Digital recording

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Audio levels display on a digital audio recorder (Zoom H4n) Zoom H4n audio recording levels.jpg
Audio levels display on a digital audio recorder (Zoom H4n)

In digital recording, an audio or video signal is converted into a stream of discrete numbers representing the changes over time in air pressure for audio, or chroma and luminance values for video. This number stream is saved to a storage device. To play back a digital recording, the numbers are retrieved and converted back into their original analog audio or video forms so that they can be heard or seen.

Contents

In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair, the analog signal is accurately reconstructed, within the constraints of the Nyquist–Shannon sampling theorem, which dictates the sampling rate and quantization error dependent on the audio or video bit depth. Because the signal is stored digitally, assuming proper error detection and correction, the recording is not degraded by copying, storage or interference.

Timeline

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 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 measurement point. The longer the word length the more precise the representation of the original audio wave level.
  3. The frequency at which the ADC measures the level of the analog wave is called the sample rate or sampling rate. The higher the sampling rate the higher the upper audio frequency of the digitized audio signal.
  4. The ADC outputs a sequence of digital audio samples that make up a continuous stream of 0s and 1s.
  5. These binary numbers are stored on recording media such as magnetic tape, a hard drive, optical drive or in solid state memory.

Playback

  1. The sequence of numbers is transmitted from storage into a digital-to-analog converter (DAC)
  2. The DAC converts the numbers back to an analog signal by sticking together the level information stored in each digital sample, thus rebuilding the original analog waveform.
  3. This signal is amplified and transmitted to the loudspeakers.

Recording of bits

Techniques to record to commercial media

For digital cassettes, the tape head moves as well as the tape, typically in a helical scan configuration, in order to maintain a high enough speed to keep the bits at a manageable size.

For optical disc recording technologies such as CD-R, a laser is used to alter the optical properties of the dye layer of the medium. A weaker laser is used to read these patterns.

Performance parameters

Word size

The number of bits used to represent an audio signal directly affects the resulting noise or distortion in a recording. [lower-alpha 1] [54]

Sample rate

As stated by the Nyquist–Shannon sampling theorem, to prevent aliasing, the audio signal must be sampled at a rate at least twice that of the highest frequency component in the signal. For music-quality audio, 44.1 and 48 kHz sampling rates are the most common.

Master recording may be done at a higher sampling rate (i.e. 88.2, 96, 176.4 or 192 kHz). High-resolution PCM recordings have been released on DVD-Audio (also known as DVD-A), DualDisc (utilizing the DVD-Audio layer), or High Fidelity Pure Audio on Blu-ray. In addition, it is possible to release a high-resolution recording as either an uncompressed WAV or lossless compressed FLAC file [55] (usually at 24 bits) without down-converting it. There remains controversy about whether higher sampling rates provide any verifiable benefit to the consumer product. [56]

When a Compact Disc (the CD Red Book standard is 44.1 kHz 16 bit) is to be made from a high-res recording, the recording must be down-converted to 44.1 kHz. This is done as part of the mastering process.

Beginning in the 1980s, music that was recorded, mixed or mastered digitally was often labeled using the SPARS code to describe which processes were analog and which were digital. Since digital recording has become near-ubiquitous the SPARS codes are now rarely used.

Error rectification

One of the advantages of digital recording over analog recording is its resistance to errors. Once the signal is in the digital format, it is not subject to generation loss from copying. Instead of the gradual degradation experienced with analog media, digital media is subject to a cliff effect.

See also

Notes

  1. Intentionally added dither in the recording process transforms quantization distortion into noise.

Related Research Articles

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