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ACOUSTIC FEEDBACK CANCELLATION

Acoustic feedback (also known as howling) is inevitable whenever sound reinforcement is necessary, for instance in large meeting rooms, court rooms, classrooms, auditoriums, theaters, etc. In such large rooms, although audience (or participants) are seated in the same room, they may be seated far from the presenter (or from each other) and may not hear the message clearly.

Sound reinforcement is necessary in such situations where microphones are used to pick up the talker voice, the microphones signals are amplified, and then played to loudspeakers placed in the same room so that audience can hear clearly. The problem, however, is that the microphones not only pick up the presenter's voice, but they also pick up the loudspeaker sound after traveling through the room back to the microphones, causing a closed electroacoustic loop. If the loop gain at any frequency becomes larger than 1, the loop becomes unstable at that frequency, and starts oscillation, producing the well known howling phenomenon.



AUDIO SAMPLES

To avoid howling, an electronic device (Acoustic Feedback Canceller or Howling Suppressor) must be used to monitor the closed loop and ensure that the loop stays stable at all times. The sample audio files below demonstrate the AFC at work.

The audio signal at the LEFT side has been recorded in a room where sound reinforcement is used at moderate amplifier gain with the AFC switched OFF. In this case the coupling between the loudspeaker and microphone is too high and oscillation at around 5KHz starts immediately, even before the loop is excited by any speech. The 5 KHz tone and other oscillations increase in amplitude rapidly resulting in saturating the amplifier and other equipment.

The sample at the right is recorded in the same room at the same amplifier gain with the AFC switched ON. This sample audio file shows that the AFC suppressed the 5KHz oscillation even before it happens and other oscillations almost immediately as they appear until the loop has been stabilized completely.



AFC OFF AFC ON

BEFORE

AFTER



DISTINCT FEATURES

  • Efficient block frequency domain processing resulting in low resource consumption and high quality loudspeaker signal.
  • Minimum distortion compared to solution relying on frequency shifting or phase modulation.
  • Robust against microphones and loudspeaker displacement since it does not rely on estimating the room transfer function.
  • Low processing delay.
  • Processing delay depends on user adjustable block length.


EVALUATING THE ACOUSTIC FEEDBACK CANCELLER

A real-time demonstrator is directly available on the OCEAN-ADSP21489 platform. This demonstrator also includes the CANEC speech enhancement, to request this demo, please fill the CANEC demonstration request form. For more information on the Acoustic Feedback Canceller algorithm we appreciate taking the time to contact us.


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