Digital recording

In digital recording, the analog signal of video or sound is converted into a stream of discrete numbers, representing the changes in chroma and luminance values (video) or air pressure (audio) through time, thus making an abstract template for the original sound or moving image.

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.

History

• 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 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 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, and New England Digital offers the Sample-to-Disk option on the Synclavier, the first commercial hard disk recording system.

• In 1990, digital radio begins in Canada, using the L-Band.

• DVD players begin selling in Japan in 1996.

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Process

Recording

1. The analog signal is transmitted from the input device to an analog-to-digital converter (ADC).
2. The ADC converts this signal to a series of binary numbers. The quantity of numbers produced per second is called the sample rate.
3. These numbers are stored onto recording media such as magnetic tape or hard drive or optical drive.

Playback

1. The sequence of numbers is transmitted from storage into a digital-to-analog converter (DAC), which converts the numbers back to an analog signal.
2. This signal 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 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

Word Size

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.

Sample rate

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.

Error Rectification

Main article: digital

One of the advantages of digital recording over analog recording is its resistance to errors.

See also

• Compact discs use Reed-Solomon error correction
• Many bits are stored on RAID storage systems.
• Parity Computation
• Cyclic redundancy check (CRC)
• Multitrack recording