[LAU] MotU Ultralite AVB observations.

[John Murphy]

On Sun, 8 Mar 2020 18:25:08 -0500 "Chris Caudle" wrote:

> On Sat, March 7, 2020 4:52 pm, John Murphy wrote:
> > I seem to have narrowed it down to dependence on the setting of
> > 'Clock Mode'. I do a lot of recordings from a 48kHz optical source
> > plugged into the S/Pdiff optical input. Seems to be that clock the
> > kernel can't use.  
> 
> Do you have a valid S/PDIF bitstream running when you start your device? 
> If the default clock is S/PDIF then there must be a valid S/PDIF  stream
> to recover the clock.

Yes. It's almost always running and should be 'valid'.

> > Probably because it has somehow changed the device
> > to 44.1kHz (via 192kHz) and then demands a 44.1kHz clock.  
> 
> So you think the ALSA driver may be configuring the device for 44.1k based
> sample rates, but you have a 48k S/PDIF stream running?
> If that is the case you should be able to figure out which software is
> opening the audio device and change the default sample rate to 48k. 
> Depending on desktop environment it is likely PulseAudio.

It is indeed. Thank you. I'm amazed that in all these years of
using Linux, I hadn't realised there's a default setting for the
sound server sample rate. I've added the following lines near
the top of /etc/pulse/daemon.conf (I'm the only user):

### Changed default sample rate to 48kHz and alt sample rate to 44.1kHz
default-sample-rate = 48000
alternate-sample-rate = 44100

I'm now convinced the problem I'm seeing is due to using it on Windows,
which puts it in proprietary mode and causes:

[pulseaudio] alsa-util.c: Device hw:2 has 64 channels, but PulseAudio
supports only 32 channels. Unable to use the device.

After writing these settings to it:
curl --data 'json={"value":"UAC"}' IP address/datastore/host/mode
curl --data 'json={"value":"24"}' IP address/datastore/host/maxUSBToHost

Then switching it off and on again. PulseAudio syslog messages then stop
at a "clock source 1 is not valid, cannot use" and a few lines up from
there it says:

pulseaudio[1319]: [pulseaudio] module-alsa-card.c: Failed to find a
working profile.
pulseaudio[1319]: [pulseaudio] module.c: Failed to load module
"module-alsa-card" (argument: "device_id="2" name="usbMOTU[...]"
card_name="alsa_card.usb-MOTU[...]" namereg_fail=false tsched=yes
fixed_latency_range=no ignore_dB=no deferred_volume=yes use_ucm=yes
card_properties="module-udev-detect.discovered=1""):
initialization failed.

But it works and all seems well now.

Without doing that (curl bit) it seems to me that anyone trying
to use one which has been used on Windows will get a variation of
'the bitcrusher effect' which today, for me, was to see and hear
thin audio on 'From computer' 1 & 2 and 9 & 10, whereas previously
I was seeing channel hopping (and hearing the crushed audio on one
pair at a time). I've seen some weird symptoms in my time, but...

Something I've found useful is to have PulseAudio Volume Control
running on the desktop on its Output Devices tab. The UltraLite
AVB Multichannel device appears there as soon as it's ready to use. 

Thanks for the help.

-- 
John.