A feedback suppressor is an

audio signal processing Audio signal processing is a subfield of signal processing that is concerned with the electronic manipulation of audio signals. Audio signals are electronic representations of sound waves—longitudinal waves which travel through air, consisting ...

device which is used in the signal path in a live

sound reinforcement system A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in Loudspeaker enclosure, enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also ...

to prevent or suppress

audio feedback Audio feedback (also known as acoustic feedback, simply as feedback) is a positive feedback situation that may occur when an acoustic path exists between an audio output (for example, a loudspeaker) and its audio input (for example, a microphon ...

. Digital feedback reduction is the application of digital techniques to

sound reinforcement A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sou ...

in order to reduce

and increase headroom.

Operation

Feedback suppressors use three main methods to control feedback, * frequency shifting, * adaptive filtering and * automatic

notch filter In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the inverse of a ''band-pass filter''. A notch filter is ...

ing Frequency shifting is the oldest feedback suppression technique dating back to the 1960s. This technique works by introducing a varying shift in frequency to the system response. This is typically implemented using a

frequency mixer In electronics, a mixer, or frequency mixer, is an electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and di ...

. Only modest improvement of gain before feedback is achieved and the technique creates noticeable pitch distortion in music program. The adaptive filter approach works by modeling the transfer function of the sound reinforcement system and subtracts the reinforced sound from the inputs to the system in the same way that an echo canceller removes echoes from a

communications system A communications system is a collection of individual telecommunications networks systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperation to form an integrated whole. Commu ...

. Parametric equalization and notch filters are commonly used by

sound engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a sound recording, recording or a Concert, live performance, balancing and adjusting sound sources using equalization (audio), equalization, Dynamic range ...

s to manually control feedback. A feedback suppressor using the automatic notch technique listens for the onset of feedback and automatically inserts a notch filter into the signal path at the frequency of the detected feedback. Feedback suppressors use several techniques for detecting feedback from non-invasive

harmonic analysis Harmonic analysis is a branch of mathematics concerned with investigating the connections between a function and its representation in frequency. The frequency representation is found by using the Fourier transform for functions on unbounded do ...

of a potential feedback signal to more invasive adaptive filtering and speculative placement of notch filters. The automatic notch technique is the most popular method and has the advantage that the sound is not colored until the system is at risk of feedback.

References

{{Reflist Sound recording technology Audio engineering