Compensating for binaural loudspeaker directivity
The directivity of a loudspeaker describes how sound produced by the speaker varies with angle and frequency. Low-frequency sound tends to be relatively omnidirectional, while high-frequency sound tends to be more strongly directional. Because the two ears of a listener are in different spatial positions, the direction-dependent performance of the speakers can produce unwanted differences in volume or spectral content between the two ears. For example, high-frequency sounds may appear to be muffled in one ear, compared to the other. A multi-speaker sound system can employ binaural directivity compensation, which can compensate for directional variations in performance of each speaker, and can reduce or eliminate the difference in volume or spectral content between the left and right ears of a listener. The binaural directivity compensation can optionally be included with spatial audio processing, such as crosstalk cancellation, or can optionally be included with loudspeaker equalization.
1. A system for producing binaural directivity-compensated sound, the system comprising:
a plurality of speakers,
each of the plurality of speakers having a characteristic directivity that describes a relative volume level output by the speaker, as a function of azimuth angle, elevation angle, and frequency,
the directivities of the speakers operationally producing a spectral content imbalance between left and right ears of a listener of the plurality of speakers;
a processor coupled to the plurality of speakers, the processor configured to:
receive an input multi-channel audio signal;
perform processing on the input multi-channel audio signal to form an output multi-channel audio signal, the processing including binaural directivity compensation to operationally reduce or eliminate the spectral content imbalance between the left and right ears of the listener; and
direct the output multi-channel audio signal to the plurality of speakers, the plurality of speakers being configured to produce sound corresponding to the output multi-channel audio signal.
2. The system of claim 1 , wherein the processing further includes spatial audio processing that:
causes the plurality of speakers to deliver sound corresponding to a specified left audio channel to a left ear location that corresponds to a left ear of the listener, and
causes the plurality of speakers to deliver sound corresponding to a specified right audio channel to a right ear location that corresponds to a right ear of the listener.
3. The system of claim 2 , further comprising a head tracker configured to actively track the left ear location and the right ear location.
4. The system of claim 2 , wherein the processor is configured to use estimated and time-invariant left and right ear locations.
5. The system of claim 2 , wherein:
the plurality of speakers includes only a left speaker and a right speaker;
the input multi-channel audio signal includes data corresponding to a left input audio signal and a right input audio signal; and
the output multi-channel audio signal includes data corresponding to a left output audio signal and a right output audio signal.
6. The system of claim 5 , wherein the processor is configured to perform the binaural directivity compensation within the spatial audio processing.
7. The system of claim 6 , wherein the processor is configured to perform the spatial audio processing to include cancelling crosstalk between the left speaker and the right ear of the listener and between the right speaker and the left ear of the listener.
8. The system of claim 7 , wherein the processor is configured to cancel the crosstalk by:
providing a first directivity value corresponding to a directivity of the left speaker at the left ear location;
providing a second directivity value corresponding to a directivity of the left speaker at the right ear location;
providing a third directivity value corresponding to a directivity of the right speaker at the left ear location;
providing a fourth directivity value corresponding to a directivity of the right speaker at the right ear location;
providing a first head-related transfer function that characterizes how the left ear of the listener, at the left ear location, receives sound from the left speaker;
providing a second head-related transfer function that characterizes how the right ear of the listener, at the right ear location, receives sound from the left speaker;
providing a third head-related transfer function that characterizes how the left ear of the listener, at the left ear location, receives sound from the right speaker;
providing a fourth head-related transfer function that characterizes how the right ear of the listener, at the right ear location, receives sound from the right speaker;
forming a modified second head-related transfer function as the second head-related transfer function, multiplied by the third directivity value, divided by the fourth directivity value;
forming a modified third head-related transfer function as the second head-related transfer function, multiplied by the first directivity value, divided by the second directivity value;
forming a compensation matrix as an inverse of a matrix that includes the first, modified second, modified third, and fourth head-related transfer functions;
forming an input matrix that includes transforms of the left input audio signal and the right input audio signal; and
forming an output matrix calculated as a product of the compensation matrix and the input matrix, the output matrix including transforms of the left output audio signal and the right output audio signal.
9. The system of claim 6 , wherein the processor is configured to further perform loudspeaker equalization downstream from the spatial audio processing and the binaural directivity compensation.
10. The system of claim 5 , wherein the processor is configured to perform the binaural directivity compensation downstream from the spatial audio processing.
11. The system of claim 10 , wherein the processor is configured to perform the spatial audio processing to include cancelling crosstalk between the left speaker and the right ear of the listener and between the right speaker and the left ear of the listener.
