Headphone off-ear detection
Disclosed is a signal processor for headphone off-ear detection. The signal processor includes an audio output to transmit an audio signal toward a headphone speaker in a headphone cup. The signal processor also includes a feedback (FB) microphone input to receive a FB signal from a FB microphone in the headphone cup. The signal processor also includes an off-ear detection (OED) signal processor to determine an audio frequency response of the FB signal over an OED frame as a received frequency response. The OED processor also determines an audio frequency response of the audio signal times an off-ear transfer function between the headphone speaker and the FB microphone as an ideal off-ear response. A difference metric si generated comparing the received frequency response to the ideal off-ear frequency response. The difference metric is employed to detect when the headphone cup is disengaged from an ear.
1. A signal processor for headphone off-ear detection, the signal processor comprising:
an audio generator to create and transmit an audio signal toward a headphone speaker in a headphone cup;
a feedback (FB) microphone input to receive a FB signal from a FB microphone in the headphone cup; and
an off-ear detection (OED) signal processor configured to:
determine an audio frequency response of the FB signal as a received frequency response,
generate a difference metric comparing the received frequency response to a modeled off-ear frequency response, and
use the difference metric to detect when the headphone cup is disengaged from an ear.
2. The signal processor of claim 1 , in which the audio signal generated by the audio generator is less than 100 Hz.
3. The signal processor of claim 1 , in which the audio signal generated by the audio generator is between 15 and 30 Hz.
4. The signal processor of claim 1 , further comprising a feedforward (FF) microphone input to receive a FF signal from a FF microphone outside of the headphone cup, wherein the OED signal processor is further configured to remove a correlated frequency response between the FF signal and the FB signal when determining the received frequency response.
5. The signal processor of claim 4 , in which the FF signal is used to determine a noise floor, and in which the audio generator is structured to create and transmit the audio signal having an amplitude greater than the noise floor.
6. The signal processor of claim 5 , in which when the audio processor is structured to increase an amplitude of the audio signal after the noise floor increases.
7. The signal processor of claim 6 , in which when the audio processor is structured to increase an amplitude of the audio signal at a rate slower than the noise floor increased.
8. The signal processor of claim 5 , further comprising:
a left feedforward (FF) microphone input to receive a left FF signal from a left FF microphone; and
a right FF microphone input to receive a right FF signal from a right FF microphone, wherein the OED signal processor is further configured to select a weaker of the FF signals to determine the noise floor when wind noise is detected in a stronger of the FF signals.
9. The signal processor of claim 1 , wherein a plurality of difference metrics, including the difference metric, are generated over an OED cycle, and the OED signal processor is further configured to determine the headphone cup is disengaged when a change between difference metrics is greater than a difference metric change threshold.
10. The signal processor of claim 1 , wherein the OED signal processor is further configured to:
determine a distortion metric based on a variance of the difference metric over a plurality of frequency bins, and
ignore the difference metric when the distortion metric is greater than a distortion threshold.
11. The signal processor of claim 1 , wherein the OED signal processor is further configured to:
determine an expected phase of the FB signal based on a phase of the audio signal, and
reduce a confidence metric corresponding to the difference metric when a difference in phase of a received frequency response associated with the FB signal and the expected phase of the received frequency response associated with the FB signal is greater than a phase margin.