Method for creating a virtual acoustic stereo system with an undistorted acoustic center

• US 10,063,984 B2
• Filed: 09/29/2015
• Issued: 08/28/2018
• Est. Priority Date: 09/30/2014
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Johnson, Martin E., Choisel, Sylvain J., Boothe, Daniel K., Lerner, Mitchell R.
• Primary Examiner(s): Paul, Disler
• Application Number:
  US15/514,813
• Publication Number:
  US 20170230772A1
• Time in Patent Office: 1,064 Days
• Field of Search: 381 17, 381303, 381 18, 381309-310
• CPC Class Codes: H04S 1/00 Two-channel systems H04S5/00, H04S7/00 take precedence
• CPC Class Codes: H04S 1/002 Non-adaptive circuits, e.g. manually adjustable or static, for enhancing the sound image or the spatial distribution control circuits for electronic adaptation of the sound field H04S7/30
• CPC Class Codes: H04S 2400/09 Electronic reduction of distortion of stereophonic sound systems
• CPC Class Codes: H04S 2400/11 Positioning of individual sound objects, e.g. moving airplane, within a sound field H04S2420/13 takes precedence
• CPC Class Codes: H04S 2400/13 Aspects of volume control, not necessarily automatic, in stereophonic sound systems
• CPC Class Codes: H04S 2420/01 Enhancing the perception of the sound image or of the spatial distribution using head related transfer functions [HRTF's] or equivalents thereof, e.g. interaural time difference [ITD] or interaural level difference [ILD]
• CPC Class Codes: H04S 7/30 Control circuits for electronic adaptation of the sound field
• CPC Class Codes: H04S 7/307 Frequency adjustment, e.g. tone control H04S7/301 takes precedence

Abstract

A system and method are described for transforming stereo signals into mid and side components xm and xs to apply processing to only the side-component xs and avoid processing the mid-component. By avoiding alteration to the mid-component _XM, the system and method may reduce the effects of ill-conditioning, such as coloration that may be caused by processing a problematic mid-component xM while still performing crosstalk cancellation and/or generating virtual sound sources. Additional processing may be separately applied to the mid and side components x_M and xs and/or particular frequency bands of the original stereo signals to further reduce ill-conditioning.

Claims

1. A method for generating a set of virtual sound sources based on a left audio signal and a right audio signal corresponding to left and right channels for a piece of sound program content, comprising:

transforming the left and right audio signals to a mid-component signal and a side-component signal;

generating a set of filter values for the mid-component signal and the side-component signal, wherein the filter values

     1) provide crosstalk cancellation between two speakers and

     2) simulate virtual sound sources for the left and right channels of the piece of sound program content;

normalizing the set of filter values to produce normalized filter values, wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values that correspond to the mid-component signal are equal to a desired value; and

applying the normalized set of filter values to one or more of the mid-component signal and the side-component signal.

2. The method of claim 1, wherein the mid-component signal is the sum of the right and left audio signals and the side-component signal is the difference between the left and right audio signals.

3. The method of claim 1, further comprising:

transforming the resulting signals produced from the application of the set of normalized filter values to the one or more of the mid-component signal and the side-component signal to produce a left filtered stereo audio signal and a right filtered stereo audio signal; and

driving the two speakers using the left filtered stereo audio signal and the right filtered stereo audio signal to generate the virtual sound sources.

4. The method of claim 3, further comprising:

band pass filtering the left audio signal using a first cutoff frequency and a second cutoff frequency to produce a band pass left signal, such that the band pass left signal includes frequencies from the left audio signal between the first and second cutoff frequencies; and

band pass filtering the right audio signal using the first and second cutoff frequencies to produce a band pass right signal, such that the band pass right signal includes frequencies from the right audio signal between the first and second cutoff frequencies,

wherein the band pass left and right signals are transformed to produce the mid-component signal and the side-component signal.

5. The method of claim 4, further comprising:

low pass filtering the left audio signal using the first cutoff frequency to produce a low pass left signal;

low pass filtering the right audio signal using the first cutoff frequency to produce a low pass right signal;

high pass filtering the left audio signal using the second cutoff frequency to produce a high pass left signal;

high pass filtering the right audio signal using the second cutoff frequency to produce a high pass right signal;

combining the low pass left signal and the high pass left signal with the left filtered stereo audio signal; and

combining the low pass right signal and the high pass right signal with the right filtered stereo audio signal, wherein the left filtered stereo audio signal after combination with the low pass left signal and the high pass left signal and the right filtered stereo audio signal after combination with the low pass right signal and the high pass right signal are used to drive the two speakers.

6. The method of claim 3, further comprising:

compressing the mid-component signal; and

compressing the side-component signal, wherein compression of the mid-component signal is performed separately from compression of the side-component signal.

7. The method of claim 1, wherein the normalized set of filter values are applied to the side-component signal, the method further comprising:

applying a delay to the mid-component signal while the side-component signal is being filtered using the normalized set of filter values such that the mid-component signal remains in sync with the side-component signal as a result of the delay.

