Abstract: A prefilter (5) for an audio system comprising a loudspeaker (1) in a room (2), which corrects both amplitude and phase errors due to the loudspeaker (1) by a linear phase correction filter response and corrects the amplitude response of the room (2) whilst introducing the minimum possible amount of extra phase distortion by employing a minimum phase correction filter stage. A test signal generator (8) generates a signal comprising a periodic frequency sweep with a greater phase repetition period than the frequency repetition period. A microphone (7) positioned at various points in the room (2) measures the audio signal processed by the room (2) and loudspeaker (1), and a coefficient calculator (6) (e.g. a digital signal processor device) derives the signal response of the room and thereby a requisite minimum phase correction to be cascaded with the linear phase correction already calculated for the loudspeaker (1). Filter (5) may comprise the same digital signal processor as the coefficient calculator (6).

Abstract: A surround sound apparatus wherein a decoder decodes directionally encoded audio signals for reproduction via a loudspeaker layout over a listening area wherein the signals are decoded by a matrix. The coefficients of the decoding matrix are such that at a predetermined listening position, the reproduced velocity vector direction and the reproduced energy vector position directions are substantially equal to each other and substantially independent of frequency in a broad audio frequency range The gain coefficients of the decoding matrix are such that the reproduced velocity vector magnitude varies systematically with encoded sound direction at frequencies in the region of and above a predetermined middle audio frequency.

Abstract: A frequency-dependent linear audio signal processor takes source signals S in input signals and provide directionally spread directionally encoded output signals. The processor directionally encodes with constant gain magnitude frequency components of the source signal S to-and-fro across a predetermined directional stage P" as frequency increases such that at least three predetermined positions within the stage P", the directional encoding has substantially zero perceived phasiness. The processor may be a frequency-dependent rotation matrix for stereo input signal and may be a unitary network using a feedback path around parallel identical all-pass networks in series with a rotation matrix and a feedforward path bypassing the all-pass networks. Successive frequencies of positioning of source signal S at a predetermined position P within the stage P" are preferably spaced approximately uniformly on a logarithmic or Bark Frequency scale.

Abstract: In a surround-sound encoding system which does not have full circular or square symmetry, a third channel is added to a basic two channel system in such a way that the third channel may be reduced in amplitude or restricted in frequency without substantially affecting important localization criteria. The encoder includes a phase-amplitude matrix such that a decoder having a phase-amplitude matrix which is the inverse of the encoder phase-amplitude matrix is such that the absence of the third channel signals at the input thereof does not affect localization in at least three, preferably six, predetermined directions.

Abstract: In a sound reproduction system of the type which enables the listener to distinguish sound from sources extending over 360.degree. of azimuth, compensation is provided for the asymmetry arising when four loudspeakers are located at the corners of a non-square rectangle. Compensation is also provided for the different localization mechanisms used by a human listener at high and low frequencies. The invention is also applicable to systems which enable the listener to distinguish sounds from different heights.

Abstract: In a sound reproduction system, the phasiness of the psychoacoustically most important signals is minimized by subtracting from the velocity signal components of the most important signals a directional bias signal comprising a fraction of the pressure signal phase shifted by 90.degree..

Abstract: A microphone assembly for simulating a plurality of coincident microphones has at least four microphone units mutually disposed at the integration points of an integration rule for the surface of a sphere. In one embodiment, four such microphone units are disposed on respective faces of a regular tetrahedron.

Abstract: In a sound reproduction system of the type which enables the listener to distinguish sound from sources extending over 360.degree. of azimuth, compensation is provided for the asymmetry arising when four loudspeakers are located at the corners of a non-square rectangle. Compensation is also provided for the different localization mechanisms used by a human listener at high and low frequencies. The invention is also applicable to systems which enable the listener to distinguish sounds from different heights.