Audio converter
Encyclopedia
In signal processing
, an audio converter or digital audio converter is a type of electronic hardware
technology which converts an analog
audio
signal to a digital audio
format, either on the input (Analog-to-digital converter
or ADC), or the output (Digital-to-analog converter
, or DAC). They are common in numerous technologies —notably in computer sound card
s, digital cellular phones, portable recording
devices, and digital audio workstation
s (DAW). Once converted to digital format, digital audio signals and file formats can be processed in any of a number of ways as allowed by software —including converting to audio CD or MP3
formats.
Different types of converter units can operate at different resolution
s which largely determines the resulting sound "quality." Depending on their quality and cost, converters also differ in their handling of:
"Resolution" generally refers to both bit and sample rates (though it may occasionally refer specifically to the sampling rate; being the more variable of the two.)
For example, an inexpensive brand consumer sound card for computer has a typical operating range of around CD quality — 44,100 samples per second, (44.1 kHz), with 65,536 allowable values for volume
(16 bits, giving
values).
, and is composed of a series of sampled sound pressure
s. Each sample has been quantized
to one out of a set of discrete values depending on the bit depth
.
The playback of the waveform reproduces the sound pressure changes in sequence, producing complex sounds.
"Bit depth" determines the number of alternative values for sampled sound pressure that can be represented. The "sampling rate
" (or frequency) refers to the number of samples (or "snapshots") per second, measured in hertz
(Hz).
A stereo
"track" is simply two isolated mono tracks within the same file, which are played back simultaneously. The illusion of spatial sound can be created through differences between the left and right channels. High-quality motion picture sound formats, like Dolby Surround
and DTS simply use more mono channels to create more complex illusions of spatial depth.
capacity for either or both bit resolution and sampling rate will increase the quality
of the sound.
Multiple tracks can enhance the experience of spatial sound and sound "quality."
"Professional" sound quality refers to highest current sound qualities for available hardware. In the past, very high resolution hardware was also highly specialized and expensive. Today, many commonly available technologies are sufficient for generating adequate or above average quality digital audio signals for use in various recording or transmission applications.
Currently it's possible to purchase hardware using reasonably high quality digital converters for well under 1000 USD. In professional uses, extremely high fidelity converters are typically sold as components within specialized rack mount multi-channel (multitrack
) recording units. These typically connect to a computer via parallel
, proprietary or specialized (i.e. PCI
card), fiber optic, USB, or Firewire connections. USB 2 and Firewire are the most common, though Firewire has lower latency
and thus is more suited for multitrack applications.
At the barebones level, audio card converters take a nominal line-level input signal and is designed to be a transparent link between the mixing board and the computer. Once converted to digital format, the sound files can be processed in any of a number of ways as allowed by software, including converting to audio CD or MP3
formats.
Both the recording and playback of sound must consider the nature of the digital medium to produce undesirable noise. The noise produced by analog tape, by comparison can be pleasing, as it is of sufficient resolution to produce "warm" natural sounding random noise. Digital recording has traditionally had the problem of sounding "cold," mainly due to the quantization
effect of digitizing a linear wave into a segmented series of adjacent samples. In the case of long wave sounds, such as bass, the wave is smooth, and therefore its playback does not "jump" from a low volume bit to an adjacent high volume bit. However, high pitched sounds like cymbal
crashes and similar "white" noise can only be represented by a virtually random array of bits. This random array exposes the weakness of digital playback to produce digital artifacts in the form of a high-pitched "harshness."
While better resolutions produce better sound and less digital harshness, an important function of digital converters is to minimize the known problems endemic to the digital format. A common solution was to use separate vacuum tube
amplifiers to "warm" the sound at both input output. This is generally expensive, and normalizes the sound to the range given by the tube amp, which negates the benefits of digital's deep low and clear high frequencies.
