Number Of Channels And Bus Load - RME Audio Fireface 400 User Manual

35.4 Number of Channels and Bus Load

As explained in chapter 35.3, 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 (record/playback). Exceeding this limit, any system activity - even out-
side the PCI bus - causes drop outs.
Transferring these experiences to FireWire and the Fireface 400 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!
To use FireWire as efficiently as possible, the Fireface helps to reduce the number of trans-
ferred channels. Limit Bandwidth provides three options, limiting the transmission internally to
18, 10 or 8 channels. This limitation is independent from the sample rate. As the Fireface offers
only 10 channels in Quad Speed mode, the option All Channels (18 channels) performs no
change compared to Analog+SPDIF (10 channels). Logically, as ADAT isn't available in this
mode anyway.
Limit Bandwidth
All Channels
Analog+SPDIF
Analog 1-8
The bus load is doubled at 96 kHz and quadrupled at 192 kHz. Limit Bandwidth sets a constant
number of channels, but those channels cause a bigger load in DS and QS mode, because
more data have to be transferred. For example the 10 channels at 192 kHz equal a FireWire
and PCI bus load of 40 channels at 48 kHz! The following table shows the real bus load in all
modes.
Limit Bandwidth
All Channels
Analog+SPDIF
Analog 1-8
The usage of multiple Firefaces in DS and QS operation can be problematic due to the in-
creased bus load. Some examples:
• 2 Firefaces are critical at 192 kHz with full track count. 2 x 10 channels 192 kHz equal 2 x 40
channels at 48 kHz = 80 channels per direction.
• 2 Firefaces at 96 kHz with full channel count are not critical. 2 x 14 equals 2 x 28 = 56 chan-
nels per direction.
• 3 Firefaces at 96 kHz are not reliable at full channel count (3 x 14 equals 3 x 28 = 84 chan-
nels per direction). The Settings dialog might show errors, audio might distort.
• To not exceed a maximum of 80 channels with 3 Firefaces at 96 kHz, a setting like Ana-
log+SPDIF is recommended to be used on at least one Fireface.
86
48 kHz (18) 96 kHz (14)
x x
x x
x x
48 kHz DS
(max 18) (max. 14)
18 28
10 20
8 16
User's Guide Fireface 400 © RME
192 kHz (10) FW-Channels
/ 18
x 12
x 8
QS
(max. 10)
40
40
32