[LAU] Does the link of 2 units decrease the DSP load? (and more questions about DSP under GNU/Linux)

Aurelien tyranorl at free.fr
Fri Sep 10 16:57:20 UTC 2010


My question is in the subject: Do the use of 2 units linked decrease the
DSP load?

I'm currently using a Focusrite Saffire Pro 40 with FFADO 2.1 and
jackdmp on a Core2Quad Q6600 processed machine with 8 Gb RAM.
The setup I use loads the DSP at 65%.
If I add a new Focusrite Saffire Pro 40, chained to the first one, will
the DSP load decrease?

*Discussion you may not want to read*

I'm at a point where I must increase the performance of my machine, as I
will pass from 8 analog ins and 10 analog outs to 24 ins and 8 outs.
I've tried my current setup (same softs, same machine) with a RME HDSP
9652, and came to a DSP load of 35%.

I now have two options:

1. buy a RME HDSP 9652, use my Focusrite Saffire Pro 40 as a 8-Pres unit
coming through ADAT to the RME, and buy 2 more Octopre or Saffire Pro
	+ : DSP load is lower
	+ : RME clock is better than the Focusrite one
	+ : use of a PCI device => no use of FireWire, less cables, use of
	stable drivers (already packaged in Debian)
	- : more expensive (about 1250 €)
	- : this setup is limited to 24 analog ins, and cannot be raised

2. buy another Saffire Pro 40, linked to the first one, and then buying
only one OctoPre.
	+ : cheaper (about 850 €)
	+ : this setup can be easily upgraded to a 32 analog ins[1]
	+ : more outs and more S/PDIF ins
	+/- ? : DSP load decreased with link of the unit?
	- : DSP load for one unit is higher

[1] One of my jobs is to make live sounds, and with such a system, I'm
able to replace a analog mixer for FOH, coming to a solution looking
like LS-9 (with better pres) and other digital mixers.

Last but not least, last time I wondered about the use of DSP in
GNU/Linux, I finally understood that the DSP of audio interfaces weren't
use under GNU/Linux.
Am I wrong?
Is it always true?
If yes, how does it come that for a very same PC/software setup the RME
(PCI, so alsa driver) DSP load is twice lower than the Focusrite
(FireWire so FFADO) one?

Thanks a lot.


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