A method of controlling data transfer in a data transmission system which includes a plurality of video and/or audio data sources, a single transmitter operable to transmit data from the data sources over at least one transmission channel, a plurality of client receivers operable to receive transmitted data, the method comprising estimating available channel capacity for each client receiver, using parameters relating to the at least one transmission channel, and using information from the transmitter and receiver, and using information relating to queuing of data between the data sources and the transmitter; determining a target bit transmit rate for each client receiver in dependence upon the estimated channel capacity, a quality parameter, and an activity parameter for the client receiver concerned; and controlling the data sources and transmitter in dependence upon the determined bit transmit rate.