1. A conferencing server, comprising: an interface for receiving a plurality of incoming bitstreams from respective conferencing endpoints and supplying at least one outgoing bitstream to the conferencing endpoints, wherein said incoming and outgoing bitstreams comprise media data, including at least one of: audio data, video data, audiovisual data and screen-sharing data; a memory with a plurality of memory spaces; a processor operable to perform operations selected from a predefined collection of primitives, wherein each operation inputs media data from one or more of said memory spaces and/or outputs media data to one or more of said memory spaces; and a pre-processor adapted to receive a mixing strategy specifying properties of the at least one outgoing bitstream and requiring at least one additive media mixing step, and repeatedly derive a run list of operations selected from said collection of primitives that realizes the received mixing strategy, wherein the pre-processor is adapted to derive the run list dynamically while taking into consideration momentary activity in each incoming bitstream, wherein the processor is configured to execute said run list for thereby supplying the at least one outgoing bitstream on the basis of said plurality of incoming bitstreams.