Digital recording
Encyclopedia
In digital recording, digital audio
and digital video
is directly recorded to a storage device as a stream of discrete numbers, representing the changes in air pressure (sound
) for audio and chroma
and luminance
values for video through time
, thus making an abstract template for the original sound or moving image.
Analog audio (sound
), or analog video
made of a continuous wave must be converted into a stream of discrete numbers, representing the changes over time in air pressure for audio, and chroma
and luminance
values for video.
Beginning in the 1980s, music that was recorded, mixed and mastered digitally was often labelled using the SPARS code
to describe which processes were analog and which were digital.
Playback
, the read/write head moves as well as the tape in order to maintain a high enough speed to keep the bits at a manageable size.
For optical disc recording technologies
such as CDs or DVD
s, a laser
is used to burn microscopic holes into the dye layer of the medium. A weaker laser is used to read these signals. This works because the metallic substrate of the disc is reflective, and the unburned dye prevents reflection while the holes in the dye permit it, allowing digital data to be represented.
s used to represent a single audio wave (the word size) directly affects the achievable noise level of a signal recorded with added dither
, or the distortion
of an undithered signal. Increasing a sample's word length by one bit doubles its possible values, likewise increasing the potential accuracy of each sample and the fidelity of the recording to the original. 24-bit recording is generally considered a current practical limit as this word length allows a signal-to-noise ratio
exceeding that of most analog circuitry, which by necessity must be used in at least two points in the recording/playback chain.
is even more important a consideration than the word size. If the sample rate is too low, the sampled signal cannot be reconstructed to the original sound signal. Hence the output will be different from the input. The process of under sampling results in aliasing
whereby the high frequency components of the sound wave are represented as being lower than they should be. This causes the output
wave shape to be severely altered.
To overcome aliasing, the sound signal (or other signal) must be sampled at a rate at least twice that of the highest frequency component in the signal. This is known as the Nyquist-Shannon sampling theorem.
For recording music-quality audio the following PCM sampling rates are the most common:
44.1 kHz
48 kHz
88.2 kHz
96 kHz
176.4 kHz
192 kHz
When making a recording, experienced audio recording and mastering engineers will normally do a master recording at a higher sampling rate (i.e. 88.2, 96, 176.4 or 192 kHz) and then do any editing or mixing at that same higher frequency. High resolution PCM recordings have been released on DVD-Audio (also known as DVD-A), DAD (Digital Audio Disc—which utilizes the stereo PCM audio tracks of a regular DVD), DualDisc (utilizing the DVD-Audio layer), or Blu-ray (Profile 3.0 is the Blu-ray audio standard, although as of mid-2009 it is unclear whether this will ever really be used as an audio-only format). In addition it is nowadays also possible and common to release a high resolution recording directly as either an uncompressed WAV or lossless compressed FLAC file (usually at 24 bits) without down-converting it .
However if a CD (the CD Red Book standard is 44.1 kHz 16 bit) is to be made from a recording, then doing the initial recording using a sampling rate of 44.1 kHz is obviously one approach. Another approach that is usually preferred is to use a higher sample rate and then downsample
to the final format's sample rate. This is usually done as part of the mastering
process. One advantage to the latter approach is that way a high resolution recording can be released, as well as a CD and/or lossy compressed file such as mp3—all from the same master recording.
is its resistance to errors.
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
and digital video
Digital video
Digital video is a type of digital recording system that works by using a digital rather than an analog video signal.The terms camera, video camera, and camcorder are used interchangeably in this article.- History :...
is directly recorded to a storage device as a stream of discrete numbers, representing the changes in air pressure (sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
) for audio and chroma
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
and luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
values for video through time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
, thus making an abstract template for the original sound or moving image.
Analog audio (sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
), or analog video
Analog video
Analog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
made of a continuous wave must be converted into a stream of discrete numbers, representing the changes over time in air pressure for audio, and chroma
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
and luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
values for video.
Beginning in the 1980s, music that was recorded, mixed and mastered digitally was often labelled using the SPARS code
SPARS Code
SPARS is an acronym for the Society of Professional Audio Recording Services. The SPARS code is a three-letter code that appears on some Compact Disc recordings telling the consumer whether analog or digital equipment was used in producing the recording...
to describe which processes were analog and which were digital.
