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Audio Signal Processing
AUDIO SIGNAL PROCESSING or AUDIO PROCESSING is the intentional alteration of audio signals often through an AUDIO EFFECT or effects unit . 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. CONTENTS * 1 History * 2 Analog signals * 3 Digital signals * 4 Application areas * 4.1 Audio broadcasting * 5 Techniques * 6 See also * 7 References * 8 Further reading HISTORYAudio signals are electronic representations of sound waves —longitudinal waves which travel through air, consisting of compressions and rarefactions. The energy contained in audio signals is typically measured in decibels . Audio processing was necessary for early radio broadcasting , as there were many problems with studio to transmitter links
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Analog (signal)
An ANALOG SIGNAL is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. For example, in an analog audio signal , the instantaneous voltage of the signal varies continuously with the pressure of the sound waves. It differs from a digital signal , in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values. The term analog signal usually refers to electrical signals ; however, mechanical , pneumatic , hydraulic , human speech, and other systems may also convey or be considered analog signals. An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information
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Digital Data
DIGITAL DATA, in information theory and information systems , are discrete, discontinuous representations of information or works, as contrasted with continuous, or analog signals which behave in a continuous manner, or represent information using a continuous function . Although digital representations are the subject matter of discrete mathematics , the information represented can be either discrete, such as numbers and letters , or it can be continuous, such as sounds, images, and other measurements. The word digital comes from the same source as the words digit and digitus (the Latin
Latin
word for finger ), as fingers are often used for discrete counting. Mathematician George Stibitz of Bell Telephone Laboratories used the word digital in reference to the fast electric pulses emitted by a device designed to aim and fire anti-aircraft guns in 1942
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Delay (audio Effect)
DELAY is an audio effect and an effects unit which records an input signal to an audio storage medium , and then plays it back after a period of time. The delayed signal may either be played back multiple times, or played back into the recording again, to create the sound of a repeating, decaying echo . Delay effects range from a subtle echo effect to a pronounced blending of previous sounds with new sounds. Delay effects can be created using tape loops, an approach developed in the 1940s and 1950s; analog effects units , which were introduced in the 1970s; digital effects pedals, introduced in 1984; and audio software plugins, developed in the 2000s
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Spring Reverb
REVERBERATION, in psychoacoustics and acoustics , is the persistence of sound after a sound is produced. A reverberation, or REVERB, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude , until they reach zero amplitude. Reverberation is frequency dependent: the length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity. In comparison to a distinct echo that is a minimum of 50 to 100 ms after the initial sound, reverberation is the occurrence of reflections that arrive in less than approximately 50 ms
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Reverberation
REVERBERATION, in psychoacoustics and acoustics , is the persistence of sound after a sound is produced. A reverberation, or REVERB, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude , until they reach zero amplitude. Reverberation
Reverberation
is frequency dependent: the length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity. In comparison to a distinct echo that is a minimum of 50 to 100 ms after the initial sound, reverberation is the occurrence of reflections that arrive in less than approximately 50 ms
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Loudness
LOUDNESS is the characteristic of a sound that is primarily a psycho-physiological correlate of amplitude . More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud." The relation of physical attributes of sound to perceived loudness consists of physical, physiological and psychological components. In different industries, loudness may have different meanings, and different standards exist, each purporting to define the measurement. Some definitions such as LKFS refer to relative loudness of different segments of electronically reproduced sounds such as for broadcasting and cinema. Others, such as ISO 532A (Stevens loudness, measured in sones ), ISO 532B (Zwicker loudness), DIN
DIN
45631 and ASA/ ANSI S3.4, have a more general scope and are often used to characterize loudness of environmental noise
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Shortwave
SHORTWAVE RADIO is radio transmission using shortwave radio frequencies , generally 1.6–30 MHz
MHz
(187.4–10.0 m), just above the medium wave AM broadcast band. Radio
Radio
waves in this band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere . Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or skip propagation . Thus shortwave radio can be used for very long distance communication, in contrast to radio waves of higher frequency which travel in straight lines (line-of-sight propagation ) and are limited by the visual horizon, about 40 miles. Shortwave radio
