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) |
|
All Channels | x | / | / | 28 |
An.+SPDIF+ADAT | x | x | / | 20 |
1 |
|
|
|
|
Analog+SPDIF | x | x | x | 12 |
Analog | 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 | 20 | 40 | 48 |
1 |
|
|
|
Analog+SPDIF | 12 | 24 | 48 |
Analog | 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.
98 | User's Guide Fireface 800 © RME |
