20 Mar
2021
20 Mar
'21
midnight
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
www.ovenwerks.net