Abstract For an audio system such as a telephone station set of the socalled ''''hands free'''' variety, the well-known echo effect known as the ''''rain barrel'''' effect is reduced and the average signal to echo power ratio is improved by employing a multiplicity of microphones and equalizing the total delays in the signal paths including the microphones at which the strongest audio signals are received. The needed variation of delay is determined by time domain correlation techniques in which one branch circuit includes a relatively fast local peak detector for a product signal and another branch circuit includes a relatively slow absolute peak detector for such a product signal. Other microphones may be similarly paired for delay equalization. The signals are then summed to produce the output signal.
A circuit for enhancing a desired signal in the presence of an undesired signal comprising first and second channels mutually arranged both to receive a desired signal and to receive an undesired signal with differing delays at the respective inputs of said channels between said desired and undesired signals, at least one of said channels having a controllable variable delay therein, means for causing the desired signal to arrive simultaneously at the outputs of both channels comprising means for correlating the instantaneous signal amplitudes in both channels and means for controlling said variable delay to yield time coincidence of like amplitudes at said outputs, and means for summing the signals at the outputs of said channels.
Claim 2
Dependent Claim
116 words
A circuit for enhancing a desired signal in the presence of an undesired signal comprising first and second channels mutually arranged both to receive a desired signal and to receive an undesired signal with differing delays at the respective inputs of said channels between said desired and undesired signals, at least one of said channels having a controllable variable delay therein, means for causing the desired signal to arrive simultaneously at the outputs of both channels comprising means for correlating the instantaneous signal amplitudes in both channels and means for controlling said variable delay to yield time coincidence of like amplitudes at said outputs, and means for summing the signals at the outputs of said channels.
Claim 3
Dependent Claim
110 words
A circuit according to claim 1 in which the correlating means comprises a first branch circuit means for detecting a local maximum of a signal that is a product of the signals in the two channels, including means for dithering the variable delay at a first frequency, a second branch circuit means for detecting the absolute maximum of a signal that is a product of the signals in the two channels, including means for dithering the variable delay at a second frequency substantially less than said first frequency, and means for intermittently resetting the output of the first branch circuit to correspond to the output of the second branch circuit.
Claim 4
Dependent Claim
25 words
A circuit according to claim 2 in which the summing means includes means for filtering the first and second frequencies from the ultimate output signal.