Outbound noise cancellation for telephonic handset

• US 5,406,622 A
• Filed: 09/02/1993
• Issued: 04/11/1995
• Est. Priority Date: 09/02/1993
• Status: Expired due to Term

• Current Assignee: American Telephone & Telegraph Company (AT&T, Inc.)
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Thompson, Norman B., Benning, Roger D., Silverberg, Michael H.
• Primary Examiner(s): Dwyer, James L.
• Assistant Examiner(s): FOURNIER, PAUL
• Application Number:
  US08/114,851
• Time in Patent Office: 586 Days
• Field of Search: 379/387, 379/411, 379/416, 379/388, 379/389, 379/392, 379/417, 379/411, 381/71, 381/94, 381/92, 381/46, 381/47
• US Class Current: 379/392.01
• CPC Class Codes: H04M 1/19 Arrangements of transmitters, receivers, or complete sets to prevent eavesdropping, to attenuate local noise or to prevent undesired transmission; Mouthpieces or receivers specially adapted therefor circuit arrangements for preventing eavesdropping H04M1/68; telephone cabinets E04H1/14
• CPC Class Codes: H04M 1/6008 in the transmitter circuit

Abstract

A telephone handset outbound noise canceller uses an omnidirectional reference microphone placed near the transmitter grille. Two adaptive filters are employed, one driven by the handset transmitter to subtract speech from a reference value to produce an enhanced reference signal; and a second adaptive filter driven by the enhanced reference signal to subtract noise from the transmitter. The weights of the two adaptive filters are allowed to adapt or are frozen according to criteria set as a result of transmitter power measurements which determine whether speech is likely to be present or not.

Claims

1. In a telephone instrument comprising a transmit microphone and means connecting said instrument to a telephone network, apparatus for reducing entry into said network of acoustic background noise comprising:

a reference microphone for picking up noise signals;

a noise-cancellation circuit comprising a first adaptive filter connected to said transmit microphone for conditioning speech signals and a second adaptive filter for receiving a signal consisting substantially of noise only, for conditioning said noise signals;

each said filter comprising;

plural transversal elements arrayed as serial delay lines each having an output tap following each said element, and each said filter further comprising weighting means for modifying the outputs of its respective taps;

said first adaptive filter comprising means for periodically sampling the output of said transmit microphone and said second adaptive filter comprising means for periodically sampling said noise-only signal;

said first adaptive filter weighting means being adapted for modifying the outputs of the corresponding output taps in accordance with the most recent transmit microphone samples;

means for additively combining the outputs of said respective first adaptive filter taps and second adaptive filter taps to produce a respective output for each said filter;

first summing means for summing the outputs of said reference microphone and said first filter;

means for connecting the output of said first summing means to the input of said second filter;

second summing means for summing the outputs of said second filter and of said transmit microphone and for feeding the result to a network;

first settable threshold means defining the presence of speech and second settable threshold means defining the absence of speech;

means including said first and said second settable threshold means for generating, when a measure of speech power exceeds said first threshold means, a first signal indicative of the presence of speech at said transmit microphone and for generating, when said measure of speech power falls below said second threshold means, a second signal indicative of the absence of speech at said transmit microphone;

means, responsive to said first signal, for updating said weighting means of said first adaptive filter while holding said weighting means of said second adaptive filter frozen; and

means, responsive to said second signal, for updating said weighting means of said second adaptive filter while holding said weighting means of said first adaptive filter frozen.

2. Apparatus in accordance with claim 1, further comprising means for holding said weighting means of both said first and second adaptive filters frozen when said measure of speech power falls between the current settings of said first and second threshold means.

3. Apparatus in accordance with claim 2, further comprising means for sensing when noise energy falls below a predetermined level;

and means responding to said sensing means for switching off said noise-cancellation circuit.

4. Apparatus in accordance with claim 3, wherein said means for generating a signal indicative of the presence or the absence of speech comprises:

means for taking samples of the output of said transmit microphone;

means for averaging a plurality of the most recent said samples to produce an indicia of total current power output;

means for calculating an estimate of noise power;

means for dividing the value of said current power output by the current noise power estimate to form a current overall power-to-noise power ratio;

means for averaging a plurality of the most recently-developed said ratios; and

means for deriving from said average an indicia of the onset of speech.

5. Apparatus for reducing the introduction of outbound noise into a communications network from a telecommunications device, said apparatus comprising:

a transmit microphone and a reference microphone closely disposed to said transmit microphone, both microphones being mounted in said device;

a speech adaptive filter comprising means for periodically sampling an output of said transmit microphone and generating an output signal replicating the total outbound signal into said network;

a noise adaptive filter comprising means for periodically sampling the sum of the output of said reference microphone and said output signal of said speech adaptive filter, and for generating from said sampling an output signal replicating said outbound noise;

said speech adaptive filter and said noise adaptive filter each comprising an updatable weighting means for modifying said respective output signals of said filters;

first settable threshold means for producing a first indication defining the presence of speech at said transmit microphone;

second settable threshold means for producing a second indication defining the absence of speech at said transmit microphone;

means responsive to said first indication for updating said weighting means of said speech adaptive filter while holding said weighting means of said noise adaptive filter frozen; and

means responsive to said second indication for updating said weighting means of said noise adaptive filter while holding said weighting means of said speech adaptive filter frozen;

whereby said respective filters are active or not depending on the presence or absence of speech energy at said transmit microphone.