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User's Guide Fireface 800 © RME
37.4 Number of Channels and Bus Load
As explained in chapter 37.3, 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 acitivity - even
outside the PCI bus - causes drop outs.
Transferring these experiences to FireWire and the Fireface 800 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 allows to reduce the number of trans-
ferred channels. Limit Bandwidth provides four options, limiting the transmission internally to 28,
20, 12 or 8 channels. This limitation is independent from the sample rate, which is why the op-
tion's descriptions are not fully correct at 96 kHz. As can be seen in the following table, in 96
kHz mode there is no difference between the setting All Channels and An.+SPDIF+ADAT1. For
a valid reduction of the bus load ADAT must be unselected completely. As the Fireface offers
only 12 channels in Quad Speed mode, the options All Channels (28 channels) down to Ana-
log+SPDIF (12 channels) perform no change at all. Logically, as ADAT isn't available in this
mode anyway.
Limit Bandwidth 48 kHz (28) 96 kHz (20) 192 kHz (12) FW-Kanäle
All Channels x / / 28
An.+SPDIF+ADAT
1
x x / 20
Analog+SPDIF x x x 12
Analog 1-8 x x x 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 12 channels at 192 kHz equal a FireWire
and PCI bus load of 48 channels at 48 kHz! The following table shows the real bus load in all
modes.
Limit Bandwidth 48 kHz
(max 28) DS (max. 20) QS (max. 12)
All Channels 28 40 48
An.+SPDIF+ADAT
1
20 40 48
Analog+SPDIF 12 24 48
Analog 1-8 8 16 32
The usage of multiple Firefaces in DS and QS operation can be problematic due to the in-
creased bus load. Some examples:
• 2 Firefaces will most likely not run stable at 192 kHz at full track count. 2 x 12 channels 192
kHz equal 2 x 48 channels at 48 kHz = 96 channels per direction.
• 2 Firefaces at 96 kHz should operate reliable at full channel count. 2 x 20 equals 2 x 40 = 80
channels per direction.
• 3 Firefaces at 96 kHz can't operate at full channel count (3 x 20 equals 3 x 40 = 120 chan-
nels per direction). The Settings dialog will show Errors, audio will sound distorted.
• 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 all Firefaces. This equals 3 x 24 = 72 channels
per direction.