Headphone off-ear detection

1. A signal processor for headphone off-ear detection, the signal processor comprising:

an audio output to 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 over an OED frame as a received frequency response,

determine 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,

generate a difference metric comparing the received frequency response to the ideal off-ear frequency response, and

employ the difference metric to detect when the headphone cup is disengaged from an ear.

2. 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.

3. The signal processor of claim 2 , wherein the OED signal processor is further configured to determine an audio frequency response of the audio signal times an on-ear transfer function between the headphone speaker and the FB microphone as an ideal on-ear response.

4. The signal processor of claim 3 , wherein the OED signal processor is further configured to normalize the difference metric based on the ideal on-ear response.

5. The signal processor of claim 4 , wherein the difference metric is determined according to:

Normalized_difference

⁢

_metric

=

log

⁢

abs

(

Received

)

abs

⁡

(

Ideal_off

⁢

_ear

)

log

⁢

abs

⁡

(

Ideal_on

⁢

_ear

)

abs

⁡

(

Ideal_off

⁢

_ear

)

where Received is the received frequency response, Ideal_off_ear is the ideal off-ear frequency response, and Ideal_on_ear is the ideal on-ear response.

6. The signal processor of claim 2 , wherein the difference metric includes a plurality of frequency bins, and the OED signal processor is further configured to weight the frequency bins.

7. The signal processor of claim 6 , wherein the OED signal processor is further configured to determine a difference metric confidence as a sum of frequency bin weights, and employ the difference metric confidence when detecting the headphone cup is disengaged from the ear.

8. The signal processor of claim 7 , wherein the OED signal processor is further configured to determine the headphone cup is engaged when difference metric confidence is above a difference metric confidence threshold and the difference metric is above a difference metric threshold.

9. The signal processor of claim 6 , further comprising a tone generator configured to generate an OED tone at a specified frequency bin to support generation of the difference metric when the audio signal drops below a noise floor.

10. The signal processor of claim 9 , wherein the OED signal processor is further configured to control the tone generator to maintain a ratio of OED tone power to noise-floor tone power with a programmable margin.

11. The signal processor of claim 9 , 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.

12. The signal processor of claim 1 , wherein the difference metric is averaged over an OED cycle, and the OED signal processor is further configured to determine the headphone cup is disengaged when the average difference metric is above a difference metric threshold.

13. 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.

14. 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.

15. 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.

16. A method comprising:

employing a tone generator to generate an off-ear detection (OED) tone at a specified frequency bin;

injecting the OED tone into an audio signal forwarded to a headphone speaker;

detecting a noise floor from a feedforward (FF) microphone signal;

adjusting a volume of the OED tone based on a volume of the noise floor;

generating a difference metric by comparing a Feedback (FB) signal from a FB microphone to the audio signal; and

employing the difference metric to detect when the headphone cup is disengaged from an ear.

17. The method of claim 16 , wherein a tone margin is maintained between the volume of the OED tone and the volume of the noise floor.

18. The method of claim 16 , wherein detecting when the headphone cup is disengaged includes determining when the difference metric exceeds a threshold.

19. The method of claim 16 , wherein the difference metric is generated by:

determining an audio frequency response of the FB signal over an OED frame as a received frequency response,

determining 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, and

generating a difference metric comparing the received frequency response to the ideal off-ear frequency response.

20. The method of claim 19 , wherein the difference metric is determined over a plurality of frequency bins, including the specified frequency bin, and the method further comprises:

weighting the frequency bins;

determining a difference metric confidence as a sum of frequency bin weights; and

employing the difference metric confidence when detecting the headphone cup is disengaged from the ear.