The main solution has generally been to use an oversampling
process, which can differ substantially depending on its application for input or output. Oversampling is the processing of sound input at higher sampling rates than the hardware actually performs. On input, this produces a digital wave which is still based on the input, but altered to record substantially fewer jumps which would produce audible artifacts. On the output, oversampling serves the same role as using a tube amp, whereby the adjacent bits are re-quantized at higher resolution to produce a smoother wave.
Given the capabilities of the digital realm to transfer exact files without analog loss, it is rare that multitrack digital to analog (DAC) converters are needed, with the exception of basic playback monitoring of stereo
or specialized surround sound
formats.
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
, an audio converter or digital audio converter is a type of electronic hardware
Electronic hardware
Electronic hardware refers to interconnected electronic components which perform analog and/or logic operations on received and locally stored information to produce as output and/or store resulting new information and/or to provide control for output actuator mechanisms.Electronic hardware can...
technology which converts an analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
signal to a digital audio
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
format, either on the input (Analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
or ADC), or the output (Digital-to-analog converter
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
, or DAC). They are common in numerous technologies —notably in computer sound card
Sound card
A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs. The term sound card is also applied to external audio interfaces that use software to generate sound, as opposed to using hardware...
s, digital cellular phones, portable recording
Recording
Recording is the process of capturing data or translating information to a recording format stored on some storage medium, which is often referred to as a record or, if an auditory medium, a recording....
devices, and digital audio workstation
Digital audio workstation
A digital audio workstation is an electronic system designed solely or primarily for recording, editing and playing back digital audio. DAWs were originally tape-less, microprocessor-based systems such as the Synclavier and Fairlight CMI...
s (DAW). Once converted to digital format, digital audio signals and file formats can be processed in any of a number of ways as allowed by software —including converting to audio CD or MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
formats.
Different types of converter units can operate at different resolution
Resolution (music)
Resolution in western tonal music theory is the move of a note or chord from dissonance to a consonance .Dissonance, resolution, and suspense can be used to create musical interest...
s which largely determines the resulting sound "quality." Depending on their quality and cost, converters also differ in their handling of:
- electronic (radio) interferenceInterference (communication)In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
- signal-to-noise ratioSignal-to-noise ratioSignal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR) - electronic noiseNoiseIn common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
shielding and rejection - digital noise floorNoise floorIn signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where the noise is defined as any signal other than the one being monitored....
handling, - oversamplingOversamplingIn signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth or highest frequency of the signal being sampled...
, (input and/or output) - simultaneous playback and recording (full-duplex)
- clock jitter
"Resolution" generally refers to both bit and sample rates (though it may occasionally refer specifically to the sampling rate; being the more variable of the two.)
For example, an inexpensive brand consumer sound card for computer has a typical operating range of around CD quality — 44,100 samples per second, (44.1 kHz), with 65,536 allowable values for volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
(16 bits, giving
values).Digital audio basics
A single ("mono") channel (or "track") of digital audio can be visualized as a waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
, and is composed of a series of sampled sound pressure
Sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
s. Each sample has been quantized
Quantization (sound processing)
In signal processing and digital audio, quantization is the process of approximating a continuous range of values by a relatively small set of discrete symbols or integer values...
to one out of a set of discrete values depending on the bit depth
Audio bit depth
In digital audio, bit depth describes the number of bits of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data...
.
The playback of the waveform reproduces the sound pressure changes in sequence, producing complex sounds.
"Bit depth" determines the number of alternative values for sampled sound pressure that can be represented. The "sampling rate
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
" (or frequency) refers to the number of samples (or "snapshots") per second, measured in hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(Hz).
A stereo
STEREO
STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...
"track" is simply two isolated mono tracks within the same file, which are played back simultaneously. The illusion of spatial sound can be created through differences between the left and right channels. High-quality motion picture sound formats, like Dolby Surround
Dolby Surround
Dolby Surround was the earliest consumer version of Dolby's multichannel analog film sound decoding format Dolby Stereo introduced to the public in 1982 during the time home video recording formats were introducing Stereo and HiFi capability...
and DTS simply use more mono channels to create more complex illusions of spatial depth.