History
- In 1937, British scientist Alec ReevesAlec ReevesAlec Harley Reeves, CBE was a British scientist best known for his invention of pulse-code modulation . He was awarded 82 patents.-Early life:...
files the first patent describing Pulse-code modulationPulse-code modulationPulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
.
- In 1943, Bell Telephone LaboratoriesBell LabsBell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
develops the first digital scrambled speech transmission system, SIGSALYSIGSALYIn cryptography, SIGSALY was a secure speech system used in World War II for the highest-level Allied communications....
.
- In 1957, Max MathewsMax MathewsMax Vernon Mathews was a pioneer in the world of computer music.-Biography:...
of Bell develops the process to digitally record sound via computerComputerA computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
.
- In 1967, the first digital audioDigital audioDigital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
magnetic tape recorder is invented. A 12-bit 30 kHz stereo device using a compander (similar to DBX Noise ReductionDbx (noise reduction)dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo...
) to extend the dynamic range.
- In the 1970s, Thomas StockhamThomas StockhamThomas Greenway Stockham was an American scientist who developed the first practical digital audio recording system, and pioneered techniques for digital audio recording and processing as well....
makes the first digital audio recordings using standard computer equipment and develops a digital audio recorder of his own design, the first of its kind to be offered commercially (through Stockham's SoundstreamSoundstream-The Company:Soundstream Inc. was founded in 1975 in Salt Lake City, Utah by Dr. Thomas G. Stockham, Jr. It was the world’s first audiophile digital audio recording company, providing commercial services for recording and computer-based editing...
company).
- In 1970, James Russell patents the first digital-to-optical recording and playback system, which would later lead to the Compact DiscCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
.
- In 1972, Denon invents the first 8-track reel to reelReel to Reel* For the audio technology, see "Reel-to-reel audio tape recording"Reel to Reel is the debut album by Grand Puba. It was Puba's first solo venture, following group projects with the likes of the short lived group Masters of Ceremony and Brand Nubian. Both of the group’s albums were critically...
digital recorder.
- In 1978, Sound 80 Records of Minneapolis records "Flim and the BB's" (S80-DLR-102) directly to digital before pressing the vinyl LP. The mastering engineer is Bob Berglund. The recording system is a 3M Digital Audio Mastering System.
- In 1979, the first digital Compact DiscCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
prototype was created as a compromise between sound quality and size of the medium.
- In 1979, the first digitally recorded album of popular music Bop 'Til You Drop by guitarist Ry CooderRy CooderRyland Peter "Ry" Cooder is an American guitarist, singer and composer. He is known for his slide guitar work, his interest in roots music from the United States, and, more recently, his collaborations with traditional musicians from many countries.His solo work has been eclectic, encompassing...
is released by Warner Bros. RecordsWarner Bros. RecordsWarner Bros. Records Inc. is an American record label. It was the foundation label of the present-day Warner Music Group, and now operates as a wholly owned subsidiary of that corporation. It maintains a close relationship with its former parent, Warner Bros. Pictures, although the two companies...
. The album was recorded in Los AngelesLos ÁngelesLos Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...
on a 32-track digital machine built by the 3M3M3M Company , formerly known as the Minnesota Mining and Manufacturing Company, is an American multinational conglomerate corporation based in Maplewood, Minnesota, United States....
corporation.
- In 1982, the first digital compact discCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
s are marketed, and New England DigitalNew England DigitalNew England Digital Corp. , founded originally in Norwich, Vermont and eventually relocated to White River Junction, Vermont, was best known for its signature product, the Synclavier Synthesizer System, which evolved into the Synclavier Digital Audio System or "Tapeless Studio." The company sold...
offers the hard disk recorderHard disk recorderA hard disk recorder is a type of direct to disk recording system that uses a high-capacity hard disk to record digital audio or digital video. Hard disk recording systems represent an alternative to more traditional reel-to-reel tape or cassette multitrack systems, and provide editing...
