Speakerphone with adaptive cancellation of room echoes
First Claim
1. A method for reducing the effect of room resonances on the quality of sound transmitted over a speakerphone including a loudspeaker-microphone pair and a handset transmitter-receiver pair, comprising the steps of:
- speaking into said transmitter of said handset at a location in said room so that a clean audio signal is generated by said transmitter which is unaffected by said room resonances, and a corresponding audio signal is generated by said microphone which is affected by said room resonances;
delaying said corresponding audio signal to produce a plurality of incrementally delayed samples of said corresponding audio signal;
multiplying each of said incrementally delayed samples of said corresponding audio signal by one of a plurality of multipliers to produce a plurality of multiplied signals;
summing together said multiplied signals to produce a sum signal;
subtracting said sum signal from said clean audio signal to produce a difference signal;
adjusting each of said plurality of multipliers so as to reduce said difference signal towards zero, whereby said sum signal approximates said clean signal;
fixing the values of each of said plurality of multipliers to produce fixed multipliers which multiply said incrementally delayed samples of said corresponding audio signal, whereby said sum signals is said corresponding audio signal corrected for said room resonances;
ceasing speech into said transmitter of said handset after said fixing step;
speaking towards said microphone after said ceasing step; and
utilizing said sum signal as the output of said speakerphone.
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Accused Products
Abstract
An arrangement for generating an electrical audio signal in response to a sound signal applied to a microphone in a reverberating room removes the reverberations with an adaptive filter. The filter learns or adapts so as to cancel the room echoes by receiving both the audio signal affected by reverberation and a clean audio signal from a highly directional local-acting microphone placed near the source of the sound. The signal to be filtered is applied to a transversal filter including a tapped delay line having variable taps or multipliers to produce incrementally delayed multiplied samples. The multiplied samples are summed, and the sum signal is subtracted from the clean signal to produce a difference signal. A μP is programmed to iteratively calculate the effects of changes to the tap values and to select tap values which tend to reduce the magnitude of the difference signal. When the filter is converged, the difference signal is near zero. At this time, the filtered signal and the clean signal are similar, and therefore the filter characteristic removes the effect of reverberation. The tap values are then fixed, and the signal with room reverberations can be made clean by the adapted filter.
67 Citations
10 Claims
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1. A method for reducing the effect of room resonances on the quality of sound transmitted over a speakerphone including a loudspeaker-microphone pair and a handset transmitter-receiver pair, comprising the steps of:
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speaking into said transmitter of said handset at a location in said room so that a clean audio signal is generated by said transmitter which is unaffected by said room resonances, and a corresponding audio signal is generated by said microphone which is affected by said room resonances; delaying said corresponding audio signal to produce a plurality of incrementally delayed samples of said corresponding audio signal; multiplying each of said incrementally delayed samples of said corresponding audio signal by one of a plurality of multipliers to produce a plurality of multiplied signals; summing together said multiplied signals to produce a sum signal; subtracting said sum signal from said clean audio signal to produce a difference signal; adjusting each of said plurality of multipliers so as to reduce said difference signal towards zero, whereby said sum signal approximates said clean signal; fixing the values of each of said plurality of multipliers to produce fixed multipliers which multiply said incrementally delayed samples of said corresponding audio signal, whereby said sum signals is said corresponding audio signal corrected for said room resonances; ceasing speech into said transmitter of said handset after said fixing step; speaking towards said microphone after said ceasing step; and utilizing said sum signal as the output of said speakerphone. - View Dependent Claims (2, 3, 4)
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5. A method for reducing the effect of room resonances on the quality of sound transmitted over a speakerphone including a loudspeaker-microphone pair and a handset transmitter-receiver pair, comprising the steps of:
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speaking into said transmitter of said handset at a location in said room so that a clean audio signal substantially unaffected by said resonances is generated by said transmitter, and a corresponding audio signal is generated by said microphone which is affected by said resonances; applying said corresponding audio signal to an apparatus including a delay line, controllable taps coupled to said delay line, and a summer coupled to said taps, whereby said corresponding audio signal is delayed to produce a plurality of incrementally delayed audio signals, each of said incrementally delayed audio signals is multiplied by one of a plurality tap values to produce multiplied signals, and said multiplied signals are summed together to produce a sum signal; subtracting said sum signal produced by said apparatus from said clean audio signal to produce a difference signal; adjusting each of said plurality of tap values to minimize said difference signal, whereby said apparatus has a transfer function having an effect tending to cancel the effect of said room resonances; fixing said tap values; ceasing application of said clean audio signal to said apparatus; and applying said corresponding audio signal to said apparatus, whereby said sum signal represents said corresponding audio signal with the effect of said room resonances substantially reduced. - View Dependent Claims (6, 7, 8)
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9. An apparatus adapted for cancellation of room echoes, comprising:
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a handset including a transmitter adapted for receiving sound at a location in a room for producing clean audio signal unaffected by room resonances; a microphone adapted for receiving corresponding sound signals affected by room resonances for producing corresponding audio signals; delay means coupled to said microphone for delaying said corresponding audio signals to form a plurality of delayed corresponding signals; controllable tap means coupled to said delay means for multiplying each of said delayed corresponding signals by one of a plurality of tap values to form multiplied corresponding signals; summing means coupled to said controllable tap means for summing together said multiplied corresponding signals to form a summed signal; subtracting means including a first input terminal coupled to said summing means and also including a second input terminal for forming a difference signal representative of the difference between said summed signal and a signal applied to said second input terminal of said subtracting means; coupling means coupled to said transmitter and to said second input terminal of said subtracting means for applying said clean audio signal to said second input terminal of said subtracting means, whereby said difference signal represents the difference between said clean audio signal and said summed signal; control means coupled to said subtracting means and to said controllable tap means for, in a learning mode of operation, selecting the values of each of said plurality of tap values in such a manner as to reduce the magnitude of said difference signal, and, in a normal mode of operation, for maintaining the values of each of said plurality of tap values at a selected value, whereby said summed signal represents said corresponding audio signal corrected for room resonances; and means for coupling said summed signal to a utilization means. - View Dependent Claims (10)
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Specification