Audio signal processing
Encyclopedia
Audio signal processing, sometimes referred to as audio processing, is the intentional alteration of auditory
signals, or sound
. As audio signals may be electronically represented in either digital
or analog
format, signal processing may occur in either domain. Analog processors operate directly on the electrical signal, while digital processors operate mathematically on the digital representation of that signal.
Coders do nothing but segment input signals into quasi-stationary frames ranging from 2 to 50 ms.The temporal and spectral component of each frame can be estimated through time-frequency analysis.This time-frequency system mapping is usually matched to the analysis properties of human auditory system.The objective of audio coding is to extract from the input audio a set of time-frequency parameters that is amenable to quantization. Based upon design parameters the section usually contains one of the following:
The choice of methodology depends upon tradeoff between time and frequency resolution requirements.
s which travel through air, consisting of compressions and rarefactions. These audio signals are measured in bels or in decibel
s. Audio processing was necessary for early radio broadcasting, as there were many problems with studio to transmitter links.
Historically, before the advent of widespread digital technology, ASP was the only method by which to manipulate a signal. Since that time, as computers and software became more advanced, digital signal processing has become the method of choice.
numbers. This permits signal processing using digital circuits such as microprocessors and computers. Although such a conversion can be prone to loss, most modern audio systems use this approach as the techniques of digital signal processing are much more powerful and efficient than analog domain signal processing.
, data compression, transmission, enhancement (e.g., equalization
, filtering
, noise cancellation, echo
or reverb removal or addition, etc.)
or audio broadcasting) is perhaps the biggest market segment (and user area) for audio processing products—globally.
Traditionally the most important audio processing (in audio broadcasting) takes place just before the transmitter. Studio audio processing is limited in the modern era due to digital audio systems (mixers, routers) being pervasive in the studio.
In audio broadcasting, the audio processor must
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...
signals, or sound
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...
. As audio signals may be electronically represented in either digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
or 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...
format, signal processing may occur in either domain. Analog processors operate directly on the electrical signal, while digital processors operate mathematically on the digital representation of that signal.
Audio coding architecture
There are several efficient signal models[ e.g. transform-based,standard filter structure,wavelet-packet]and compression standard for digital audio reproduction.Coders do nothing but segment input signals into quasi-stationary frames ranging from 2 to 50 ms.The temporal and spectral component of each frame can be estimated through time-frequency analysis.This time-frequency system mapping is usually matched to the analysis properties of human auditory system.The objective of audio coding is to extract from the input audio a set of time-frequency parameters that is amenable to quantization. Based upon design parameters the section usually contains one of the following:
- Unitary transform
- Harmonic/sinusoidal analyzer
- Source system analysis(low pulse and multi pulse excitation)
- Signal adaptive bank of critically sampled,uniform/non-uniform bandpass filter
- Time-invarient bank of critically sampled,uniform/non-uniform bandpass filter.
- Hybrid versions of the above
The choice of methodology depends upon tradeoff between time and frequency resolution requirements.
History
Audio signals are sound waves—longitudinal waveLongitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
s which travel through air, consisting of compressions and rarefactions. These audio signals are measured in bels or in decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s. Audio processing was necessary for early radio broadcasting, as there were many problems with studio to transmitter links.
Audio coding architecture
There are several efficient signal models (e.g. transform based, standard filter structure, wavelet-packet) and compression standard for digital audio reproduction. Coders are nothing but segment of input signal into quasi-stationary frames ranging from 2 to 50 ms. The temporal and spectral component of each frame can be estimated through time-frequency analysis. This time-frequency system mapping is usually matched to the analysis properties of human auditory system.Analog signals
"Analog" indicates something that is mathematically represented by a set of continuous values; for example, the analog clock uses constantly-moving hands on a physical clock face, where moving the hands directly alters the information that clock is providing. Thus, an analog signal is one represented by a continuous stream of data, in this case along an electrical circuit in the form of voltage, current or charge changes (compare with digital signals below). Analog signal processing (ASP) then involves physically altering the continuous signal by changing the voltage or current or charge via various electrical means.Historically, before the advent of widespread digital technology, ASP was the only method by which to manipulate a signal. Since that time, as computers and software became more advanced, digital signal processing has become the method of choice.
Digital signals
A digital representation expresses the pressure wave-form as a sequence of symbols, usually binaryBinary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
numbers. This permits signal processing using digital circuits such as microprocessors and computers. Although such a conversion can be prone to loss, most modern audio systems use this approach as the techniques of digital signal processing are much more powerful and efficient than analog domain signal processing.
Application areas
Processing methods and application areas include storage, level compressionAudio level compression
Dynamic range compression, also called DRC or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or "compressing" an audio signal's dynamic range...
, data compression, transmission, enhancement (e.g., equalization
Equalization (audio)
Equalization is the process commonly used in sound recording and reproduction to alter the frequency response of an audio system using linear filters. Most hi-fi equipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more...
, filtering
Audio filter
An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Many types of filters exist for applications including graphic equalizers, synthesizers, sound effects, CD players and virtual reality systems.Being a frequency dependent...
, noise cancellation, echo
Echo (phenomenon)
In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...
or reverb removal or addition, etc.)
Audio Broadcasting
Audio broadcasting (be it for televisionTelevision
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
or audio broadcasting) is perhaps the biggest market segment (and user area) for audio processing products—globally.
Traditionally the most important audio processing (in audio broadcasting) takes place just before the transmitter. Studio audio processing is limited in the modern era due to digital audio systems (mixers, routers) being pervasive in the studio.
In audio broadcasting, the audio processor must
- prevent overmodulationOvermodulationOvermodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always considered a fault condition. In layman's terms, the...
, and minimize it when it occurs - compensate for non-linear transmitters, more common with medium wave and shortwaveShortwaveShortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
broadcasting - adjust overall loudness to desired level
- correct errors in audio levels