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 digitized, converting 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. The digitized number streams themselves are never actually heard or seen, being hidden by the process.

Contents

In a properly matched analog-to-digital converter (ADC) and digital-to-analog converter (DAC) pair, there is one and only one analog output which must, by definition, exactly match the analog input. Because the signal is stored digitally, the recording is not degraded by copying, not degraded by storage (assuming proper error detection and correction), and not degraded by 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 measuring point.
  3. The frequency at which the ADC measures the level of the analog wave is called the sample rate or sampling rate.
  4. A digital audio sample with a given word length represents the audio level at one moment.
  5. The longer the word length the more precise the representation of the original audio wave level.
  6. The higher the sampling rate the higher the upper audio frequency of the digitized audio signal.
  7. The ADC outputs a sequence of digital audio samples that make up a continuous stream of 0s and 1s.
  8. These binary numbers are stored on recording media such as 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), 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.
  2. 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.

Parameters of digital audio recording

Word size

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. [56]

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. [57]

Sample rate

If the sampling rate is too low, the original audio signal cannot be reconstructed from the sampled signal.

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 recording music-quality audio, the following PCM sampling rates are the most common: 44.1, 48, 88.2, 96, 176.4, and 192 kHz, each with an upper-frequency limit half the sampling frequency.

When making a recording, experienced audio recording and mastering engineers will often 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 to avoid aliasing errors. 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 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 [58] (usually at 24 bits) without down-converting it. There remains some controversy whether higher sampling rates actually provide any verifiable benefit in the consumer product when using modern anti-aliasing filters. [59]

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, or originally recorded at that rate. This is done as part of the mastering process.

Beginning in the 1980s, music that was recorded, mixed and/or mastered digitally was often labelled 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 will not be degraded (add noise or distortion) from copying or storage.

See also

Related Research Articles

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Compact Disc Digital Audio, also known as Digital Audio Compact Disc or simply as Audio CD, is the standard format for audio compact discs. The standard is defined in the Red Book, one of a series of Rainbow Books that contain the technical specifications for all CD formats.

MiniDisc Magneto-optical storage medium, mainly used for audio

The MiniDisc (MD) is a magneto-optical disc-based data storage format offering a capacity of 60, 74, and later, 80 minutes of digitized audio or 1 gigabyte of Hi-MD data. Sony brand audio players went on the market in September 1992.

Digital audio Technology that records, stores, and reproduces sound

Digital audio is a representation of sound recorded in, or converted into, digital form. In digital audio, the sound wave of the audio signal is typically encoded as numerical samples in a continuous sequence. For example, in CD audio, samples are taken 44,100 times per second, each with 16-bit sample depth. Digital audio is also the name for the entire technology of sound recording and reproduction using audio signals that have been encoded in digital form. Following significant advances in digital audio technology during the 1970s and 1980s, it gradually replaced analog audio technology in many areas of audio engineering and telecommunications in the 1990s and 2000s.

Digital Audio Tape Digital audio cassette format developed by Sony

Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a Compact Cassette, using 3.81 mm / 0.15" magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. The recording is digital rather than analog. DAT can record at sampling rates equal to, as well as higher and lower than a CD at 16 bits quantization. If a comparable digital source is copied without returning to the analogue domain, then the DAT will produce an exact clone, unlike other digital media such as Digital Compact Cassette or non-Hi-MD MiniDisc, both of which use a lossy data reduction system.

Super Audio CD Read-only optical disc for high-fidelity audio storage

Super Audio CD (SACD) is a read-only optical disc format for audio storage, introduced in 1999. It was developed jointly by Sony and Philips Electronics, and intended to be the successor to the Compact Disc (CD) format.

Sound can be recorded and stored and played using either digital or analog techniques. Both techniques introduce errors and distortions in the sound, and these methods can be systematically compared. Musicians and listeners have argued over the superiority of digital versus analog sound recordings. Arguments for analog systems include the absence of fundamental error mechanisms which are present in digital audio systems, including aliasing and quantization noise. Advocates of digital point to the high levels of performance possible with digital audio, including excellent linearity in the audible band and low levels of noise and distortion.

CD player an electronic device that plays audio compact discs

A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or audiobooks. CD players may be part of home stereo systems, car audio systems, personal computers, or portable CD players such as CD boomboxes. Most CD players produce an output signal via a headphone jack or RCA jacks. To use a CD player in a home stereo system, the user connects an RCA cable from the RCA jacks to a hi-fi and loudspeakers for listening to music. To listen to music using a CD player with a headphone output jack, the user plugs headphones or earphones into the headphone jack.

Digital Compact Cassette Philips-developed system with digital audio on compact cassette

The Digital Compact Cassette (DCC) is a magnetic tape sound recording format introduced by Philips and Matsushita in late 1992 and marketed as the successor to the standard analog Compact Cassette. It was also a direct competitor to Sony's MiniDisc (MD), but neither format toppled the then-ubiquitous analog cassette despite their technical superiority, and DCC was discontinued in October 1996.

Direct Stream Digital

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PCM adaptor encodes digital audio as video

A PCM adaptor is a device that encodes digital audio as video for recording on a videocassette recorder. The adapter also has the ability to decode a video signal back to digital audio for playback. This digital audio system was used for mastering early compact discs.

Compact Disc recordings contain two channels of 44.1-kHz 16-bit linear PCM audio. However, creators of the CD originally contemplated a four-channel, or quadraphonic, mode as well.

Thomas Stockham

Thomas Greenway Stockham was an American scientist who developed one of the first practical digital audio recording systems, and pioneered techniques for digital audio recording and processing.

Soundstream Inc. was the first United States audiophile digital audio recording company, providing commercial services for recording and computer-based editing.

History of sound recording

The history of sound recording - which has progressed in waves, driven by the invention and commercial introduction of new technologies — can be roughly divided into four main periods:

The dbx Model 700 Digital Audio Processor was a professional audio ADC/DAC combination unit, which digitized a stereo analog audio input into a bitstream, which was then encoded and encapsulated in an analog composite video signal, for recording to tape using a VCR as a transport. Unlike other similar pieces of equipment like the Sony PCM-F1, the Model 700 used a technique called Companded Predictive Delta Modulation, rather than the now-common pulse-code modulation. At the time of its introduction in the mid-1980s the device was the first commercial product to use this method, although it had been proposed in the 1960s and prototyped in the late '70s.

Audio bit depth The number of bits of information recorded for each digital audio sample

In digital audio using pulse-code modulation (PCM), bit depth is the number of bits of information in each sample, and it directly corresponds to the resolution of each sample. Examples of bit depth include Compact Disc Digital Audio, which uses 16 bits per sample, and DVD-Audio and Blu-ray Disc which can support up to 24 bits per sample.

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitude of the analog signal is sampled regularly at uniform intervals, and each sample is quantized to the nearest value within a range of digital steps.

In digital audio, 44,100 Hz is a common sampling frequency. Analog audio is often recorded by sampling it 44,100 times per second, and then these samples are used to reconstruct the audio signal when playing it back.

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