Abstract: Within a digital PBX system or the like, an appartaus permits provision of digital composite tone alerting particularly suitable for piezoelectric transducers with one arbitrarily selectable tone, by means of logic circuits for latching the sign-bit in a received PCM signal that exceeds a validity threshold, and by combining the toggling sign-bit with another tone to yield the composite tone.

Abstract: The oil-shale is burned in a combustion chamber (24) having a chimney, and the method comprises:providing a drying gas by mixing hot gas taken from the combustion chamber via a duct (4) with relatively cool gas taken from the chimney via a duct (5), the proportions of said gasses being adjusted as a function quantity of water in said damp shale;crushing lumps of said shale in a crusher (6) and in the presence said drying gas;injecting the resulting grains of crushed shale together with the hot gas into an expansion chamber (8) in which the major portion of the grains falls out from the drying gas, while the lightest portion of grains is entrained by the gas;passing the drying gas together with said lightest portion of grains entrained thereby via a duct (9) through filter means (10) to separate said lightest portion of grains from the drying gas;riddling said major portion of the grains (at 17A, 17B) to provide a larger grain fraction and a smaller grain fraction;recycling said larger grain fraction via a duc