Noise cancellation using segmented, frequency-dependent phase cancellation
Noise abatement within a signal stream containing unwanted signal referred to as noise is performed by acquiring a digitized noise signal and using a digital processor circuit to subdivide the acquired noise signal into different frequency band segments and thereby generate a plurality of segmented noise signals. Then individually for each segmented noise signal, the processor shifts in time the segmented noise signal by an amount dependent on a selected frequency of the segmented noise signal to produce a plurality of shifted segmented noise signals. The precise time shift applied to each noise segment considers the frequency content of the segment and the system processing time. Individually for each segmented noise signal, amplitude scaling is applied. The shifted and amplitude-scaled segmented noise signals are then combined to form a composite anti-noise signal which is output into the signal stream to abate the noise through destructive interference.
1. A method for calibrating a noise abatement system for an audio device, comprising:
disabling the noise abatement system;
acquiring, by the audio device, a known signal from an environment surrounding the audio device;
determining an in-air propagation time of the known signal while the noise abatement system is disabled, where the in-air propagation time is transit time for a signal to travel through air from an input microphone of the audio device to a speaker of the audio device;
enabling the noise abatement system; and
determining a system propagation time of the known signal through the noise abatement system while the noise abatement system is enabled, where the system propagation time is throughput speed of a digital processor circuit comprising the noise abatement system.
2. The method of claim 1 further comprises
computing a system offset time by subtracting the system propagation time from the in-air propagation time; and
storing the system offset time in a computer memory of the audio device, where the computer memory is accessible to the noise abatement system.
3. The method of 1 further comprises outputting, by the noise abatement system, an anti-noise signal into a signal stream output by the audio device, where the anti-noise signal is constructed using the system offset time.
4. The method of claim 3 further comprises
segmenting the known signal into a plurality of signal segments, where each signal segment in the plurality of signal segments correlates to a different frequency range;
for each signal segment, calculating a segment time delay, where the segment time delay causes a 180 degree phase shift at center frequency of the frequency range associated with a given signal segment;
for each signal segment, calculating an applied time delay as an absolute value of the segment time delay minus the system offset time; and
for each signal segment, storing the applied time delay in the computer memory of the audio device.
5. The method of 1 wherein determining in-air propagation time of the known signal further comprises
determining a time the known signal is captured by the input microphone;
determining a time the known signal is captured by a feedback microphone disposed proximate to the speaker of the audio device; and
calculating the in-air propagation time by subtracting the time the known signal is captured by the input microphone from the time the known signal is captured by the feedback microphone.
6. The method of claim 1 wherein determining the system propagation time further comprises
determining a time the known signal is captured by the input microphone;
determining a time the known signal is captured by a feedback microphone disposed proximate to the speaker of the audio device; and
calculating the system propagation time by subtracting the time the known signal is captured by the input microphone from the time the known signal is captured by the feedback microphone.
7. The method of claim 6 further comprises altering the known signal after the known signal is acquired by the audio device but prior to inputting the known signal into the noise abatement system.