Patent ID: 6173061
Filing Date: 2001-01-09
Classification: H04R

Abstract:
Apparatus for steering the apparent direction relative to a listener of a monaural sound source signal reproducible on headphones using left and right audio filters wherein the filter coefficients are derived from the poles and zeros of acoustical head-related transfer functions (HRTFs) by summing and differencing said HRTFs for left and right ears to obtain sigma and delta transfer functions, said apparatus comprising:an input terminal for receiving said monaural sound source signal;sigma filter means for receiving said monaural sound source signal from said input terminal and filtering said monaural on source signal with said sigma transfer function;inverting means for selectively inverting or not inverting the polarity of said monaural sound source signal received from said input terminal;delta filter means for receiving said monaural sound source signal from said inverting those values which produce poles and zeros at the correct frequencies to generate said sigma means and filtering said monaural sound source signal with said delta transfer function;means for presetting the coefficients of each of said sigma and delta filter means to and delta transfer functions appropriate for said apparent sound source direction and for selecting the polarity of the input to said delta filter means;summing means for summing the output signals from sigma and delta filter means to produce a left output signal;differencing means for subtracting the output signal of said delta filter means from that of said sigma filter means to produce a right output signal;said apparatus being operative to produce from said monaural sound source signal a left and a right output signal suitable for application to headphones so that a listener hears the acoustical analog of said left and right output signals and perceives said left and right output signals to be acoustically equivalent to hearing said monaural sound source at said apparent direction;wherein said signal and delta filter and said summing, differencing and inverting means are accomplished in digital signal processing (""DSP"") means and said coefficients of said filter means are stored in memory associated with said DSP means;wherein said coefficients of said filter means are stored in the form of pole and zero locations for a multiplicity of directions for which HRTFs have been measured, the apparatus further comprising:means for generating additional coincident pole-zero pairs among the pole and zero locations for one of said multiplicity of directions such that the number of poles and zeros is equal to that for an adjacent one of said multiplicity of directions; andmeans for interpolating between the pole and zero locations for said one and said adjacent one of said multiplicity of directions to obtain approximate pole and zero locations for a direction intermediate between said adjacent directions;said pole and zero locations for said intermediate direction providing sufficient information to approximate HRTFs for said intermediate direction and hence to compute appropriate coefficients for said sigma and delta filter means.