Abstract: A conference system (1) comprises a central unit (2) and speaker units (3) which are connectable to the central unit. The central unit (2), which serves to combine speech signals from the speaker units (3) and to distribute the combined speech signals to said units, comprises an adaptive filter (23) for suppressing feedback. Each speaker unit (3) comprises a microphone (33), a loudspeaker (34), an activation switch (35) and an adaptive filter (36) coupled between the microphone (33) and the loudspeaker (34). When the speaker unit is not activated, the adaptive filter (36) serves as an echo canceller, while serving as a feedback suppressor when the speaker unit is activated. By keeping the loudspeaker (34) always on, any transients due to mis-adaptations of the filter (36) are avoided.

Abstract: A communication system comprises a central processing unit (1) and a plurality of N communication stations (2.1, 2.2, . . . , 2N), in which the central processing unit and the communication stations are coupled over an uplink signal line (3) and a downlink signal line (4). The downlink signal (FIG. 3) on the downlink signal line has the form of successive frames, each frame comprising M signal blocks (SB.sub.m), the first signal block in a frame comprising a synchronizing word (sync word) (22), the signal blocks all having space for storing audio information in the form of p audio words (21), the M-1 signal blocks not comprising a sync word further having space for storing control information, the control information in a signal block belonging to a communication station containing downlink assign information (a.dl+m) denoting which audio words in the signal blocks in a frame are destined for said communication station. The uplink signal (FIG.

Abstract: The invention relates to a circuit for a light-sensitive element (1), for example an infrared diode. The anode (a) of the light-sensitive element is coupled to a point of constant potential (earth). The cathode (c) of the light-sensitive element (1) is coupled to the collector of a pnp transistor (7) and, via a resistor (3), to the base of this transistor. The emitter of the transistor is coupled to a second point (6) of constant potential (+). The base is also coupled to this point (6) via a series arrangement of a resistor (5) and a capacitor (4). The desired sensitivity of the circuit is maintained even in the case of a high background light level.