Patent
1977-05-25
1978-03-14
Stellar, George G.
H04R 302
Patent
active
040791998
ABSTRACT:
A device for detecting the oscillating condition caused by the acoustic coupling between a loud speaker and a microphone at those frequencies which do not satisfy the Nyquist stability criteria, and for thereafter decreasing the system loop gain. The detection circuitry consists of a bank of phase lock loop detectors. Each phase lock loop within the bank covers a limited frequency range such that together the bank covers the entire audio band. Upon detecting an oscillation in the particular phase lock loop frequency passband, that phase lock loop control voltage is automatically internally adjusted to align the phase lock loop frequency with the oscillating frequency. An abnormal phase lock loop control voltage signifies the presence of an unwanted oscillating signal and is detected by circuitry which includes a diode peak detector followed by a pair of voltage comparators. Upon detecting the ringing condition, the loop gain is decreased until the ringing condition ceases. A digital up-down counter is employed to establish the loop gain. The output of this counter is applied to a digital-to-analog converter which in combination with an operational amplifier, a light emitting diode, and a photoresistor form an attenuator circuit which controls the loop gain. Provision is also made for automatically counting down the up-down counter by applying down clocks to this counter at predetermined intervals such that after a certain time period the loop gain is returned to a particular desired setting.
REFERENCES:
patent: 2157177 (1939-05-01), Kellogg
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