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Have you tested surround21?

Yes, front speakers work fine, however LFE doesn't seem to work. Instead

when I try to test LFE (from sound UI) I have weird noise in one of rear speakers. There are no such problems with any other mode though (I see that officially only 5.1 mode is supported, however 4.1, 5.0 and 5.1 all work fine), I decided to leave 2.1 mode for now, not sure whose fault is that.

I: [pulseaudio] (alsa-lib)pcm_hw.c: Invalid PCM state for chmap_get: OPEN I: [pulseaudio] (alsa-lib)pcm_hw.c: SNDRV_PCM_IOCTL_HW_PARAMS failed (-5)

http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=test/chmap.c;hb=HEAD

Do snd-usb-audio provide channel map ?

http://cgit.freedesktop.org/pulseaudio/pulseaudio/log/?qt=grep&q=lfe-fil...

http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/conf/cards/USB-Audi...

# If a device does not use the first PCM device for digital data, the device # number for the iec958 device can be changed here. USB-Audio.pcm.iec958_device

Do the red light in the optical spdif alway turn on ?

Do the red light turn on when pulseaudio probe the devices ?

card 2: S51 [SB Omni Surround 5.1], device 0: USB Audio [USB Audio] Subdevices: 1/1 Subdevice #0: subdevice #0 card 2: S51 [SB Omni Surround 5.1], device 1: USB Audio [USB Audio #1] Subdevices: 1/1 Subdevice #0: subdevice #0 card 2: S51 [SB Omni Surround 5.1], device 2: USB Audio [USB Audio #2] Subdevices: 1/1 Subdevice #0: subdevice #0