(Sample-to-Disk) option on the SynclavierSynclavierThe Synclavier System was an early digital synthesizer, polyphonic digital sampling system, and music workstation, manufactured by New England Digital Corporation, Norwich, VT. The original design and development of the Synclavier prototype occurred at Dartmouth College with the collaboration of...
, the first commercial hard diskHard diskA hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
(HDD) recording system.
- In 1985 AkaiAkaiAkai is a consumer electronics brand, founded by Saburo Akai as , a Japanese manufacturer in 1929. It is now headquartered in Singapore as a subsidiary of Grande Holdings, a Hong Kong-based conglomerate, which also owns the formerly Japanese brands Nakamichi and Sansui. The Akai brand is now used...
announced the 12 track mixer and tape recorder. This tape format used for simultaneously recording 12 tracks of digital audioDigital audioDigital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
at once, onto Super VHS magnetic tapeMagnetic tapeMagnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
- a format similar to that used by consumer VCRs. Recorder used the dbxDbx (noise reduction)dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo...
type II for better head-room.
- In 1990, digital radioDigital radioDigital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
begins in CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, using the L-Band.
- In 1991 AlesisAlesisAlesis is a company based in Cumberland, Rhode Island, that designs and markets electronic musical instruments, digital audio processors, audio mixers, digital audio interfaces, recording equipment, drum machines, professional audio and electronic percussion products...
Digital Audio Tape or ADATADATAlesis Digital Audio Tape or ADAT is a magnetic tape format used for the simultaneous digital recording of eight analog audio or digital audio tracks at once, onto a Super VHS tape that is used by consumer VCRs.- History :...
is a tape format used for simultaneously recording eight tracks of digital audioDigital audioDigital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
at once, onto Super VHS magnetic tapeMagnetic tapeMagnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
- a format similar to that used by consumer VCRs. The product was announced in January 1991 at the NAMM convention in Anaheim, CaliforniaCaliforniaCalifornia is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. The first ADAT recorders shipped over a year later in February or March 1992.
- In 1996 optical discOptical discIn computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
s and DVD playerDVD playerA DVD player is a device that plays discs produced under both the DVD-Video and DVD-Audio technical standards, two different and incompatible standards. These devices were invented in 1997 and continue to thrive...
s begin selling in JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
.
Process
Recording- The analog signal is transmitted from the input deviceInput deviceIn computing, an input device is any peripheral used to provide data and control signals to an information processing system such as a computer or other information appliance...
to an analog-to-digital converterAnalog-to-digital converterAn analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
(ADC). - The ADC converts this signal by repeatedly measuring the momentary level of the analog (audio) wave and then assigning a binary number with a given quantity of bits (word length) to each measuring point.
- The frequency at which the ADC measures the level of the analog wave is called the sample rate or sampling rate.
- A digital audio sample with a given word length represents the audio level at one moment.
- The longer the word length the more exact is the representation of the original audio wave levelwise.
- The higher the sampling rate the higher the upper cutoff frequency of the digitized audio signal.
- The ADC outputs a sequence of samples that make up a continuous stream of 0s and 1s.
- These numbers are stored onto recording media such as magnetic tapeMagnetic tapeMagnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
, hard drive, optical drive or solid state memory.
Playback
- The sequence of numbers is transmitted from storage into a digital-to-analog converterDigital-to-analog converterIn electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
(DAC), which converts the numbers back to an analog signal by sticking together the level information stored in each digital sample, thus rebuilding the original analog wave form. - This signal is amplified and transmitted to the loudspeakerLoudspeakerA loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s or video screen.
Getting the bits recorded
Even after getting the signal converted to bits, it is still difficult to record: the hardest part is finding a scheme that can record the bits fast enough to keep up with the signal. For example, to record two channels of audio at 44.1 kHz sample rate with a 16 bit word size, the recording software has to handle 1,411,200 bits per second.Techniques to record to commercial media
For digital cassettesDigital cassettes
Digital audio cassette formats introduced to the professional audio and consumer markets:*Digital Audio Tape is the most well-known, and had some success as an audio storage format among professionals and "prosumers" before the prices of hard drive and solid-state flash memory-based digital...
