[linux-audio-user] FA-66, slackware, freebob, thinkpad R40 - cracks on recordings

[Pieter Palmers]

mea wrote:
> Hello from Prague,
> I bought the firewire soundcard Edirol FA-66. Yesterday morning I 
> compiled freebob, recompiled jack and within half an hour I already saw 
> all 6 inputs and outputs in qjackctl. What? Recording on linux is hard? 
> Pha! I thought but unfortunately my moment of victory didn't last long. 
> On all recordings there is a strange cracking sound, not all the time, 
> but every couple of seconds. No xruns though. Before I bought this 
> device I used to record through the internal soundcard of my thinkpad 
> R40, running all programs as root and this 'sort of' worked. So I 
> thought I should first of all make sure all my apps can run as user. 
> After a lot of reading (I spent all day on this yesterday before I 
> decided to consult this list) I figured the best approach for me would 
> be to use set_rlimits, because slackware doesn't have PAM, and I never 
> patched a kernel before. As I understand these are the 3 possibilities 
> for programs to run in realtime right? Well, in short I can now run 
> qjackctl, jack and ardour as user without xruns, but this weird cracking 
> is still on the recordings. To make sure it was not an ardour issue I 
> tried sooperlooper, and the noise is still there. I suspect the trouble 
> is probably freebob, because I didn't have this issue before and maybe I 
> should have written on their list, but I am not completely sure and I 
> found very useful information here before, so here I am, asking for any 
> advice, because I don't know what to do anymore. Here is some more 
> information about my setup>

The crackles you hear are due to libfreebob not being able to process 
the firewire packets fast enough. This means lost packets and hence lost 
audio data. You would expect xruns when this happens, but freebob-1.0 
doesn't report dropped packets as xruns, this could be called a bug. 
We'll fix this in the next version.

In order to use FreeBoB, it is really recommended to use a realtime 
patched kernel. We are doing in userspace what is normally done in 
kernel space, and hence we need to meet very strict deadlines to be able 
to. When using a realtime patched kernel that is configured correctly 
(see 
http://freebob.sourceforge.net/index.php/System_Configuration_Hints), no 
packets are dropped and this problem does not occur.

Having said this, you can probably improve things on a standard kernel 
if you allow user processes to request SCHED_FIFO scheduling (realtime 
scheduling). As a test you can run jackd+freebob as root and see if the 
problem persists. Remember to start jackd in realtime mode:
e.g.
jackd -R -p60 -d freebob -p512

> 
> Thinkpad R40, Pentium4, 512MbRAM
You should check the brand of the firewire controller that you are 
using. (some) Thinkpads have a Ricoh firewire controller built in, and 
that is a very buggy piece of hardware. It will never work properly with 
freebob because it fails on isochronous traffic. (check this using lspci -v)

The solution for that is to use a cardbus firewire controller. Chipsets 
from Via, Texas Instruments and Nec are working fine for me.

> Slackware 10.2
> uname -r    2.6.13
Should it be possible, maybe consider switching to a more audio friendly 
distro...

Greets,

Pieter Palmers
FreeBoB developer