits on PC motherboards
• ADAT Lightpipe interface
• AES3 interface with XLR connectors, common in professional audio equipment
• AES47 - professional AES3-style digital audio over Asynchronous Transfer Mode networks
• Intel High Definition Audio - modern replacement for AC'97
• I²S (Inter-IC sound) interface between integrated circuits in consumer electronics
• MADI (Multichannel Audio Digital Interface)
• MIDI - low-bandwidth interconnect for carrying instrument data; cannot carry sound but can carry digital sample data in non-real time
• S/PDIF - either over coaxial cable or TOSLINK, common in consumer audio equipment and derived from AES3
• TDIF, TASCAM proprietary format with D-sub cable