# US 2019/0356984 A1 · Reducing acoustic feedback over variable-delay pathway > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2019/0356984 A1 - Document ID: US20190356984A1 - Status: Published application - Kind: A1 - Application number: 16/412,863 - Filed: 2019-05-15 - Published: 2019-11-21 - Earliest claimed: 2018-05-15 - Assignees: LogMeIn, Inc. - Applicants: LogMeIn, Inc. - Inventors: Carlotta Anemüller; Florian Heese; Patrick Vicinus - Web page: https://patentgrep.com/patent/US20190356984A1 ## CPC - H04R 3/02: for preventing acoustic reaction {, i.e. acoustic oscillatory feedback (specially adapted for hearing aids H04R25/453)} - H04R 3/04: for correcting frequency response - H04R: LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS (producing sounds with frequency not determined by supply frequency G10K) ## Abstract A technique for reducing acoustic feedback in audio communications includes measuring variations in round-trip delay over an audio signal pathway. The technique varies a delay interval of an adjustable-delay element in real time based on the measured variations in round-trip delay, effectively canceling the delay variations. Further techniques are disclosed for detecting and eliminating howling frequencies which arise as a result of acoustic feedback in the audio signal pathway. ## Claims ### Claim 1 A method of reducing acoustic feedback in audio communications, the method comprising: measuring changes in round-trip delay along an audio signal pathway that extends from a microphone of a first computing device, to a computer network, over the computer network to a second computing device, to a speaker of the second computing device, and through an acoustic medium from the speaker back to the microphone, the microphone having an output that produces a microphone signal; modeling the audio signal pathway with a path emulator that includes (i) an adaptive filter configured to emulate an impulse response of the audio signal pathway but not the changes in round-trip delay and (ii) an adjustable-delay element, coupled in series with the adaptive filter and configured to emulate the changes in round-trip delay based on the measured changes; and generating, by the path emulator in response to receipt of an audio signal by the path emulator, a prediction signal that emulates effects of the audio signal pathway on the audio signal, the audio signal generated as a difference between the microphone signal and the prediction signal and providing a representation of the microphone signal corrected for acoustic feedback. ### Claim 2 (depends on 1) The method of claim 1, wherein measuring the changes in round-trip delay includes measuring multiple instances of round-trip delay at respective times, and wherein modeling the audio signal pathway includes configuring, in real time, the adjustable-delay element to establish delay changes that match the measured changes in round-trip delay. ### Claim 3 (depends on 2) The method of claim 2, wherein measuring each instance of round-trip delay includes: identifying a repeating pattern in the microphone signal; and generating the instance of round-trip delay as a time difference between a first occurrence of the repeating pattern and a second occurrence of the repeating pattern. ### Claim 4 (depends on 3) The method of claim 3, wherein identifying the repeating pattern includes detecting a set of howling frequencies in the microphone signal, each howling frequency being a frequency at which the microphone signal exhibits unstable oscillatory behavior. ### Claim 5 (depends on 4) The method of claim 4, wherein generating the instance of round-trip delay includes: generating multiple frequency transforms of the microphone signal at respective times; performing an autocorrelation operation on a selected frequency bin across the frequency transforms, the autocorrelation operation providing a measure of correlation among magnitudes of the selected frequency bin over time; and identifying the instance of round-trip delay as a time at which the autocorrelation operation produces a maximum value, wherein generating the instance of round-trip delay is based at least in part on measurements of at least one of the set of howling frequencies. ### Claim 6 (depends on 5) The method of claim 5, wherein configuring, in real time, the adjustable delay element includes establishing a delay setting of the delay element based at least in part on the identified instance of round-trip delay. ### Claim 7 (depends on 5) The method of claim 5, wherein detecting the set of howling frequencies includes: identifying multiple sets of frequency bins across the frequency transforms, each set of frequency bins corresponding to a respective frequency range, different sets of frequency bins corresponding to different frequency ranges; and for each set of frequency bins, performing a power test on that set of frequency bins, the power test passing in response to a peak-to-average power ratio (PAPR) of the set of frequency bins exceeding a predetermined PAPR threshold, the power test failing in response to the PAPR of the set of frequency bins falling below the predetermined PAPR threshold. ### Claim 8 (depends on 7) The method of claim 7, further comprising disqualifying frequency bins as candidates for containing a howling frequency in response to the power test failing. ### Claim 9 (depends on 7) The method of claim 7, wherein detecting the set of howling frequencies further includes, for each set of frequency bins for which the power test passes, performing an autocorrelation test on that set of frequency bins, the autocorrelation test passing in response to an autocorrelation operation performed on the set of frequency bins producing a maximum value that exceeds a predetermined autocorrelation threshold, the autocorrelation test failing in response to the autocorrelation operation performed on the set of frequency bins producing a maximum value that falls below the predetermined autocorrelation threshold; and detecting a howling frequency in the frequency range that corresponds to the set of frequency bins, in response to both the power test passing and the autocorrelation test passing. ### Claim 10 (depends on 4) The method of claim 4, further comprising, once the set of howling frequencies has been detecting, implementing a set of notch filters in line with the audio signal pathway, the set of notch filters configured to selectively attenuate the set of howling frequencies. ### Claim 11 (depends on 2) The method of claim 2, further comprising realizing the path emulator entirely within the first computing device. ### Claim 12 A computerized apparatus, comprising control circuitry that includes a set of processors coupled to memory, the control circuitry constructed and arranged to: measure changes in round-trip delay along an audio signal pathway that extends from a microphone of a first computing device, to a computer network, over the computer network to a second computing device, to a speaker of the second computing device, and through an acoustic medium from the speaker back to the microphone, the microphone having an output that produces a microphone signal; model the audio signal pathway with a path emulator that includes (i) an adaptive filter configured to emulate an impulse response of the audio signal pathway but not the changes in round-trip delay and (ii) an adjustable-delay element, coupled in series with the adaptive filter and configured to emulate the changes in round-trip delay based on the measured changes; and generate, by the path emulator in response to receipt of an audio signal by the path emulator, a prediction signal that emulates effects of the audio signal pathway on the audio signal, the audio signal generated as a difference between the microphone signal and the prediction signal and providing a representation of the microphone signal corrected for acoustic feedback. ### Claim 13 A computer program product including a set of non-transitory, computer-readable media having instructions which, when executed by control circuitry of a computerized apparatus, cause the computerized apparatus to perform a method for reducing acoustic feedback in audio communications, the method comprising: measuring changes in round-trip delay along an audio signal pathway that extends from a microphone of a first computing device, to a computer network, over the computer network to a second computing device, to a speaker of the second computing device, and through an acoustic medium from the speaker back to the microphone, the microphone having an output that produces a microphone signal; modeling the audio signal pathway with a path emulator that includes (i) an adaptive filter configured to emulate an impulse response of the audio signal pathway but not the changes in round-trip delay and (ii) an adjustable-delay element, coupled in series with the adaptive filter and configured to emulate the changes in round-trip delay based on the measured changes; and generating, by the path emulator in response to receipt of an audio signal by the path emulator, a prediction signal that emulates effects of the audio signal pathway on the audio signal, the audio signal generated as a difference between the microphone signal and the prediction signal and providing a representation of the microphone signal corrected for acoustic feedback. ### Claim 14 (depends on 13) The computer program product of claim 13, wherein measuring the changes in round-trip delay includes measuring multiple instances of round-trip delay at respective times, and wherein modeling the audio signal pathway includes configuring, in real time, the adjustable-delay element to establish delay changes that match the measured changes in round-trip delay, and wherein measuring each instance of round-trip delay includes (i) identifying a repeating pattern in the microphone signal and (ii) generating the instance of round-trip delay as a time difference between a first occurrence of the repeating pattern and a second occurrence of the repeating pattern. ### Claim 15 (depends on 14) The computer program product of claim 14, wherein identifying the repeating pattern includes detecting a set of howling frequencies in the microphone signal, each howling frequency being a frequency at which the microphone signal exhibits unstable oscillatory behavior, and wherein generating the instance of round-trip delay is based at least in part on measurements of at least one of the set of howling frequencies. ### Claim 16 A method of detecting howling frequencies in audio communications, the method comprising: generating a frequency transform of a microphone signal; generating an autocorrelation function of the microphone signal; and identifying a set of howling frequencies based on both the frequency transform and the autocorrelation function. ### Claim 17 (depends on 16) The method of claim 16, further comprising: generating a centroid frequency that represents a weighted average of magnitude values of the frequency transform; computing a sum of magnitude values of frequency bins within a predetermined range of the centroid frequency; and confirming the centroid frequency as a howling frequency based at least in part on a ratio of the sum of magnitude values to a sum of all magnitude values of the frequency transform exceeding a predetermined threshold. ### Claim 18 (depends on 16) The method of claim 16, further comprising: generating multiple frequency transforms of a microphone signal at respective times; identifying multiple sets of frequency bins across the frequency transforms, each set of frequency bins corresponding to a respective frequency range, different sets of frequency bins corresponding to different frequency ranges; and for each set of frequency bins, performing a power test on that set of frequency bins, the power test passing in response to a peak-to-average power ratio (PAPR) of the set of frequency bins exceeding a predetermined PAPR threshold, the power test failing in response to the PAPR of the set of frequency bins falling below the predetermined PAPR threshold. ### Claim 19 (depends on 16) The method of claim 16, further comprising, once the set of howling frequencies has been identified, implementing a set of notch filters in line with the audio signal pathway, the set of notch filters configured to selectively attenuate the set of howling frequencies.