Shortwave radio
is used for broadcasting of voice and music to shortwave listeners over very large areas; sometimes entire continents or beyond
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Reverb
REVERBERATION, in psychoacoustics and acoustics , is the persistence of sound after a sound is produced. A reverberation, or REVERB, is created when a sound or signal is reflected causing a large number of reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude , until they reach zero amplitude. Reverberation
Reverberation
is frequency dependent: the length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity. In comparison to a distinct echo that is a minimum of 50 to 100 ms after the initial sound, reverberation is the occurrence of reflections that arrive in less than approximately 50 ms
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Mixing Console
In audio , a MIXING CONSOLE is an electronic device for combining (also called "mixing "), routing, and changing the volume level , timbre (tone color) and/or dynamics of many different audio signals, such as microphones being used by singers , mics picking up acoustic instruments such as drums or saxophones , signals from electric or electronic instruments such as the electric bass or synthesizer , or recorded music playing on a CD player . In the 2010s, a mixer is able to control analog or digital signals , depending on the type of mixer. The modified signals (voltages or digital samples ) are summed to produce the combined output signals, which can then be broadcast, amplified through a sound reinforcement system or recorded (or some combination of these applications)
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Overmodulation
OVERMODULATION 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 signal is going "off the scale". Overmodulation results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal. This means that the envelope of the output waveform is distorted. Although overmodulation is sometimes considered permissible, it should not occur in practice; a distorted waveform envelope will result in a distorted output signal of the receiving medium. REFERENCES * ^ "Overmodulation". Retrieved 10 January 2013
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Medium Wave
MEDIUM WAVE (MW) is the part of the medium frequency (MF) radio band used mainly for AM radio broadcasting . For Europe
Europe
the M W band ranges from 526.5 kHz to 1606.5 kHz, using channels spaced every 9 kHz, and in North America
North America
an extended MW broadcast band ranges from 525 kHz to 1705 kHz, using 10 kHz spaced channels
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Flanging
FLANGING /ˈflændʒɪŋ/ is an audio effect produced by mixing two identical signals together, one signal delayed by a small and gradually changing period, usually smaller than 20 milliseconds . This produces a swept comb filter effect: peaks and notches are produced in the resulting frequency spectrum , related to each other in a linear harmonic series . Varying the time delay causes these to sweep up and down the frequency spectrum. A FLANGER is an effects unit that creates this effect. Part of the output signal is usually fed back to the input (a "re-circulating delay line "), producing a resonance effect which further enhances the intensity of the peaks and troughs. The phase of the fed-back signal is sometimes inverted, producing another variation on the flanger sound. Example of flanging A short sample followed by two flanging versions
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Tachometer
A TACHOMETER (REVOLUTION-COUNTER, TACH, REV-COUNTER, RPM GAUGE) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes from Greek ταχος (tachos "speed") and metron ("measure"). Essentially the words tachometer and speedometer have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine and the other for vehicle speed. In formal engineering nomenclature, more precise terms are used to distinguish the two
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High-pass Filter
A HIGH-PASS FILTER is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. A high-pass filter is usually modeled as a linear time-invariant system . It is sometimes called a LOW-CUT FILTER, BASS-CUT FILTER or SUBSONIC FILTER with the latter found on amplifiers equipped with a phono stage, in place to curb needle rumble. High-pass filters have many uses, such as blocking DC from circuitry sensitive to non-zero average voltages or radio frequency devices. They can also be used in conjunction with a low-pass filter to produce a bandpass filter
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Low-pass Filter
A LOW-PASS FILTER is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design . The filter is sometimes called a HIGH-CUT FILTER, or TREBLE CUT FILTER in audio applications. A low-pass filter is the complement of a high-pass filter . Low-pass filters exist in many different forms, including electronic circuits such as a HISS FILTER used in audio , anti-aliasing filters for conditioning signals prior to analog-to-digital conversion , digital filters for smoothing sets of data, acoustic barriers, blurring of images, and so on. The moving average operation used in fields such as finance is a particular kind of low-pass filter, and can be analyzed with the same signal processing techniques as are used for other low-pass filters
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