Diffusing acoustical crosstalk

Abstract

When two loudspeakers play the same signal, a “phantom center” image is produced between the speakers. However, this image differs from one produced by a real center speaker. In particular, acoustical crosstalk produces a comb-filtering effect, with cancellations that may be in the frequency range needed for the intelligibility of speech. Methods for using phase decorrelation to fill in these gaps and produce a flatter magnitude response are described, reducing coloration and potentially enhancing dialogue clarity. These methods also improve headphone compatibility and reduce the tendency of the phantom image to move toward the nearest speaker.

1 . A method of decorrelating a signal using phase diffusion at high frequencies, the method comprising:
 separating a mono input signal into a high-frequency signal and a low-frequency signal;   processing the high-frequency signal using a first diffusion means to create a high-frequency left channel signal and a second diffusion means to create a high-frequency right channel signal, wherein a frequency-dependent delay is created between the high-frequency left channel signal and the high-frequency right channel signal;   processing the low-frequency signal to create a delayed low-frequency signal; and   combining the delayed low-frequency signal with the high-frequency left channel signal and combining the delayed low-frequency signal with the high-frequency right channel signal, thereby producing a stereo response with phase diffusion at high frequencies.   
     
     
         2 . A method as recited in  claim 1  further comprising accepting a mono input signal. 
     
     
         3 . A method as recited in  claim 1  wherein separating the mono input signal further comprises:
 using a pair of magnitude-complementary filters.   
     
     
         4 . A method as recited in  claim 1  wherein the first diffusion means comprises a first allpass filter and the second diffusion means comprises a second allpass filter. 
     
     
         5 . A method as recited in  claim 4  further comprising:
 applying in one of the first allpass filter or the second allpass filter, a positive feedback gain and a negative feedforward gain, concurrently applying in the other allpass filter a negative feedback gain and a positive feedforward gain, thereby creating a frequency-dependent delay between the high-frequency right channel signal and the high-frequency left channel signal.   
     
     
         6 . A method as recited in  claim 1  wherein separating a mono input signal further comprises using a high pass filter and a low pass filter. 
     
     
         7 . A method as recited in  claim 1  wherein the second diffusion means is different from the first diffusion means. 
     
     
         8 . A method as recited in  claim 1  wherein the delay of the delayed low-frequency signal is substantially the same as an average of delays of the high-frequency left channel signal and the high-frequency right channel signal. 
     
     
         9 . A method as recited in  claim 1  wherein combining the delayed low-frequency signal with the high-frequency left channel signal further comprises creating a left channel output signal. 
     
     
         10 . A method as recited in  claim 1  wherein combining the delayed low-frequency signal with the high-frequency right channel signal further comprises creating a right channel output signal. 
     
     
         11 . A method as recited in  claim 1  wherein the frequency-dependent delay does not cause significant temporal smearing of impulsive sounds. 
     
     
         12 . A method of decorrelating a signal using phase diffusion at high frequencies, the method comprising:
 separating a left input signal into a left high-frequency signal and a left low-frequency signal and separating a right input signal into a right high-frequency signal and a right low-frequency signal;   applying a first diffusion means to the left high-frequency signal, thereby creating a diffused left high-frequency signal;   applying a second diffusion means to the right high-frequency signal, thereby creating a diffused right high-frequency signal;   creating a delayed left low-frequency signal and a delayed right low-frequency signal;   combining the delayed left low-frequency signal with the diffused left high-frequency signal; and   combining the delayed right low-frequency signal with the diffused right high-frequency signal,   thereby producing a stereo response with phase diffusion at high frequencies.   
     
     
         13 . A method as recited in  claim 12  further comprising accepting a left input signal and a right input signal. 
     
     
         14 . A method as recited in  claim 12  wherein the first diffusion means includes a first allpass filter and the second diffusion means includes a second allpass filter. 
     
     
         15 . A method as recited in  claim 14  further comprising:
 applying in one of the first allpass filter or the second allpass filter, a positive feedback gain and a negative feedforward gain, concurrently applying in the other allpass filter a negative feedback gain and a positive feedforward gain, thereby creating a frequency-dependent delay between the diffused left high-frequency signal and the diffused right high-frequency signal.   
     
     
         16 . A method as recited in  claim 12  wherein the first diffusion means is different from the second diffusion means. 
     
     
         17 . A method as recited in  claim 12  wherein a delay of the delayed left low-frequency signal is substantially the same as an average of delays of the diffused left high-frequency signal and the diffused right high-frequency signal; 
     
     
         18 . A method as recited in  claim 12  wherein a delay of the delayed right low-frequency signal is substantially the same as an average of delays of the diffused left high-frequency signal and the diffused right high-frequency signal. 
     
     
         19 . A method as recited in  claim 12  wherein combining the delayed left low-frequency signal with the diffused left high-frequency signal creates a left channel output signal and combining the delayed right low-frequency signal with the diffused right high-frequency signal creates a right channel output signal. 
     
     
         20 . A system for decorrelating a mono input signal using phase diffusion at high frequencies, the system comprising:
 a high pass filter for outputting a high-frequency signal from the mono input signal;   a low pass filter for outputting a low-frequency signal from the mono input signal;   a first diffusion means for creating a high-frequency left channel signal;   a second diffusion means for creating a high-frequency right channel signal; and   a delay component for creating a delayed low-frequency signal.   
     
     
         21 . A system as recited in  claim 20  further comprising:
 a first adder for combining the delayed low-frequency signal and the high-frequency left channel signal.   
     
     
         22 . A system as recited in  claim 20  further comprising:
 a second adder for combining the delayed low-frequency signal and the high-frequency right channel signal.   
     
     
         23 . A system as recited in  claim 20  further comprising:
 a first gain component and a second gain component.   
     
     
         24 . A system as recited in  claim 20  wherein the first diffusion means includes a first allpass filter and the second diffusion means includes a second allpass filter. 
     
     
         25 . A system as recited in  claim 20  wherein the first diffusion means is different from the second diffusion means. 
     
     
         26 . A system as recited in  claim 20  wherein a frequency-dependent delay is created between the high-frequency left channel and the high-frequency right channel and wherein the delay of the delay component is substantially the same as an average of delays of the first diffusion means and the second diffusion means. 
     
     
         27 . A system for decorrelating a stereo input signal having a left input and a right input using phase diffusion at high frequencies, the system comprising:
 a first low pass filter and a first high pass filter, each for processing the left input;   a second low pass filter and a second high pass filter, each for processing the right input;   a first diffusion means for creating a high-frequency left channel signal;   a second diffusion means for creating a high-frequency right channel signal; and   a first delay component for creating a delayed low-frequency left channel signal and a second delay component for creating a delayed low-frequency right channel signal.   
     
     
         28 . A system as recited in  claim 27  further comprising:
 a first adder for combining the high-frequency left channel signal and the delayed low-frequency left channel signal.   
     
     
         29 . A system as recited in  claim 27  further comprising:
 a second adder for combining the high-frequency right channel signal and the delayed low-frequency right channel signal.   
     
     
         30 . A system as recited in  claim 27  wherein the first diffusion means includes a first allpass filter and the second diffusion means includes a second allpass filter. 
     
     
         31 . A system as recited in  claim 27  wherein the first diffusion means is different from the second diffusion means.