[LAU] Low recording volume in Pulseaudio

Len Ovens len at ovenwerks.net
Fri Mar 19 22:59:55 CET 2021

On Fri, 19 Mar 2021, Antony Gelberg wrote:

> For the purposes of this discussion, I am playing guitar through an Axe-Fx
> II into a Behringer UMC1820 USB rack soundcard. OS is Xubuntu Focal 20.04.
> This works fine for general jamming, I can play a backing track from the
> computer and use the hardware monitoring to mix in the Axe-Fx as necessary.
> But when I want to record, the levels are low. I record using my Android
> phone, and activate Audacity to record the guitar tracks from the Axe-Fx.
> But the levels are so low, the waveform is tiny. I can apply a plugin to
> increase the volume, but you can imagine the quality.

Android phone? This is confusing to me. How do you go from Xubuntu to 
android phone?

> I see the same thing in the Pulseaudio Volume Control Input Devices tab, so
> it's not an Audacity issue. With jackd active, the input levels are fine, I
> can see them in Ardour. But I don't want to have to activate jack for more
> simple stuff like this, it's a pain.

Ardour and Audacity are two different animals. Audacity as it comes by 
default shows levels as voltages Ardour shows levels as dBfs (in the 
tracks) and -20dB in master (as a quick summary)
In Audacity Edit->Preferences->Tracks->Default Waveform scale, change the 
setting from linear to Logarithmic and see if that doesn't look more 
normal to you. With linear, when the wave is at half height, it is only 
6dB down from full scale... not half volume when you listen by ear. Also, 
compare audacity using pulse, alsa and jack. They should all be about the 
same (unless pavucontrol has played with the level). Alsa check the level 
using alsamixer as well as pavucontrol. I do not know if your device shows 
any controls to alsamixer or not (some don't) but it is good to at least 
find that out.

> For what it's worth, the output level of the Axe-Fx is around 75%, and the
> input gain knobs on the soundcard at about 25%. Any higher and the clip
> lights come on when I play hard.

75% and 25% have little meaning except where calibrated... in which case 
no one would read them as a %  :)  However, if that is just below clip 
level that is where you want them anyway.

> Any clues on how to debug / fix this? I'm suspecting a driver issue, because
> there simply aren't that many moving parts.

Drivers? The only things that might be refered to as drivers would be the 
kernel modules for USB audio devices. They serve merely to transfer Audio 
data from your USB TX to which ever application is using that data. Both 
jackd and Pulseaudio run on top and use alsa as their back end. SO it 
would be hard to explain how the kernel module or alsa would be able to be 
different for one over the other. Probably don't spend your time trying to 
find driver issues as your experimentation has pretty much ruled that out.

I would expect that the main problem is that your waveforms in audacity 
are linear giving you a faulse sense of low levels when they are not 
really. Also, be aware that recording a 24 bit signal at -10 or even -20dB 
is normal and still leaves lots of signal to noise for a clean signal when 
it is turned up. Also be aware that the music you are playing along with 
has been artifically squashed (sorry compressed) to make it sound as loud 
as possible compared to the music it is compared to and as such, is not a 
good signal to compare real audio to.

Len Ovens

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