Metric Halo Mobile I/O - Manual (Page 327)

FireWire
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speed of any bus traffic that flows through them. In order to maximize performance, you want to ensure that
low speed devices are not used to join high speed devices. In practice this generally means that you should
attach your interface directly to your computer or through a high speed hub.
To connect the interface to your computer simply plug a FireWire cable into the interface and into the com-
puter. The FireWire bus provides a path for all communication between the computer and box – audio, control
and meter data.
Metric Halo’s audio transport takes advantage of FireWire’s support for isochronous transmission, in which
the hardware can reserve a dedicated amount of bandwidth on the bus for moving audio samples. Since the
audio must be transmitted on a regular basis to ensure continuous playback and recording, the isochronous
model is perfect for this task.
Control changes and meter data are transmitted using asynchronous transactions on the FireWire bus. This
transmission approach makes use of the unreserved bandwidth on the bus and competes with things like
FireWire hard disk accesses for time. Under normal circumstances this is completely transparent to the user.
If the bus becomes overloaded, you may find that disk accesses and meter updates slow down. If you are
experiencing bus overloads, you can always add a second FireWire bus with a third-party FireWire card (PC-
Card or PCI card depending on your machine), and offload one or more devices to the second bus.
FireWire FAQ
There is a lot of “wisdom” on the Internet regarding FireWire and how it does (and doesn’t) work. Unfortunately,
much of it is incorrect and leads to a lot of confusion. Let’s take a look at some common statements about
FireWire:
GENERAL:
• “You can’t use a FW400 device on a computer with FW800 ports.” -False
All you need is a FW400 to FW800 cable or adapter. Your computer and peripherals will do the rest.
• “My computer has two FW ports, so I have two FW busses.” -False
Your computer has one FireWire bus and an internal hub. To have more than one FW bus you need
to use a PCI card or ExpressCard expansion card.
• “If I connect a FW400 device to a FW800 bus, it will slow the entire bus down to 400 Mbs.” -False
In a mixed speed bus, everything after the slowest device runs at the slower speed.
For example, in this FW800 chain:
FW800 FW400 FW800 host
computer
FW800
device
FW400
device
Figure D.1: Correctly Mixed FW800/400 Chain
The FW800 device communicates with the computer at 800 Mbs, and the FW400 device communicates
at 400 Mbs. However, in this chain:
FW400 FW800 host
computer
FW800
device
FW400
device
FW400
Figure D.2: Incorrectly Mixed FW800/400 Chain