, the read/write head moves as well as the tape in order to maintain a high enough speed to keep the bits at a manageable size.
For optical disc recording technologies
Optical disc recording technologies
Optical disc authoring requires a number of different optical disc recorder technologies working in tandem, from the optical disc media to the firmware to the control electronics of the optical disc drive...
such as CDs or DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
s, a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is used to burn microscopic holes into the dye layer of the medium. A weaker laser is used to read these signals. This works because the metallic substrate of the disc is reflective, and the unburned dye prevents reflection while the holes in the dye permit it, allowing digital data to be represented.
Word Size
The number of bitBit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s used to represent a single audio wave (the word size) directly affects the achievable noise level of a signal recorded with added dither
Dither
Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...
, or the distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
of an undithered signal. Increasing a sample's word length by one bit doubles its possible values, likewise increasing the potential accuracy of each sample and the fidelity of the recording to the original. 24-bit recording is generally considered a current practical limit as this word length allows a signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
exceeding that of most analog circuitry, which by necessity must be used in at least two points in the recording/playback chain.
Sample rate
The sample rateSampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
is even more important a consideration than the word size. If the sample rate is too low, the sampled signal cannot be reconstructed to the original sound signal. Hence the output will be different from the input. The process of under sampling results in aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
whereby the high frequency components of the sound wave are represented as being lower than they should be. This causes the output
Output
Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...
wave shape to be severely altered.
To overcome aliasing, the sound signal (or other signal) must be sampled at a rate at least twice that of the highest frequency component in the signal. This is known as the Nyquist-Shannon sampling theorem.
For recording music-quality audio the following PCM sampling rates are the most common:
44.1 kHz
48 kHz
88.2 kHz
96 kHz
176.4 kHz
192 kHz
When making a recording, experienced audio recording and mastering engineers will normally do a master recording at a higher sampling rate (i.e. 88.2, 96, 176.4 or 192 kHz) and then do any editing or mixing at that same higher frequency. High resolution PCM recordings have been released on DVD-Audio (also known as DVD-A), DAD (Digital Audio Disc—which utilizes the stereo PCM audio tracks of a regular DVD), DualDisc (utilizing the DVD-Audio layer), or Blu-ray (Profile 3.0 is the Blu-ray audio standard, although as of mid-2009 it is unclear whether this will ever really be used as an audio-only format). In addition it is nowadays also possible and common to release a high resolution recording directly as either an uncompressed WAV or lossless compressed FLAC file (usually at 24 bits) without down-converting it .
However if a CD (the CD Red Book standard is 44.1 kHz 16 bit) is to be made from a recording, then doing the initial recording using a sampling rate of 44.1 kHz is obviously one approach. Another approach that is usually preferred is to use a higher sample rate and then downsample
Sample rate conversion
Sample rate conversion is the process of converting a signal from one sampling rate to another, while changing the information carried by the signal as little as possible...
to the final format's sample rate. This is usually done as part of the mastering
Audio mastering
Mastering, a form of audio post-production, is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device ; the source from which all copies will be produced...
process. One advantage to the latter approach is that way a high resolution recording can be released, as well as a CD and/or lossy compressed file such as mp3—all from the same master recording.
Error rectification
One of the advantages of digital recording over analog recordingis its resistance to errors.
See also
- Analog recording vs. digital recording
- Compact discCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
s use Reed-Solomon error correction - Cyclic redundancy checkCyclic redundancy checkA cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
(CRC) - Direct to disk recordingDirect to Disk RecordingDirect-to-disk recording refers to methods by which analog signals and digital signals such as digital audio and digital video are digitally recorded to optical disc recording technologies such as DVDs, and CD optical discs...
- Magnetic storageMagnetic storageMagnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using...
- Multitrack recordingMultitrack recordingMultitrack recording is a method of sound recording that allows for the separate recording of multiple sound sources to create a cohesive whole...
- Parity ComputationParity bitA parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
- Many bits are stored on RAID storage systemsRAIDRAID is a storage technology that combines multiple disk drive components into a logical unit...
.