A spread spectrum system is used for transmitting data to devices in a distributed system. Each device has a respective spread spectrum code, and has a corresponding encoder in a central control system operating the same spread spectrum codes, the encoded data relating to the devices being aggregated over a shared channel. An additional broadcast spread spectrum coding sequence is allocated to a broadcast channel readable by a plurality of the devices using a command extraction function and used to transmit general commands for operation by the plurality of devices. Individual actuators may be arranged to respond in different ways to such a broadcast command, for example switching some on and switching others off. The broadcast may also be used to change the coding sequences allocated to individual devices, allowing flexible use of the available spread-spectrum coding sequences.