Adaptive

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

cancellation is a common method of cancelling

audio feedback Audio feedback (also known as acoustic feedback, simply as feedback) is a positive feedback situation which may occur when an acoustic path exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for examp ...

in a variety of electro-acoustic systems such as digital

hearing aid A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers su ...

s. The time varying acoustic feedback leakage paths can only be eliminated with adaptive feedback cancellation. When an electro-acoustic system with an adaptive feedback canceller is presented with a correlated input signal, a recurrent

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

artifact,

entrainment Entrainment may refer to: * Air entrainment, the intentional creation of tiny air bubbles in concrete * Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency * Entrainment (biomusicology), the synchronization o ...

is generated. There is a difference between the system identification and feedback cancellation. Adaptive feedback cancellation has its application in

echo cancellation Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression i ...

. The error between the desired and the actual output is taken and given as feedback to the adaptive processor for adjusting its coefficients to minimize the error. In hearing aids, feedback arises when a part of the receiver (loudspeaker) signal is captured by the hearing aid microphone(s), gets amplified in the device and starts to loop around through the system. When feedback occurs, it results in a disturbingly loud tonal signal. Feedback is more likely to occur when the hearing aid volume is increased, when the hearing aid fitting is not in its proper position or when the hearing aid is brought close to a reflecting surface (e.g. when using a mobile phone). Adaptive feedback cancellation algorithms are techniques that estimate the transmission path between loudspeaker and microphone(s). This estimate is then used to implement a neutralizing electronic feedback path that suppresses the tonal feedback signal.

History

Adaptive feedback cancellation originated during the evolution of the hearing aid. The hearing aid became digital, and as such feedback cancellation was needed. In 1980 a

directional microphone A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...

was introduced in the digital hearing aid, and adaptive feedback cancellation was created to block external noise that the microphone picked up. Today, adaptive feedback cancellation is in nearly every digital hearing aid and current research is still ongoing.

Phases

Adaptive feedback cancellation follows the following process: # The process begins with background noise picked by a microphone getting amplified by a speaker in that same device. # This noise is audio feedback which is stored to later be cancelled # An adaptive filter uses an algorithm to maximize the amount of the stored audio feedback that can be cancelled # The adaptive filter is implemented in an acoustic device, and the repetition of this process is adaptive feedback cancellation

Applications

Hearing Aids

Hearing aids use adaptive feedback cancellation to improve the amount of gain. When hearing aids are misplaced or turned to loud volumes they can have large feedback. Noise picked up through the hearing aid's microphone creates this feedback, which is then amplified creating a ringing noise. Adaptive feedback cancellation cancels the noise with an adaptive filter. The product of the adaptive filter cancels the feedback from the microphone creating clear sound from the hearing aid.

Echo Cancellation

Echo cancellation Echo suppression and echo cancellation are methods used in telephony to improve voice quality by preventing echo from being created or removing it after it is already present. In addition to improving subjective audio quality, echo suppression i ...

is a form of adaptive feedback cancellation used in telephones and

teleconferencing A teleconference is the live exchange of information among several people remote from one another but linked by a telecommunications system. Terms such as audio conferencing, telephone conferencing and phone conferencing are also sometimes used t ...

devices. Much like adaptive feedback cancellation in hearing aids, echo cancellation uses an adaptive filter to cancel echo reverberations from a microphone.

Jammer Suppression

Jammer suppression is a way to reject interference with large signals that are much stronger than traditional signals. This process uses an adaptive filter, and its algorithm has applications in all types of signal suppression.

Current Research

Prediction Error Method

Prediction error method is an adaptive feedback cancellation algorithm that focuses on using

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...

s instead of speech signals. This change proposes to improve adaptive feedback cancellation in objects like hearing aids and other audio applications. This approach works closely with echo cancellation, and looks to bring the techniques in echo cancellation to hearing aids.

Sub-band Feedback Cancellation

Sub-band feedback cancellation is a type of adaptive feedback cancellation that relies less on computations and algorithms, but rather uses the signals already in the device to optimize the feedback cancellation. Sub-band feedback cancellations purpose is to make adaptive feedback cancellation cheaper and more widespread.

Digital Signal Processing

Digital Signal Processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...

pledges to reduce the effect of adaptive feedback cancellation on sound quality with the use of feedback tests. The method also reports more gain in the hearing aid, and cites a figure of 10

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...

Adaptive Feedback Cancellation in Smartphones

Research from 2018 is ongoing into adaptive feedback cancellation on smartphone speakers and microphones. Current research intends to use digital signal processing to mimic the cancellation in hearing aids in smartphones.{{cite conference , last=Mishra , first=Parth , last2=Tokgoz , first2=Serkan , last3=Panahi , first3=Issa M. S. , title=Robust real-time implementation of adaptive feedback cancellation using noise injection algorithm on smartphone , conference=Proceedings of Meetings on Acoustics, publisher=Acoustical Society of America , year=2018 , doi=10.1121/2.0000836 , volume=33, page=055003

References

External links

Diagram
Acoustics