[LAU] Sound Devices USBPre2, UAC2 status

shal at free.fr shal at free.fr
Mon Jan 7 19:25:29 UTC 2013

----- Mail original -----
> De: "alexander" <axeldenstore at gmail.com>
> À: linux-audio-user at lists.linuxaudio.org
> Envoyé: Lundi 7 Janvier 2013 18:27:42
> Objet: Re: [LAU] Sound Devices USBPre2, UAC2 status
> I also own this interface, recently it has started to work in uac2
> mode,
> I still cant raise the samplerate, but it runs.. however, It still
> runs
> alot better in uac1.. when in uac2 jack mysteriously shuts down for
> no
> appearant reason. I run archlinux, allways uptodated with vanilla
> kernel.
> This is when I connect it without pressing 1. eg, uac2
> http://pastebin.com/BFJTwXnJ
> And as with pressing 1. http://pastebin.com/ZJ1jbPUK


In my understanding, you are not in UAC2 , you are in high-speed mode but the usb audio protocol remains UAC1.

In the kernel source in the file include/uapi/linux/usb/audio.h is defined two values:
/* bInterfaceProtocol values to denote the version of the standard used */
#define UAC_VERSION_1			0x00
#define UAC_VERSION_2			0x20

So if the card is in UAC2 , you should see bInterfaceProtocol at 32 (or 0x20)
By example, now the RME babyface is recognize as UAC2 device, you can view its "lsub -v" here  : http://minilien.fr/a0m3kc

But for the usbPre2 device with ou without pressing the "1" button,  we have bInterfaceProtocol = 0. So we stays in UAC1, even the speed increase to high-speed.

So IMHO, it's why, we are already limited to 48khz framerate.

If a usb audio guru can confirm my idea...

I have no problem in high speed since kernel 3.2. Ubuntu kernel or vanilla kernel are OK (today, I use a 3.7.1 vanilla kernel).

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