8. The method of claim 1 wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values corresponding to the mid-component are equal to one.

9. The method of claim 1 further comprising:

equalizing the mid-component signal; and

equalizing the side-component signal, wherein equalization of the mid-component signal is performed separately from equalization of the side-component signal.

10. A system for generating a set of virtual sound sources based on a left audio signal and a right audio signal corresponding to left and right channels for a piece of sound program content, comprising:

a first set of filters to transform the left and right audio signals to a mid-component signal and a side-component signal;

a processor to;

generate a set of filter values for the mid-component signal and the side-component signal, wherein the filter values

     1) provide crosstalk cancellation between two speakers and

     2) simulate virtual sound sources for the left and right channels of the piece of sound program content, and

normalize the set of filter values to produce normalized filter values, wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values that correspond to the mid-component signal are equal to a desired value; and

a second set of filters to apply the normalized set of filter values to one or more of the mid-component signal and the side-component signal.

11. The system of claim 10, wherein the mid-component signal is the sum of the right and left audio signals and the side-component signal is the difference between the left and right audio signals.

12. The system of claim 10, wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values corresponding to the mid-component are equal to one.

13. The system of claim 10, further comprising:

a third set of filters to transform the resulting signals produced from the application of the set of filter values to one or more of the mid-component signal and the side-component signal to produce left and right filtered audio signals; and

a set of drivers to drive the two speakers using the left and right filtered audio signals to generate the virtual sound sources.

14. The system of claim 13, further comprising:

a band pass filter to

     1) filter the left audio signal using a first cutoff frequency and a second cutoff frequency to produce a band pass left signal, such that the band pass left signal includes frequencies from the left audio signal between the first and second cutoff frequencies and

     2) filter the right audio signal using the first and second cutoff frequencies to produce a band pass right signal, such that the band pass right signal includes frequencies from the right audio signal between the first and second cutoff frequencies,

wherein the band pass left and right signals are transformed by the first set of filters to produce the mid-component signal and the side-component signal.

15. The system of claim 14, further comprising:

a low pass filter to filter

     1) the left audio signal using the first cutoff frequency to produce a low pass left signal and

     2) the right audio signal using the first cutoff frequency to produce a low pass right signal;

a high pass filter to filter

     1) the left audio signal using the second cutoff frequency to produce a high pass left signal and

     2) the right audio signal using the second cutoff frequency to produce a high pass right signal;

a summation unit to combine

     1) the low pass left signal and the high pass left signal to the left filtered audio signal and

     2) the low pass right signal and the high pass right signal to the right filtered audio signal, wherein the left filtered audio signal after combination with the low pass left signal and the high pass left signal and the right filtered audio signal after combination with the low pass right signal and the high pass right signal are used to drive the two speakers.

16. The system of claim 12, wherein first set of filters, the second set of filters, and the third set of filters are finite impulse response (FIR) filters.

17. An article of manufacture for generating a set of virtual sound sources based on a left audio signal and a right audio signal corresponding to left and right channels for a piece of sound program content, comprising:

a non-transitory machine-readable storage medium that stores instructions which, when executed by a processor in a computing device,

transform the left and right audio signals to a mid-component signal and a side-component signal;

generate a set of filter values for the mid-component signal and the side-component signal, wherein the filter values

     1) provide crosstalk cancellation between two speakers and

     2) simulate virtual sound sources for the left and right channels of the piece of sound program content;

normalize the set of filter values to produce normalized filter values, wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values that correspond to the mid-component signal are equal to a desired value; and

apply the normalized set of filter values to one or more of the mid-component signal and the side-component signal.

18. The article of manufacture of claim 17, wherein the mid-component signal is the sum of the right and left audio signals and the side-component signal is the difference between the left and right audio signals.

19. The article of manufacture of claim 17, wherein the non-transitory machine-readable storage medium stores further instructions which when executed by the processor:

transform the resulting signals produced from the application of the set of filter values to one or more of the mid-component signal and the side-component signal to produce left and right filtered audio signals; and

drive the two speakers using the left and right filtered audio signals to generate the virtual sound sources.

20. The article of manufacture of claim 17, wherein normalizing the set of filter values comprises dividing each non-zero filter value by the filter value corresponding to the mid-component signal such that the normalized filter values corresponding to the mid-component are equal to one.

21. The article of manufacture of claim 20, wherein the non-transitory machine-readable storage medium stores further instructions which when executed by the processor:

equalize the mid-component signal; and

equalize the side-component signal, wherein equalization of the mid-component signal is performed separately from equalization of the side-component signal.

22. The article of manufacture of claim 20, wherein the non-transitory machine-readable storage medium stores further instructions which when executed by the processor:

compress the mid-component signal; and

compress the side-component signal, wherein compression of the mid-component signal is performed separately from compression of the side-component signal.