Browse Prior Art Database

Digital Autosquelch Circuit

IP.com Disclosure Number: IPCOM000117748D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Beers, GE: AUTHOR [+2]

Abstract

Disclosed is a circuit for reducing the extraneous noise coupled into the audio beeper signals for a notebook computer. The design involves a MOSFET squelching circuit which is comprised of a logic control circuit, a frequency dependent attenuator, and a multiple circuit summing node.

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Digital Autosquelch Circuit

      Disclosed is a circuit for reducing the extraneous noise
coupled into the audio beeper signals for a notebook computer.  The
design involves a MOSFET squelching circuit which is comprised of a
logic control circuit, a frequency dependent attenuator, and a
multiple circuit summing node.

      In the audio sections of some notebook computers, digitally
generated audio signals are resistively summed into the left channel
of a power amplifier input stage.  These signals, AUD1 and AUD2, are
sourced from potentially noisy digital sections of the computer.  A
circuit or filter is required to suppress this digitally generated
noise to a low level.  Just using a low pass filter is not nearly
ideal because the low pass cutoff frequency would need to be so low
as to render the normal input signals too quiet to hear and the
volume would be frequency dependent; higher frequencies would be at
lower volume levels.

      The Digital Autosquelch Circuit is used to attenuate the AUD1
signal and the AUD2 signal when not active so digital noise is not
feed into the analog circuitry.  When either AUD signal is active it
will mask any digital noise riding on the signal due to the fact that
the amplitude is higher than the noise.  Refer to Fig. 1 for a
drawing of the signals vs. time showing how the AUDx signals may
have noise superimposed upon them.  Also observe from the drawing
that when the squelch circuit is engaged that the noise is sup...