[LAU] Crackles in audio - how to troubleshoot ?

jonetsu at teksavvy.com jonetsu at teksavvy.com
Sat Aug 27 21:47:42 UTC 2016

On Fri, 26 Aug 2016 20:50:43 -0700 (PDT)
Len Ovens <len at ovenwerks.net> wrote:

Thanks for the comments.

I'm afraid the following are rather random notes at this point.

There is some progress in that now when there's a hint of crackle
happening it is possible to augment it a lot by enabling the audio
interface in Bitwig (icon at the top left corner).  No need to have any
project loaded.  As soon as the Activate Audio Engine icon is press, a
lot of crackles are heard.  As soon as the Audio Engine is
deactivated, the crackling almost disappear.  At the same tine Mixbus
32C is playing a project and the sound out of Mixbus has no crackle
whatsoever.  But vlc has, when the Audio Engine in Bitwig is enabled.

At this point Bitwig has already played a project for some time,
perhaps 30 minutes or so.  It seems that at some point the system gets
into a state that promotes the production of crackling sounds.  

As I write this there was no audio whatsoever being played.  Bitwig is
running, audio enabled, as well as Mixbus.  vlc is now launched and
there's no crackle at all.  At the present moment it has disappeared.
Bitwig then plays a project and why not, Mixbus also.  No crackling.

Looks like that state the audio system was in has now moved to
something else.

> That sounds like there are two dbus running and two pulse. normally
> pulse is run across dbus which should ensure only one of them. Do you
> have two sessions open? (one with a dfferent user?)

No, only one user.  OTOH, it seems there's some kind of dbus fiesta
there, with no less than 5 /bin/dbus-daemon processes running.  I do
not know dbus, although it seems that instances are run depending of
applications connecting to it.  Having 5 might just make sense.  See
below for a list.

> try running these two commands while the noise is there:
> pactl unload-module module-udev-detect
> pactl unload-module module-alsa-card

I ran them, as well as the other one for jackbus, and that did not have
any effect. Running them again shows that the modules were effectively
unloaded as they now give error msgs attempting to unload them again.

> In other words I use pulse only as a front end for jack... nothing
> more. If I want pulse to see some other AI, I start a zita-ajbridge
> to make that interface show on the jack graph and attach the pa
> bridge to that. 

First time I hear about zita.  What is it about ?

> So in pavucontrol I can see both a Jack sink (PA-M66) and a Jack sink 
> (PA-PCH) and they are connected to the AI of the same name. jack_lsp
> -c shows:
> PCH-in:capture_1
>     PA_PCH:front-left
> PCH-in:capture_2
>     PA_PCH:front-right
> PA-PCH:front-left
>     PCH-out:playback_1
> PA-PCH:front-right
>     PCH-out:playback_2

Here also Jack Sink is shown running the same command, as well as many
other inputs/outputs.

I have to try again to remove the autospawn in pa's config file,  Last
attempt did not yield good results, I have to confirm this.

To sum up this pass in broad lines:

1) Bitwig audio enable can provoke a plethora of crackling once the
audio system as reached an unknown state in which these can happen in
both Bitwig and vlc.

2) The problem kind of mysteriously went away without having to reboot,
while writing this reply.

3) During that time the output from Mixbus 32C remained clean.

4) The pactl unload commands were run immediately when the crackling
started to manifest itself.  That did not stop the crackles but the
modules remained unloaded for the rest of the time, up to now.

Thanks again for the comments - much appreciated.


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