Audio levels display on a digital audio recorder (Zoom H4n) 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.
- In 1937, British scientist Alec Reeves files the first patent describing Pulse-code modulation.
- In 1943, Bell Telephone Laboratories develops the first digital scrambled speech transmission system, SIGSALY.
- In 1957, Max Mathews of Bell develops the process to digitally record sound via computer.
- In 1967, the first digital audio magnetic tape recorder is invented. A 12-bit 30 kHz stereo device using a compander (similar to DBX Noise Reduction) to extend the dynamic range.
- In the 1970s, Thomas Stockham 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 Soundstream company).
- In 1970, James Russell patents the first digital-to-optical recording and playback system, which would later lead to the Compact Disc.
- In 1972, Denon invents the first 8-track reel to reel 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 Disc 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 Cooder is released by Warner Bros. Records. The album was recorded in Los Angeles on a 32-track digital machine built by the 3M corporation.
- In 1982, the first digital compact discs are marketed by Sony and Philips, and New England Digital offers the hard disk recorder (Sample-to-Disk) option on the Synclavier, the first commercial hard disk (HDD) recording system. Also that same year, Peter Gabriel releases, Security, which was an early full digital recording.
- In 1990, digital radio begins in Canada, using the L-Band.
- In 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.
- In 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 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 tape, hard drive, optical drive or 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.
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 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 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.
The sample rate 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.
One of the advantages of digital recording over analog recording is its resistance to errors.
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