12. The system of claim 11 , wherein the processor is configured to cancel the crosstalk by:
providing a first head-related transfer function that characterizes how the left ear of the listener, at the left ear location, receives sound from the left speaker;
providing a second head-related transfer function that characterizes how the right ear of the listener, at the right ear location, receives sound from the left speaker;
providing a third head-related transfer function that characterizes how the left ear of the listener, at the left ear location, receives sound from the right speaker;
providing a fourth head-related transfer function that characterizes how the right ear of the listener, at the right ear location, receives sound from the right speaker;
forming a compensation matrix as an inverse of a matrix that includes the first, second, third, and fourth head-related transfer functions;
forming an input matrix that includes transforms of the left input audio signal and the right input audio signal; and
forming an output matrix calculated as a product of the compensation matrix and the input matrix, the output matrix including transforms of the left output audio signal and the right output audio signal.
13. The system of claim 10 , wherein the processor is configured to further perform loudspeaker equalization downstream from the spatial audio processing, and perform the binaural directivity compensation within the loudspeaker equalization.
14. A method for producing binaural directivity-compensated sound, the method comprising:
receiving an input multi-channel audio signal at a processor;
performing, with the processor, processing on the input multi-channel audio signal to form an output multi-channel audio signal, the processing including binaural directivity compensation to compensate for directional variations in performance of each speaker of a plurality of speakers, each of the plurality of speakers having a characteristic directivity that describes a relative volume level output by the speaker, as a function of azimuth angle, elevation angle, and frequency, the directivities of the speakers operationally produce a spectral content imbalance between left and right ears of a listener of the plurality of speakers, the binaural directivity compensation operationally reducing or eliminating the spectral content imbalance between the left and right ears of the listener;
directing the output multi-channel audio signal to the plurality of speakers; and
producing sound corresponding to the output multi-channel audio signal with the plurality of speakers.
15. The method of claim 14 , wherein the processing further includes spatial audio processing that:
causes the plurality of speakers to deliver sound corresponding to a specified left audio channel to a left ear location that corresponds to a left ear of the listener, and
causes the plurality of speakers to deliver sound corresponding to a specified right audio channel to a right ear location that corresponds to a right ear of the listener.
16. A system for producing binaural directivity-compensated sound, the system comprising:
a left speaker having a characteristic left directivity that describes a relative volume level output by the left speaker, as a function of azimuth angle, elevation angle, and frequency;
a right speaker having a characteristic right directivity that describes a relative volume level output by the right speaker, as a function of azimuth angle, elevation angle, and frequency, the left directivity and the right directivity operationally producing a spectral content imbalance between left and right ears of a listener of the left speaker and the right speaker; and
a processor coupled to the left speaker and the right speaker, the processor configured to:
receive an input multi-channel audio signal;
perform processing on the input multi-channel audio signal to form an output multi-channel audio signal, the processing including spatial audio processing that operationally causes the plurality of speakers to deliver sound corresponding to a specified left audio channel to a left ear location that corresponds to a left ear of the listener, and operationally causes the plurality of speakers to deliver sound corresponding to a specified right audio channel to a right ear location that corresponds to a right ear of the listener, the processing further including binaural directivity compensation to operationally reduce or eliminate the spectral content imbalance between the left and right ears of the listener; and
direct the output multi-channel audio signal to the left speaker and the right speaker, the left speaker and the right speaker being configured to produce sound corresponding to the output multi-channel audio signal.
17. The system of claim 16 , wherein:
the processing further includes spatial audio processing that causes the plurality of speakers to deliver sound corresponding to a specified left audio channel to a left ear location that corresponds to a left ear of the listener, and causes the plurality of speakers to deliver sound corresponding to a specified right audio channel to a right ear location that corresponds to a right ear of the listener;
the processor is configured to perform the binaural directivity compensation within the spatial audio processing; and
the processor is configured to further perform loudspeaker equalization downstream from the spatial audio processing and the binaural directivity compensation.
18. The system of claim 16 , wherein:
the processing further includes spatial audio processing that causes the plurality of speakers to deliver sound corresponding to a specified left audio channel to a left ear location that corresponds to a left ear of the listener, and causes the plurality of speakers to deliver sound corresponding to a specified right audio channel to a right ear location that corresponds to a right ear of the listener;
the processor is configured to perform the binaural directivity compensation downstream from the spatial audio processing; and
the processor is configured to further perform loudspeaker equalization downstream from the spatial audio processing, and perform the binaural directivity compensation within the loudspeaker equalization.