User's Guide Fireface 400
© RME
95
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 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 allows 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 to
SPDIF
(10 channels). Logically, as ADAT isn't available in this
mode anyway.
Limit Bandwidth
48 kHz (18) 96 kHz (14)
192 kHz (10)
FW-Kanäle
All Channels
x
x
/
18
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 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
48 kHz
(max 18)
DS
(max. 14)
QS
(max. 10)
All Channels
18
28
40
SPDIF 10
20
40
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 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 can't are not reliable at full channel count (3 x 14 equals 3 x 28 = 84
channels 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.
Summary of Contents for Fireface 400
Page 5: ...User s Guide Fireface 400 RME 5 User s Guide Fireface 400 General ...
Page 12: ...12 User s Guide Fireface 400 RME ...
Page 13: ...User s Guide Fireface 400 RME 13 User s Guide Fireface 400 Installation and Operation Windows ...
Page 63: ...User s Guide Fireface 400 RME 63 ...
Page 86: ...86 User s Guide Fireface 400 RME ...
Page 87: ...User s Guide Fireface 400 RME 87 User s Guide Fireface 400 Technical Reference ...
Page 99: ...User s Guide Fireface 400 RME 99 38 Diagrams 38 1 Block Diagram Fireface 400 ...
