Field of the Invention
The present invention relates to acoustics, and, in particular but not exclusively, to techniques for the capture of the spatial sound field on mobile devices, such as laptop computers, cell phones, and cameras.
Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Due to the low cost of high-performance matched microphones and the commensurate increase in digital signal processing capabilities in mobile communication devices, realistic high-quality spatial audio pick-up from mobile devices is now becoming possible. Recording of spatial audio signals has been known since the invention of stereo recording at Bell Labs in the early 1930's. Gibson, Christensen, and Limberg in 1972, gave a fundamental description of three-dimensional audio spatial playback. See J. J. Gibson, R. M. Christensen, and A. L. R. Limberg, “Compatible FM Broadcasting of Panoramic Sound,” J. Audio Eng. Soc., vol. 20, pp. 816-822, December 1972, the teachings of which are incorporated herein by reference in their entirety. It is interesting that these authors discussed higher-order playback systems.
A first-order three-dimensional spatial recording was later proposed by Fellgett and Gerzon in 1975 who described a first-order “B-format ambisonic” SoundField® microphone array constructed of four cardioid capsules mounted in a tetrahedral arrangement. See Peter Fellgett, “Ambisonics, Part One: General System Description,” Studio Sound, vol. 17, no. 8, pp. 20-22, 40, August 1975; Michael Gerzon, “Ambisonics, Part Two: Studio Techniques,” Studio Sound, vol. 17, no. 8, pp. 24, 26, 28-30, August 1975; and U.S. Pat. No. 4,042,779, the teachings of all three of which are incorporated by reference in their entirety.
Later, Elko proposed a spherical microphone array with six pressure microphones mounted on a rigid sphere that utilized first-order spherical harmonics. See G. W. Elko, “A steerable and variable first-order differential microphone array,” IEEE ICASSP proceedings, April 1997, and U.S. Pat. No. 6,041,127, the teachings of both of which are incorporated herein by reference in their entirety.
More-accurate spatial recording using higher-order spherical harmonics or, equivalently, Higher-Order Ambisonics (HOA) was thought to be difficult to construct due to the required measurement of higher-order spatial derivative signals of the acoustic pressure field. The measurement of higher-order spatial derivatives is problematic due to the loss of SNR due to the natural high-pass nature of the acoustic pressure derivative signals and the commensurate need in post-processing to equalize these high-pass signals with a corresponding low-pass filter. Since the uncorrelated microphone self-noise and electrical noises of preamplifiers are invariant under differential processing, the low-pass equalization filter can amplify these noise components greatly, especially at lower frequencies and higher differential orders. One practical solution to extracting the higher-order differential modes by employing many pressure microphones mounted on a rigid spherical baffle and associated signal processing to extract the higher-order spatial spherical harmonics was proposed and patented by Meyer and Elko. See U.S. Pat. No. 7,587,054 (the “'054 patent”) and U.S. Pat. No. 8,433,075 (the “'075 patent”), the teachings of both of which are incorporated herein by reference in their entirety.
A mathematical series representation of a three-dimensional (3D) scalar pressure field is based on signals that are proportional to the zero-order and the higher-order pressure gradients of the field up to the desired highest order of the field series expansion. The basic zero-order omnidirectional term is the scalar acoustic pressure that can be measured by one or more of the pressure microphone elements. For all three first-order components, the acoustic pressure field is sufficiently sampled so that the three Cartesian orthogonal differentials can be resolved along with the acoustic pressure. Three first-order spatial derivatives in mutually orthogonal directions can be used to estimate the first-order gradient of the scalar pressure field. The smallest number of pressure microphones that span 3D space for up to first-order operation is therefore four microphones, preferably in a tetrahedral arrangement.