Method and apparatus for reducing periodic interference in audio signals

In call handling system having a telecommunications switch, and a plurality of operator computer stations connected to the telecommunications switch, each computer station including a communication device having a microphone and a speaker permitting the operator at the computer station to participate in voice communications with one or more parties via calls established through the telecommunications switch, a method of processing an audio signal including voice and periodic interference in an audible frequency band delivered to the operator by way of the speaker where a decibel level of the periodic interference output by the speaker may exceed a threshold that is undesirable to the operator listening to the communications device, comprising the steps of:

• sampling the audio signal at a predetermined sampling rate to generate a signal sequence;
• determining a signal energy of the signal sequence;
• based on a previous sequence of samples, predicting a current value of the periodic interference by delaying the signal sequence by a predetermined delay and filtering the delayed signal sequence to generate a filter output;
• and
• removing the predicted current value of the periodic interference to ensure that the decibel level of the periodic interference present in the current signal is less than the threshold by subtracting the filtered output from the signal sequence to generate a difference signal;
• wherein the method further comprises:
• adapting filter parameters used in the filtering step based on the difference signal, and
• determining a step size used in adapting the filter parameters based on the signal energy.

A call handling system comprising:

• a telecommunications switch, and
• a plurality of operator computer stations connected to the telecommunications switch, each computer station including a communication device having a microphone and a speaker permitting the operator at the computer station to participate in voice communications with one or more parties via calls established through the telecommunications switch, some of the calls including one or more audible tones, and an audio signal processor,
• wherein the audio processor includes:
• an analog to digital converter for converting received audio signals from the telecommunications switch into digitized audio samples;
• an adaptive filter connected to the analog to digital converter substantially reducing an amplitude of a tone or other periodic interference occurring in a current audio sample while substantially preserving an amplitude of the voice information in the current audio sample;
• a filter adaptor determining (1) a signal energy associated with the current audio sample and (2) an adjustment factor based on that determined signal energy and adapting the adaptive filter using the determined adjustment factor;
• a digital to analog converter converting an output signal from the adaptive filter into an analog audio signal;
• and
• an amplifier amplifying the analog audio signal and forwarding the amplified signal to drive the speaker.

A digital filter for use in processing audio signals in a call handling system, comprising:

• a delay that receives and delays a digitized sequence of samples corresponding to an audio signal, the audio signal having both voice and interference;
• series-connected filter taps connected to the delayed signal sequence;
• plural multipliers, each multiplier multiplying a signal from a corresponding one of the filter taps with a corresponding filter coefficient;
• a summer summing outputs from the multipliers to generate a signal predicting periodic interference in a current sample;
• a combiner for subtracting the predicted periodic interference from the current sample to obtain a difference signal;
• an adaptive controller recursively modifying the filter coefficients in response to the difference signal and a step size;
• and
• a signal energy detector determining the energy of the sequence of samples,
• wherein the step size is determined based on the determined signal energy and a number of the plural filter taps.