[Jack-Devel] Bitrate from Jack or USB driver

[Chris Caudle]

On Fri, December 14, 2018 1:47 am, liebrecht at grossmann-venter.com wrote:
> My presonus 1818vsl seemingly defaults to 44100 Hz never mind if you
> configure it to 96000Hz with the windows mixer.
> It seems as soon as you close down the windows based presonus audiobox
> software it defaults back to 44100Hz and doesn't keep it in memory when
> you use it on Linux.

Most USB devices will use whatever the driver requests if possible.  The
driver will request whatever the application which  opens the ALSA driver
requests if the driver accepts the request as supported by the card (USB
devices return a parameter list which shows what rates are supported).

> Questions:
> 1) I need to confirm the above.
> Is there any way jack can report the bitrate of the data it receives
> from usb driver and therefore the bitrate by which the presonus 1818vsl
> send the data.

Check the /proc/asound/ entry for your card.  See below, in my case it was
card0/pcm0p/info, depending on your card configuration it may be something
other than card0 and pcm0.
The script that Robin mentioned will dump that info for you.

> 2) If the 1818vsl does revert back after a windows config at a higher
> rate, is there any current facility in jack to configure the presonus to
> work with 96000Hz ?

Sure, just specify what sample rate you want to use when you start jackd
and it will send that request to the ALSA driver, which should send that
request to the hardware.

Startup with 48k requested:

$ jackd -dalsa -dhw:Lambda -r48000
jackdmp 1.9.12
...startup messages, copyright, blah blah...
Acquire audio card Audio0
creating alsa driver ...
hw:Lambda|hw:Lambda|1024|2|48000|0|0|nomon|swmeter|-|32bit
configuring for 48000Hz, period = 1024 frames (21.3 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 24bit little-endian in
3bytes format

Startup with 44.1k requested:
$ jackd -dalsa -dhw:Lambda -r44100
...startup messages, copyright, blah blah...
Acquire audio card Audio0
creating alsa driver ...
hw:Lambda|hw:Lambda|1024|2|44100|0|0|nomon|swmeter|-|32bit
configuring for 44100Hz, period = 1024 frames (23.2 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 24bit little-endian in
3bytes format

Startup with 96k requested (note this is not supported, so defaults to
closest supported value, which in this case is 48k):
$ jackd -dalsa -dhw:Lambda -r96000
...startup messages, copyright, blah blah...
Acquire audio card Audio0
creating alsa driver ...
hw:Lambda|hw:Lambda|1024|2|96000|0|0|nomon|swmeter|-|32bit
configuring for 96000Hz, period = 1024 frames (10.7 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 24bit little-endian in
3bytes format

Note that jackd does not warn the rates do not match, which is not what I
expected.

$ cat /proc/asound/card0/pcm0p/sub0/hw_params
access: MMAP_INTERLEAVED
format: S24_3LE
subformat: STD
channels: 2
rate: 48000 (48000/1)
period_size: 1024
buffer_size: 2048

I thought that the ALSA driver would return the value of the actual sample
rate used, and jackd would check that and display if the rate in use did
not match the requested rate.  Apparently not, at least not for all cards,
maybe driver dependent.

-- 
Chris Caudle