W i r e l e s s   t r a n s m i s s i o n   for multi-station topology

A short description by Emmanuel Fléty.

Interactive sound and/or video Installations have (succité) a lot of interest in the last 10 years, as powerful computers allow us to "program" complex behaviour and manipulate multimedia contents. Recently the Installations have become more and more dense as have the sensors. To suppress the need for thousands of feet of wire we have developed a wireless system that allows several entities in the Installation to "talk" with the central computer. In this context the entities are QuickTime movies/ still images and lighting effects but in essence could be (m)any other things (such as animatronics or sculptural objetcs). The aim of the central computer is to manage (ie make moving) the images synchronously with sound, music and the audience.

Thus, it needs two things :
- to send orders to an entity (ie "QuickTime movie projection, lighting changes ")
- to receive information about the entity itself (where is the motion ?) and about visitors moving around the entity (proximity sensor, floor pressure sensor etc).

Building certain electronic hardwares that send and receive information through radiowaves is not that difficult. Cheap OEM R.F. modules exist on the market and would solve the problem if and when only one emitter (the performer) and one receiver (the central computer) were involved. However, as soon as you step to more than one pair things change. Two or more entities cannot "speak" at the same time on a single radio frequency. Radio frequencies are ruled and obtaining a channel is difficult. Thus, OEM modules use the same RF carrier (433 MHz in France, 868 MHz in the UK for instance). [building custom electronic devices that emit on other RF channels is illegal] To achieve multiple device communication, the channel must be shared between the different entities. We have chosen to experiment with a "question-answer" communication, since it is the simplest way to start. In this case, a central RF station (further called "master station") is an interface between the computer and the entities. On each entity, a "slave station" listens permanently to the RF carrier.

The central station is connected to the computer through a MIDI (Musical Interface Digital Instrument) link, a standard communication protocol for musical application. Each time the computer wants to talk to a slave station, a MIDI command is sent to the master station, then forwarded through RF. All slave stations receive the message but only the concerned entity "takes" it thanks to an ID embedded in the message. The concerned entity then answers by sending a message to the master station which forwards it to the computer (under the form of a MIDI message). The computer is looping towards all declared slave stations, asking each if it has something to send. The concerned station _has_ to answer even if saying "nothing to send".

Further work : We are now experimenting with a collision avoidance algorithm. In this topology each slave station is autonomous and listens to whether the RF carrier is busy or not. If not, it "takes it" to transmit. This approach is much more efficient and realtime oriented.

Emmanuel Fléty, Hardware Engineer, IRCAM