In digital recording, audio signals picked up by a microphone or other transducer or video signals picked up by a camera or similar device are converted into a stream of discrete numbers, representing the changes over time in air pressure for audio, and chroma and luminance values for video, then recorded to a storage device. To play back a digital sound recording, the numbers are retrieved and converted back into their original analog waveforms so that they can be heard through a loudspeaker. To play back a digital video recording, the numbers are retrieved and converted back into their original analog waveforms so that they can be viewed on a video monitor, television or other display.
- October 3, 1938: British telephone engineer Alec Harley Reeves files at the French Patent Office the first patent describing the technique known today as pulse-code modulation (PCM). Later, Reeves files also in the USA on November 22, 1939. It was first proposed as a telephony technology.
- 1943: Bell Telephone Laboratories develops the first PCM-based digital scrambled speech transmission system, SIGSALY, in response to German interception of military telephone traffic during World War II. The twelve transmission points were retired after the war.
- June 1950: Differential pulse-code modulation (DPCM) developed by C. Chapin Cutler at Bell Labs.
- 1957: Max Mathews of Bell develops the process to digitally record sound on a computer.
- November 1959: Metal-oxide-semiconductor field-effect transistor (MOSFET, or MOS transistor) invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs. MOS technology is the basis of digital technologies such as large-scale integration (LSI) chips, microprocessors, image sensors used in digital imaging, and digital signal processor (DSP) chips used in audio signal processing and digital image processing.
- 1966: Linear predictive coding (LPC), an audio signal processing and speech coding technology, first proposed by Fumitada Itakura of Nagoya University and Shuzo Saito of Nippon Telegraph and Telephone (NTT). LPC was the basis for voice-over-IP (VoIP) technology, as well as speech processing chips, such as the Texas Instruments LPC Speech Chips used in the Speak & Spell toys from 1978.
- 1967: The first monaural PCM encoder was developed by NHK's research facilities in Japan. The 30 kHz 12-bit device used a compander (similar to DBX Noise Reduction) to extend the dynamic range, and stored the signals on a video tape recorder.
- 1969: NHK expands the PCM encoder's capabilities to 2-channel stereo and 32 kHz 13-bit resolution.
- 1969: The charge-coupled device, the first image sensor used in digital imaging, invented by Willard S. Boyle and George E. Smith at Bell Labs, based on MOS capacitor technology.
- 1970: American inventor James Russell patents the first digital-to-optical recording and playback system, which would later lead to the Compact Disc.
- January 1971: Using NHK's experimental PCM recording system, Dr. Takeaki Anazawa, an engineer at Denon, records the world's first commercial digital recordings, The World Of Stomu Yamash'ta 1 & 2 by Stomu Yamash'ta (January 11, 1971) and Something by Steve Marcus & Jiro Inagaki (January 25, 1971). Both have to be recorded live, without edits. Marcus is released first (in February 1972), making it the first released digital recording. On January 27 Yamash'ta records Metempsychosis in the Nippon Columbia studio, Tokyo, with percussion and a brass section.
- 1972: Using lessons learned from the NHK encoder, Denon unveils the first 8-channel PCM encoder, the DN-023R, which uses 47.25 kHz 13-bit PCM resolution and 4-head open reel broadcast video tape recorder. The first recording with this new system is the Smetana Quartet performing Mozart's String Quartets K.458 and K.421, recorded in Tokyo April 24–26 and released that October. At least six other Denon-recorded digital LP records are released in October, including jazz, classical and traditional Japanese music.
- 1972: Discrete cosine transform (DCT), an important data compression technique, first proposed by Nasir Ahmed, while working at Kansas State University. DCT compression later became fundamental to digital media. It is the basis for modern multimedia compression standards, for digital images (such as JPEG), digital video (such as H.26x and MPEG), digital audio (such as Dolby Digital and MP3), and digital television (such as HDTV).
- 1973: Adaptive differential pulse-code modulation (ADPCM) developed by P. Cummiskey, Nikil Jayant and James L. Flanagan at Bell Labs.
- December 2–3, 1974: The Paillard Chamber Orchestra records the first digital recording outside Japan, in Paris' Notre Dame Cathedral, using Denon's DN-023R. Bach's "The Musical Offering" (BWV 1079) is released on LP May 1975.
- December 12–19, 1974: Helmuth Rilling records three Bach organ works inside the Gedächtniskirche, Stuttgart Germany using the DN-023R.
- May 1975: University of Utah professor Thomas Stockham develops a PCM digital audio recorder of his own design, using computer tape drives as the storage system. He founds the company Soundstream to offer it commercially. Between 1977 and 1980 a total of eighteen 4-channel 50 kHz 16-bit units were manufactured, of which seven were sold (at about $150,000 each). Over 200 recordings were made on his equipment, almost half of all digital classical recordings made in the 1970s.
