Head diffraction compensated stereo system with optimal equalization

An audio processing system including equalization to simulate an acoustic process that imposes head-related transfer-function characteristics upon a plurality of audio signals, comprising:

• source means for providing a plurality of audio signals, each designated as corresponding to a respective incidence direction of a plurality of incidence directions relative to a front-reference incidence direction;
• a plurality of simulation means for imposing head-related transfer-function characteristics corresponding to each said designated incidence direction upon each respective signal from said source means, and each simulation means characterized by a two-port input, nd two-port output, cross-coupled filter means whose transfer function simulates the acoustic transfer functions for a source incidence direction to a listener's ear and for said source incidence direction to the listener's other ear, each simulating means including filters in feedback arrangements simulating approximations of said acoustic transfer functions, to produce a left-ear-designated signal and a right-ear-designated signal;
• summing means for summing left-ear-designated signals together and for summing right-ear-designated signals together from the said plurality of simulation means to provide two combined outputs;
• a plurality of equalization filters for simulating the reciprocal of an equalization transfer function whose magnitude is approximately proportional to the square root of the sum of squares of the magnitudes of the said acoustic transfer functions determined for a reference incidence direction other than the front direction;
• and
• means for modifying signals at least at one of the input and the output of each of said simulation means by transmission of each signal through one of the equalization filters that is substantially the same for each of the ports.

An audio recording method for recording equalized audio signals suitable for reproduction to a listener through a loudspeaker system in conjunction with a crosstalk compensation circuit, the method comprising the step of:

• providing two channels of audio signals having head related transfer function imposed thereon;
• filtering the channels of audio signals by processing the respective signals through a filter means which simulates an equalization transfer function whose magnitude is approximately proportional to the square root of the sum of the squares of the magnitudes of a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from a loudspeaker to a first ear of the listener and a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from the loudspeaker to a second ear of the listener to create equalized audio signals;
• and
• recording the equalized audio signals suitably for subsequent reproduction.

An equalization system for earphones, comprising:

• a designated pair of essentially identical earphones for which equalization is intended;
• measurement means for determining the free-space acoustic transfer functions from a sound source to each of two predetermined corresponding points at two ears of a representative natural or artificial head;
• signal source means for providing two essentially identical signals;
• signal modifying means for imposing head-related transfer functions for a designated source direction upon said signals;
• means for determining an acoustic transfer function from a free-space acoustic source to one ear from said source direction and an acoustic transfer function from the source to the other ear, and for determining a first equalization transfer function whose magnitude is the square root of the sum of squares of the magnitudes of the acoustic transfer functions;
• means for determining an earphone equalization transfer function that is the first equalization transfer function divided by a second equalization transfer function,
• filter means for simulating an approximation to said earphone equalization transfer function to modify two signals each in essentially the same manner,
• means for coupling the two modified signals to the earphones.

An audio processing system that generates compensated and equalized audio signals suitable for reproduction to a listener through a loudspeaker system, comprising:

• source means for providing two channels of audio signals having head related transfer functions imposed thereon;
• compensation means having two input channels and two output channels for providing an inverse crosstalk in the audio signals to correct for the acoustic crosstalk characteristic of loudspeaker to listener ear transmission paths having a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from a loudspeaker to a first ear of the listener and a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from the loudspeaker to the second ear of the listener;
• and
• filter means, coupled to the compensation means, for simulating an equalization transfer function whose magnitude is approximately proportional to the square foot of the sum of the squares of the magnitudes of said acoustic transfer functions.

A method of audio processing that generates compensated and equalized audio signals suitable for reproduction to a listener through a loudspeaker system, the method comprising the steps of:

• providing two channels of audio signals having head related transfer functions imposed thereon;
• providing an inverse crosstalk of the audio signals to correct for the acoustic crosstalk characteristic of loudspeaker to listener ear transmission paths, having a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from a loudspeaker to a first ear of the listener and a transfer function which approximately simulates a free field acoustic transfer function of the propagation path from the loudspeaker to a second ear of the listener;
• and
• simulating an equalization transfer function whose magnitude is approximately proportional to the square root of the sum of the square of the magnitudes of said acoustic transfer functions.