Abstract: In an encoder, multiple channels of audio information representing multidimensional sound fields are split into subband signals and the subband signals in one or more subbands are combined to form composite signals. The composite signals, the subband signals not combined into a composite signal and information describing the spectral levels of subband signals combined into composite signals are assembled into an encoded output signal. The spectral level information conveys either the amplitude or power of the combined subband signals or the apparent direction of the sound field represented by the combined subband signals. In digital implementations, adaptive bit allocation may be used to reduce the informational requirements of the encoded signal.

Abstract: A multi-channel signal compressor for compressing digital sound signals in the respective channels of a multi-channel sound system. The apparatus comprises a first-stage compression system and a second-stage compression system. In the first-stage compression system, a coupling circuit performs coupling between the digital sound signals of at least two of the channels to generate coupling-processed signals, one for each of the channels. A compressor circuit receives the coupling-processed signals from the coupling circuit and frequency divides each coupling-processed signal into frequency range signals in respective frequency ranges, and compresses the frequency range signals obtained by dividing each coupling-processed signal to generate a first-stage compressed signal.

Abstract: An adaptive high-fidelity perceptual subband encoder for spatially directional multichannel audio signals, using a mixture of individual-channel subband coding with subband-steered composite-channel signal coding that combines the spectral components of the individual channel subbands selected for steering. The composite channel representation includes a subband steering control signal which either conveys the levels of the spectral components from all the steered channels, or represents the apparent directions (net directional vectors) of the spectral components from all the steered channels.

Abstract: In a stereophonic reproduction system, especially for a cinema, a center channel signal is derived by combining left and right channel signals. In order to harden the center image, particularly in speech, the left and right channel signals are compared and, when they include significant amounts of correlated information, the relative gain of the center channel is enhanced by boosting the gain of that channel and/or cutting the gain of the left and right channels. The test for correlation of information is preferably restricted to speech frequencies and the enhancement of relative gain may be performed only at middle frequencies.

Abstract: The invention concerns a dynamic range compressor type encoder or expander type decoder, in which a main signal component in a main path is boosted or bucked by a further signal component derived from a point in the main path by a further path having the characteristics of so restricting the further signal component that the boosting or bucking action is only appreciable below a low level threshold. In the present invention the further signal component is a difference signal formed between a direct signal derived from a point in the main path and a delayed version of either the same signal or of another signal derived from another point in the main path. At the frequency equal to the reciprocal of the delay, and at harmonics of this frequency, the direct and delayed signals cancel. The compressor or expander action, and hence noise reduction action, takes place only at intervening frequencies.

Abstract: A signal processing system for providing compressor and expander operation which system includes a main, or straight through, signal path and a further signal path in parallel therewith. The further path derives its input from the input to the main path or from some later point in the signal path. The output of the further signal path is combined additively with that of the main path for compressor operation and subtractively for expander operation, such further signal path output being appropriately limited so that it can only make a noticeable contribution to the resultant signal level at low input signal levels. True complementarity is attainable by the use of a compressor and expander together to provide an overall noise reduction action without introducing defects into the signal being processed.

Abstract: A compressor or expander circuit is constructed by connecting a plurality of impedance networks in series across input terminals, at least one network being frequency selective. Output terminals are connected across one or more of the networks. The frequency selective network varies its parameters in response to the level of components within a restricted frequency band determined by this network so as to narrow the band when the level of such components increases. This includes such components from the restricted band and thereby from the compression or expansion action.

Abstract: Compressors and expanders for effecting dynamic range modification are constructed by connecting reactive networks in series for voltage dividing action or in parallel for current dividing action. The output signal is derived from the voltage across or current through one of the networks which includes a series or parallel variable resistance. The variable resistance is controlled in dependence upon the voltage thereacross in the sense required to achieve compression or expansion, as the case may be. The resistance change shifts a turnover frequency of the circuit so as to exclude large amplitude components from the amplitude increase or reduction which is applied to low level components within a restricted frequency band to create the compressor or expander action.