RME Audio Fireface 802, Руководство пользователя

Страница: 103 / 112

User's Guide Fireface 802 © RME 103
¾ Connect the Fireface to a USB port
¾ Start the Device Manager, View set to Devices by Connection
¾ Select ACPI x86-based PC, Microsoft ACPI-Compliant System, expand PCI Bus
This branch normally includes two entries of a
USB2 Enhanced Host Controller
. A USB Root
Hub can be seen, which then connects all USB devices, including the Fireface. By reconnecting
to a different port this view immediately shows at which of the two controllers the Fireface is
connected. With multiple devices it can also be checked if they are connected to the same con-
troller.
Furthermore this information can be used to operate an external USB drive without disturbing
the Fireface, by simply connecting the drive to the other controller.
Especially with notebooks it can happen that all internal devices and all the sockets/ports are
connected to the same controller, with the second controller not used at all. In that case all de-
vices have to use the same bus and interfere with each other.
35.4 FireWire Audio
FireWire audio is in several ways different from PCI audio interfaces. RME’s PCI data transmis-
sion happens per channel, while FireWire is working interleaved. A direct communication with
the application (Zero CPU load) is not possible with FireWire, because communication has to
be established by the operating system's FireWire driver. Compared to PCI cards, the FireWire
subsystem creates an additional CPU load at lower latencies.
Due to insufficient buffering within FireWire controllers, single peak loads on the PCI bus can
already cause loss of one or more data packets. This is independent of the manufacturer and
no RME problem.
FireWire Audio does not reach the same performance as PCI audio. On a standard computer
with modern single PCI bus, about 100 audio channels can be transmitted per direction (re-
cord/playback). Exceeding this limit, any system activity - even outside the PCI bus - causes
drop outs.
The Fireface 802 features a unique data
checking, detecting errors during
transmission via PCI/FireWire and
displaying them in the Settings dialog.
Additionally the Fireface provides a special mechanism which allows to continue record and
playback in spite of drop-outs, and to correct the sample position in real-time. Detailed informa-
tion on this topic can be found in the Tech Info
FireWire Audio by RME – Technical Background
on the RME website: http://www.rme-audio.com/english/techinfo/fwaudio_rme.htm
Transferring these experiences to FireWire and the Fireface 802 means that besides the num-
ber of channels the
bus load
has to be taken into account too. One channel at 96 kHz causes
the same load to the system as two channels at 48 kHz, the bus load is doubled at 96 kHz and
quadrupled at 192 kHz.
Limit Bandwidth
sets a constant number of channels, but those chan-
nels cause a bigger load in DS and QS mode, because more data have to be transferred. For
example the 18 channels at 192 kHz equal a FireWire and PCI bus load of 72 channels at 48
kHz:
Limit Bandwidth 48 kHz (max 30) DS (max. 22) QS (max. 18)
All Channels 30 44 72
An.+AES+ADAT1 22 40 64
AES 14 28 56
Analog 1-8 8 16 32