5.2 Audio Over FireWire (IEEE 1394)

5.2 Audio Over FireWire (IEEE 1394)

5.2.1 Basic FireWire Principles

FireWire is an international standard serial data interface specified in IEEE 13941995 ² . One of its key applications has been as a replacement for SCSI (Small Computer Systems Interface) for connecting disk drives and other peripherals to computers. It is extremely fast, running at rates of 100, 200 and 400Mbit/s in its original form, with higher rates appearing all the time up to 3.2Gbit/s. It is intended for optical fibre or copper interconnection, the copper 100Mbit/s (S100) version being limited to 4.5m between hops (a hop is the distance between two adjacent devices). The S100 version has a maximum realistic data capacity of 65Mbit/s, a maximum of 16 hops between nodes and no more than 63 nodes on up to 1024 separate buses. On the copper version there are three twisted pairs data, strobe and power and the interface operates in half duplex mode, which means that communications in two directions are possible, but only one direction at a time. The 'direction' is determined by the current transmitter which will have been arbitrated for access to the bus. Connections are 'hot pluggable' with auto-reconfiguration in other words one can connect and disconnect devices without turning off the power and the remaining system will reconfigure itself accordingly . It is also relatively cheap to implement.

Unlike, for example, the AES3 audio interface, data and clock (strobe) signals are separated. A clock signal can be derived by exclusive-OR'ing the data and strobe signals, as shown in Figure 5.1. FireWire combines features of network and point-to-point interfaces, offering both asynchronous and isochronous communication modes, so guaranteed latency and bandwidth are available if needed for time-critical applications. Communications are established between logical addresses, and the end point of an isochronous stream is called a 'plug'. Logical connections between devices can be specified as either 'broadcast' or 'point-to-point'. In the broadcast case either the transmitting or receiving plug is defined, but not both, and broadcast connections are unprotected in that any device can start and stop it. A primary advantage for audio applications is that point-to-point connections are protected only the device that initiated a transfer can interfere with that connection, so once established the data rate is guaranteed for as long as the link remains intact. The interface can be used for real-time multichannel audio interconnections, file transfer, MIDI and machine control, carrying digital video, carrying any other computer data and connecting peripherals (e.g. disk drives).

Figure 5.1: Data and strobe signals on the 1394 interface can be exclusive-OR'ed to create a clock signal.

Data is transferred in packets within a cycle of defined time (125