Audio spatialization and environment simulation
A method and apparatus for processing an audio sound source to create four-dimensional spatialized sound. A virtual sound source may be moved along a path in three-dimensional space over a specified time period to achieve four-dimensional sound localization. A binaural filter for a desired spatial point is applied to the audio waveform to yield a spatialized waveform that, when the spatialized waveform is played from a pair of speakers, the sound appears to emanate from the chosen spatial point instead of the speakers. A binaural filter for a spatial point is simulated by interpolating nearest neighbor binaural filters chosen from a plurality of pre-defined binaural filters. The audio waveform may be processed digitally in overlapping blocks of data using a Short-Time Fourier transform. The localized sound may be further processed for Doppler shift and room simulation.
1. A computer-implemented method for simulating a binaural filter for a spatial point, the method comprising:
in a signal processing system including a processor,
accessing a plurality of pre-defined binaural filters, wherein each binaural filter further comprises a left ear head related transfer function filter and a right ear head related transfer function filter;
selecting at least two nearest neighbor binaural filters from the plurality of predefined binaural filters; and
performing an interpolation among the nearest neighbor binaural filters to obtain a new binaural filter, wherein the operation of performing an interpolation among the nearest neighbor binaural filters further comprises:
determining an inter-aural time difference for each nearest neighbor head related transfer function filter;
removing the inter-aural time difference of each nearest neighbor head related transfer function filter prior to the interpolation;
interpolating the inter-aural time differences of the nearest neighbor binaural filters to obtain a new inter-aural time difference; and
including the new inter-aural time difference in the new binaural filter.
2. A method according to claim 1 , wherein each pre-defined binaural filter is located on a unit sphere.
3. A method according to claim 2 , wherein the nearest neighbor binaural filter is spatially closer to the spatial point than the other pre-defined binaural filters.
4. The method of claim 2 , wherein the pre-defined binaural filters are uniformly spaced around the unit circle.
5. The method of claim 2 , wherein the unit sphere is scaled from 0 to 100 units and wherein 0 represents a center of a virtual room and 100 represents a periphery of the virtual room.
6. A method according to claim 3 , wherein the selection of each nearest neighbor binaural filter is based, at least in part, on a distance between the nearest neighbor binaural filter and the spatial point.
7. A method according to claim 6 , wherein the distance is a minimum Pythagorean distance.
8. A method according to claim 1 , wherein the left head related transfer function filter is a left head related transfer function approximated by an impulse response filter having a first plurality of coefficients and the right head related transfer function filter is a right head related transfer function approximated by an impulse response filter having a second plurality of coefficients.
9. A method according to claim 1 , wherein the inter-aural time difference comprises a left inter-aural time difference and a right inter-aural time difference.
10. A method according to claim 1 , further comprising accounting for the spatial point position when determining the inter-aural time difference.
11. The method of claim 1 , wherein the interpolation is selected from a set consisting of sync interpolation, linear interpolation, and parabolic interpolation.
12. The method of claim 1 , wherein the plurality of pre-defined binaural filters comprises 7,337 pre-defined binaural filters, each binaural filter at a discrete location on a unit sphere.
13. The method of claim 1 , further comprising:
calculating a discrete Fourier transform of the new binaural filter;
setting the frequency response to a fixed amplitude when the frequency is less than a lower cutoff frequency or greater than an upper cutoff frequency; and
setting the phase response to a fixed phase when the frequency is less than the lower cutoff frequency or greater than the upper cutoff frequency.