GR581

Adaptive Highpass Filter with Independent Gain Block GR581 DATA SHEET FEATURES unity gain 12 dB/oct. highpass Butterworth filter adaptive frequency range from 200 Hz to 2 kHz rectifier threshold set to 82 dBSPL (adjustable) low noise, typically 2.6 µV 300 µA typical current drain operates from 1.1 to 3 VDC preamplifier with 56 dB open loop gain DESCRIPTION The GR581 is a level-sensitive, adaptive highpass filter comprised of a current controlled highpass filter, a half- wave rectifier, a voltage regulator for microphone bias and a separate preamplifier. The internal rectifier senses the long term average signal level and controls the filter response by setting the control current of the filter. The rectifier sensitivity, with respect to the input signal, can be increased by placing the internal preamplifier (A) before the filter section of the GR581. The preamplifier has an internal 265 kΩ feedback resistor from its input to output, to facilitate in setting the gain. An increase of 20 dB above the rectifier threshold will automatically increase the corner frequency one decade above the initial corner frequency. The filter will only allow an increase in corner frequency of one decade, regardless of further increases in the input signal level. It is recommended that the attack and release times be set to 30 ms and 90 ms respectively. The average time period of a speech frame is about 15 ms which is not long enough to activate the filter, however long term average ...