Sound quality
Increasing the I/OI/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
capacity for either or both bit resolution and sampling rate will increase the quality
Sound quality
Sound quality is the quality of the audio output from various electronic devices. Sound quality can be defined as the degree of accuracy with which a device records or emits the original sound waves...
of the sound.
Multiple tracks can enhance the experience of spatial sound and sound "quality."
"Professional" sound quality refers to highest current sound qualities for available hardware. In the past, very high resolution hardware was also highly specialized and expensive. Today, many commonly available technologies are sufficient for generating adequate or above average quality digital audio signals for use in various recording or transmission applications.
Currently it's possible to purchase hardware using reasonably high quality digital converters for well under 1000 USD. In professional uses, extremely high fidelity converters are typically sold as components within specialized rack mount multi-channel (multitrack
Multitrack recording
Multitrack recording is a method of sound recording that allows for the separate recording of multiple sound sources to create a cohesive whole...
) recording units. These typically connect to a computer via parallel
Parallel port
A parallel port is a type of interface found on computers for connecting various peripherals. In computing, a parallel port is a parallel communication physical interface. It is also known as a printer port or Centronics port...
, proprietary or specialized (i.e. PCI
Peripheral Component Interconnect
Conventional PCI is a computer bus for attaching hardware devices in a computer...
card), fiber optic, USB, or Firewire connections. USB 2 and Firewire are the most common, though Firewire has lower latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
and thus is more suited for multitrack applications.
At the barebones level, audio card converters take a nominal line-level input signal and is designed to be a transparent link between the mixing board and the computer. Once converted to digital format, the sound files can be processed in any of a number of ways as allowed by software, including converting to audio CD or MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
formats.
Both the recording and playback of sound must consider the nature of the digital medium to produce undesirable noise. The noise produced by analog tape, by comparison can be pleasing, as it is of sufficient resolution to produce "warm" natural sounding random noise. Digital recording has traditionally had the problem of sounding "cold," mainly due to the quantization
Quantization (signal processing)
Quantization, in mathematics and digital signal processing, is the process of mapping a large set of input values to a smaller set – such as rounding values to some unit of precision. A device or algorithmic function that performs quantization is called a quantizer. The error introduced by...
effect of digitizing a linear wave into a segmented series of adjacent samples. In the case of long wave sounds, such as bass, the wave is smooth, and therefore its playback does not "jump" from a low volume bit to an adjacent high volume bit. However, high pitched sounds like cymbal
Cymbal
Cymbals are a common percussion instrument. Cymbals consist of thin, normally round plates of various alloys; see cymbal making for a discussion of their manufacture. The greater majority of cymbals are of indefinite pitch, although small disc-shaped cymbals based on ancient designs sound a...
crashes and similar "white" noise can only be represented by a virtually random array of bits. This random array exposes the weakness of digital playback to produce digital artifacts in the form of a high-pitched "harshness."
While better resolutions produce better sound and less digital harshness, an important function of digital converters is to minimize the known problems endemic to the digital format. A common solution was to use separate vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
amplifiers to "warm" the sound at both input output. This is generally expensive, and normalizes the sound to the range given by the tube amp, which negates the benefits of digital's deep low and clear high frequencies.
The main solution has generally been to use an oversampling
Oversampling
In signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth or highest frequency of the signal being sampled...
process, which can differ substantially depending on its application for input or output. Oversampling is the processing of sound input at higher sampling rates than the hardware actually performs. On input, this produces a digital wave which is still based on the input, but altered to record substantially fewer jumps which would produce audible artifacts. On the output, oversampling serves the same role as using a tube amp, whereby the adjacent bits are re-quantized at higher resolution to produce a smoother wave.
Given the capabilities of the digital realm to transfer exact files without analog loss, it is rare that multitrack digital to analog (DAC) converters are needed, with the exception of basic playback monitoring of stereo
Stereophonic sound
The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...
or specialized surround sound
Surround sound
Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...
formats.