- 1976: the prototype Soundstream 37.5 kHz, 16-bit, 2-channel recorder is used to record the Santa Fe Opera performing Virgil Thomson's opera The Mother of Us All for New World Records, making it the first US digital recording. However, the digital recorder is just a backup to the main analog multi-track recorder, and the analog recording is deemed superior and thus used for the LP release. The backup digital tape was presented at the October 1976 AES Convention in New York, but never commercially released.
- 1977: Denon develops the smaller portable PCM recording system, the DN-034R. Like the DN-023R it records 8 channels at 47.25 kHz, but it uses 14-bits "with emphasis, making it equivalent to 15.5 bits." It also allowed for overdubbing for the first time, crucial for professional recording.
- August 28–31, 1977: Soundstream's second-generation PCM system runs in the background of a California direct to disc recording session by organist Virgil Fox for Crystal Clear Records. When initially released the resulting LPs were pressed from the direct-to-disc acetate, though the later CD reissue (1987) comes from the digital backup tapes when the acetates were no longer usable. The CD reissue was made by Bainbridge Records.
- September 1977: Sony introduces the PCM-1 Audio Unit ($4400 street price) (44.056 kHz, 14-bit), the first consumer (well-heeled) PCM encoder. It required the use of a home video tape recorder for storage.
- November 4–7, 1977: 3M demonstrates a prototype 2-channel 50.4 kHz 16-bit digital recorder running on 1-inch tape at 45 ips at the New York AES Convention. As no true 16-bit converters were available, it combined separate 12-bit and 8-bit converters to create 16-bit performance.
- November 28, 1977: Denon brings their DN-034R to New York City's Sound Ideas Studios and records Archie Shepp's On Green Dolphin Street, making it America's first released digitally-recorded commercial album. The following two days, November 29–30, Frank Foster and the Loud Minority record Manhattan Fever which is released April 1978. Five other jazz albums are recorded with the DN-034R in New York before it returns to Japan in December.
- March 1978: Sony introduces the professional-grade PCM-1600 Audio Processor (44.056 kHz, 16-bit) (list price $40,000) used with an external U-matic tape drive, making digital recording commercially available to recording studios for the first time. PCM-1610 and PCM-1630 follow.
- April 4–5, 1978: Telarc uses Soundstream's PCM system to record Frederick Fennell and his Eastman Wind Ensemble playing Gustav Holst's Suites for Military Band and George Frideric Handel's Music for the Royal Fireworks. When released on LP this became the first US-recorded digital classical release.
- June 2, 1978: Sound 80 studios in Minneapolis records the Saint Paul Chamber Orchestra performing Aaron Copland's Appalachian Spring. This session is set up as a direct to disc recording, with the prototype 3M 50.4 kHz digital recorder running in the background. There is some disagreement, but it appears the resulting LP record (Sound80 Records S80-DLR-101) was taken from the digital backup tapes rather than the direct-to-disc acetate. In 1984 the session is re-released on Compact Disc by ProArte. This recording was nominated for three Grammy Awards, winning "Best Chamber Music Performance" (1980), making it the first digital recording so honored.
- Early June 1978: Sound 80 records Flim and the BB's debut self-titled album as another direct to disc recording again with the experimental 3M recorder in the background. Again the acetate is deemed not as good as the digital backup, so the digital master is used for the LP record (Sound80 Records S80-DLR-102). This makes it the first U.S. non-classical digital release. Within 6 months the hand-built ("very bulky and finicky") 3M digital recorder is disassembled, rendering the non-standard master tape unplayable. Therefore, no Compact Disc reissue is possible. The compact disc issue of the St. Paul Chamber Orchestra is unexplained.
- March 8, 1979: the first Compact Disc prototype was demonstrated by Philips in Eindhoven with the respective player nicknamed "Pinkeltje".
- July 11, 1979: the first U.S.-recorded digitally-recorded LP of popular music (with vocals), Bop 'Til You Drop by guitarist Ry Cooder, was released by Warner Bros. Records. The album was recorded in Los Angeles on a 32-track digital machine built by the 3M corporation.
- October 12, 1979: Fleetwood Mac's Tusk (album) is released. It, and Live (Fleetwood Mac album), December 8, 1980, were mastered on the Soundstream PCM from analog multi-tracks.
- October 30, 1979: Stevie Wonder releases his soundtrack album, Journey Through the Secret Life of Plants recorded and mixed on a Sony PCM-1600.
