Electrical audio signal processing systems and devices – Sound effects – Reverberators
Patent
1989-11-20
1992-08-25
Isen, Forester W.
Electrical audio signal processing systems and devices
Sound effects
Reverberators
H03G 300
Patent
active
051425866
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a a method and electro-acoustical system for processing the sound emitted by one or more sound sources in a listening room, by recording said sound by means of a number of microphones, the signals (S) of which are processed in a processor according to the matrix relation P=T S, in which (P) represents the processed signals supplied from the processor to a number of loudspeakers distributed across the listening room, and wherein T represents the following transfer matrix: ##EQU1## wherein M and N represent the number of microphone signals and loudspeaker signals respectively. Such a method is known from a preprint of a lecture before the Audio Engineering Society on the 82nd convention, Mar. 10-13, 1988 in London.
This preprint introduces a generalized description of electro-acoustical systems designed to improve the reproduction of sound in a room or, in other terms, to change or improve the acoustic conditions in a listening room. This description is based on the consideration that each linear transfer, whereby sound is picked up by microphones (S) and, after being processed, is emitted by loudspeakers (P), can be represented by the above matrix relation P=T S. reflected sound, or both. direct sound, reflected sound, or both.
The working of an electro-acoustical system is determined by the selection of the elements in the transfer matrix T. The above preprint does not teach how to make such selection.
A complete development of the relation P=T S results in: ##EQU2## wherein S.sub.1, S.sub.2 . . . S.sub.M define the microphone signals, which represent the direct sound or the reverberant sound or both and P.sub.1, P.sub.2 . . . P.sub.N define the loudspeaker signals which reproduce the desired output sound. It is to be noted that a number of microphone signals may be equal due to the fact that they are emitted by the same microphone. Similarly a number of loudspeaker signals may be supplied to the same loudspeaker. The properties of the system are defined by the transfer coefficient loudspeaker n and A.sub.nm (.omega.) represents the frequency dependent amplification (or attenuation) between microphone m and loudspeaker n.
A number of well-known electro-acoustical systems will now be considered in the light of the above general matrix notation:
1. In a so-called `public address` (PA) system the microphones are located close to the sound source and they largely pick up the direct sound. The delays are generally zero. For a simple single channel PA system M=N=1, .tau..sub.11 =0 and A.sub.11 (.omega.) equals the desired frequency dependent amplification,
A more advanced PA system with a mixing console and e.g. six microphones and two loudspeakers, can be represented by ##EQU3##
2. In reveberation enhancement systems, such as the well-known MCR system of Phillips, the microphones largely pick up the reverberant sound field, which means that S.sub.1, S.sub.2 . . . S.sub.M principally define reverberant sound signals (vide Fransen, N. V.; Sur amplification des champs Acoustiques, Acoustica vol. 18, pp 315-223 (1968)). Moreover, the transfer coefficients are delay-free and .tau..sub.nm (.omega.)=A.sub.nm (.omega.) represents the frequency dependent channel amplification between microphone m and loudspeaker n. Microphones and loudspeakers that are located close to another must have very small (or zero) amplification to avoid colouration or even howl-back. An optimum choice of all A.sub.nm (.omega.) values, such that enough reverberant energy is generated on the one hand and colouration is avoided on the other hand, is difficult and requires many channels.
3. In reflection generation systems, such as the system disclosed in EP 0075615, the response can be described by the above matrix relation, with a diagonal matrix ##EQU4## where amplitude A.sub.mn and delay .tau..sub.mn simulate a reflection, having the desired amplitude and travel time and coming from the direction of loudspeaker position n.
As a special example, very early reflections may be generated to support the direct sound, such a
REFERENCES:
patent: 4105864 (1978-08-01), Berkovitz
M. R. Schroeder, "Natural Sounding Artificial Reverberations", Jul. '62, Jour. Audio Eng. Soc., vol. 10.
A. J. Berkhout, "A New Direction in Electro-Acoustic Reverb. Control", Mar. '87, Audio Eng. Soc. Preprint 2441.
A. J. Berkhout, "A Holographic Approach to Acoustic Control", Dec. '88, pp. 977-995, Jour. Audio Eng. Soc., vol. 36, No. 12.
A. J. Berkhout, "Applied Seismic Wave Theory", 1987, Elsevier Science Publishing Co., Inc., N.Y., pp. XI-XIV, 20-23 (Green's theorem), 231-233, 240 (Chapter 8), and 301-303 (Chapter 10).
Birch Wood Acoustics Nederland B.V.
Isen Forester W.
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