Digital Audio Interfacing and Networking 14872 km. The maximum number of units is dependent on
the distance, but they say that usually up to fifteen units
can be cascaded in a single chain.
Even though Soundweb uses Cat 5 cabling and the
same “RJ-45” connectors as used for Ethernet
networking, it is important to note that Soundweb is not
using the Ethernet protocol, just the same cable and
connectors as used by Ethernet.
Referring to Fig. 39-21, there are several things to
note. First, the physical wiring must always be from
output to input. Although signal flows in both directions
over each cable, the output and input terminology
shows the direction of primary signal flow. Second,
even though the physical topology is a chain, the virtual
signal flow topology is a loop. Up to eight audio chan-
nels may be passed from device to device over each
virtual signal flow link. This requires planning since if,
for example, you wanted to get a signal from B to A
above, you would have to pass the signal from B to C,
connect it inside C to an output channel, pass it from C
to D, connect it inside D to an output channel, and then
pass it from D back to A. As a result you are using one
of the eight available channels on each of three links in
order to pass the signal back one box. Planning your
circuit topology carefully will reduce the need to send
signals backwards.
39.9 Nexus
The Stage Tec Nexus is another proprietary digital
audio networking system. It is a very high-quality
system using fiber optic interconnections, providing a
very flexible system. Redundant interconnections allow
very high reliability. A very wide variety of input and
output devices are available to insert into the Nexus
frames. Both analog and digital inputs and outputs are
available.
One of the most interesting aspects of the Nexus
system is the ability of the programmable devices in a
system to learn what they should be doing from other
devices in the system. If a device fails in a running
system, when the replacement device is plugged in, it
determines what it should be doing from the other
devices in the network. The instructions for all the
devices are stored in multiple places in the system to
enable this capability.
39.10 IEEE 1394 (FireWire)
FireWire is an attractive networking scheme since it
provides both isochronous and asynchronous capa-
bilities in the same network. However, this same diver-
sity of capabilities that makes it attractive as a home
networking scheme has also lead to a wide variety of
incompatible protocols that are all carried over
IEEE 1394.
Equipment from different manufacturers, and some-
times even different models from the same manufac-
turer are not compatible. James Snider, chairman of the
1394 Trade Association wrote in March 2001 that users
of 1394 enabled equipment “want to know exactly what
will work together and what will not, so they do not
have unreasonable performance expectations.” The
1394 Trade Association is working on this issue,
however, the issues are not totally resolved.
FireWire has a maximum distance limitation for a
given link of 4.5 m and has a bandwidth limitation of
400 MBit/s for the entire network. In this sense it is
similar to an Ethernet repeater-based network.
FireWire’s proponents claim it can be cheaper than
Ethernet, but market forces have dropped the cost of
Ethernet and increased the capabilities to the point one
could question if FireWire will ever catch up.
Lastly, 1394 has been mostly applied to consumer
video and gaming application to this point. While it can
certainly carry audio, there are few if any professional
digital audio devices currently using this protocol.39.11 EthernetEthernet is the most common digital networking stan-
dard in the world, with over 50 million nodes installed.
The huge volumes in the computer industry are
constantly driving the price down and the capabilities
upward. Many professional audio devices use Ethernet
for control and programming, and both AMX andFigure 39-21. Soundweb. The physical interconnection is a
chain, but the audio signal path is a loop.A B CD
in out in out in out in outABCD8 channels8 channels 8 channels8 channelsAudio signal flow that results from
physical interconnection shown