- December 1, 1979: The Grammy-award winning self-titled Christopher Cross album is released. Cross' album becomes the first digitally recorded album to chart (recorded on the 3M system) in the United States, eventually winning 5 Grammys. Digital recording is now mainstream.
- 1980: The Red Book standard (44.1 kHz, 16-bit) is established for Compact Disc Digital Audio.
- 1980: Mitsubishi Electric introduces the X-80 ProDigi open reel 1/4" tape 15 ips 50.4 kHz 16-bit digital recorder ($5000). Only 200 are sold worldwide.
- 1980: Soundstream merges with Digital Recording Corporation, becoming DRC/Soundstream, to develop and market 50 kHz PCM recording to an optical card. Eclipsed by the rise of the 44.1 kHz Compact Disc, the company is out of business after 1983.
- 1981: Sony releases the PCM-F1 Digital Audio Processor ($1900)(44.056 kHz, 16-bit) and matching SL-2000 Betamax VCR ($700) as a complete affordable portable (with optional batteries) home digital recording system
- August 17, 1982: The first compact disc manufactured, ABBA's The Visitors (because it was "mostly digitally recorded") is produced in Hanover, Germany. However due to production problems with it the third version didn't actually hit stores until later 1982 or early 1983. Billy Joel's 52nd Street actually becomes the first CD to hit the market, on October 1, 1982.
- September 5, 1982: Peter Gabriel releases his fourth studio album (titled Security in North America and Peter Gabriel IV elsewhere). When released on CD in October 1984 it becomes the first full-digital DDD release. It was recorded on Sony's Mobile One digital studio and mixed with a Sony PCM-1610.
- October 1, 1982: The Nightfly by Donald Fagen is released, recorded and mixed on 3M's 32-track recorder. When the CD is issued in 1984 it becomes another early DDD release.
- October 1, 1982: The first digital compact disc players are marketed by Sony (CDP-101, $900) and Philips (CD-100, $700).
- October 1982: New England Digital offers the hard disk recorder (Sample-to-Disk) option on the Synclavier, the first commercial hard disk (HDD) recording system.
- 1984: Sony releases the PCM-501ES digital audio processor (44.1 kHz, 16-bit) ($895 list price) which is used with an external VHS or Beta video recorder.
- September 1984: Bruce Springsteen's Born in the U.S.A. becomes the first US-manufactured CD to be released.
- 1987: Sony develops Digital Audio Tape.
- 1989: Test broadcasts for NICAM stereo digital audio for broadcast TV began in the UK.
- 1990: digital radio begins in Canada, using the L-Band.
- 1991: Alesis Digital Audio Tape or ADAT is a tape format used for simultaneously recording eight tracks of digital audio at once, onto Super VHS magnetic tape – a format similar to that used by consumer VCRs. The product was announced in January 1991 at the NAMM convention in Anaheim, California. The first ADAT recorders shipped over a year later in February or March 1992.
- 1993: RADAR (audio recorder) Random Access Digital Audio Recorder or RADAR is the first single box device used for simultaneously recording 24 tracks of digital audio at once, onto hard disk drives. The product, manufactured by Creation Technologies (iZ Technology Corporation) was announced in October 1993 at the AES convention in New York, New York. The first RADAR recorders shipped in August 1994.
- 1996: optical discs and DVD players begin selling in Japan.
- The analog signal is transmitted from the input device to an analog-to-digital converter (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 precise the representation of the original audio wave level.
- The higher the sampling rate the higher the upper audio frequency of the digitized audio signal.
- The ADC outputs a sequence of digital audio samples that make up a continuous stream of 0s and 1s.
- These binary numbers are stored on recording media such as a hard drive, optical drive or in solid state memory.
- The sequence of numbers is transmitted from storage into a digital-to-analog converter (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 loudspeakers or video screen.
Recording of bits
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 cassettes, 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 DVDs, 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.
Concerns with digital audio recording
The number of bits used to represent a sampled audio wave (the word size) directly affects the resulting noise in a recording after intentionally added dither, or the distortion of an undithered signal.
The number of possible voltage levels at the output is simply the number of levels that may be represented by the largest possible digital number (the number 2 raised to the power of the number of bits in each sample). There are no “in between” values allowed. If there are more bits in each sample the waveform is more accurately traced, because each additional bit doubles the number of possible values. The distortion is roughly the percentage that the least significant bit represents out of the average value. Distortion (as a percentage) in digital systems increases as signal levels decrease, which is the opposite of the behavior of analog systems.
The sample rate is just as important a consideration as the word size. If the sample rate is too low, the sampled signal cannot be reconstructed to the original sound signal.
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, 48, 88.2, 96, 176.4, and 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.
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.
One of the advantages of digital recording over analog recording is its resistance to errors.
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