[LAU] Record levels lower in Alsa than Jack

[John Murphy]

On Thu, 5 Nov 2020 22:31:39 +0100 Fons Adriaensen <fons at linuxaudio.org> wrote:

Many thanks for the fast reply. Recording the old way in progress
currently, so I can't try anything else today.

> On Thu, Nov 05, 2020 at 09:06:19PM +0000, John Murphy wrote:
> 
> > Wondering if there's a command line (scriptable) recorder for Jack
> > which will record for a configurable duration. Because:
> > 
> > I've been using Jack TimeMachine to record in its default 'w64' format
> > and then converting to wav with sox like:
> > 
> > sox --no-clobber -S tm-2020-10-27T22:00:15.w64 -b 16 /home/john/2710.wav
> > 
> > It all works well and sox stat shows min and max amplitude -0.9 and 0.9
> > 
> > I need to make the recordings automatic and arecord seems to fit the bill
> > with its ability to record for a number of seconds with -d, but the level
> > seems much lower. I used PulseAudio Volume Control to get something like
> > a good level with 150% on the input sliders (10.53dB) and I still only
> > see around -0.5 and 0.5 using sox stat. They only go up to 11dB.  
> 
> First advice: get rid of PA and its braindead controls. If you disable
> PA when using Jack, do the same when using arecord.

Can't seem to do that, unfortunately.

> Then recording using arecord or jack with the ALSA backend (using the
> same sound card in both cases of course) should give the same result.
> In both cases that will depend on the alsamixer settings if there any
> for your sound card. 

Alsamixer shows nothing configurable for the (USB) card and using the
same arecord command (arecord -f dat -d 10 test.wav) records silence.

> The difference between +/-0.9 and +/-0.5 is actually quite small (less
> than 6 dB). It doesn't tell you anything unless you can ensure you 
> recorded exactly the same signal in both cases. Normal audio signals
> (speech, music,...) have level variations that are much bigger than 
> 6 dB.

Good point. It's mainly that the level will just look wrong in the
visual editor I use. My tests have been much shorter, but sox stat
is suggesting Volume adjustment: 1.868 and 1.662 on new files made
with +10dB (according to Pulse) and 1.000 for a previous recording
made with jack TimeMachine.

-- 
Thanks again,
John.