Alsa passthrough Digital In to Analog Out

Question

I have a LogiLink USB Soundbox 7.1 connect to a raspberry pi running rasbian and I want to passthrough the input of S/PDIF In to Speaker Front.

I found this: Possible to route audio directly from usb-audio line-in to same usb audio line-out?

But can't get it to work. I also can't find any good documentations on how to set the exact in and output of aplay and arecord

aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Headphones [bcm2835 Headphones], device 0: bcm2835 Headphones [bcm2835 Headphones]
  Subdevices: 8/8
  Subdevice #0: subdevice #0
  Subdevice #1: subdevice #1
  Subdevice #2: subdevice #2
  Subdevice #3: subdevice #3
  Subdevice #4: subdevice #4
  Subdevice #5: subdevice #5
  Subdevice #6: subdevice #6
  Subdevice #7: subdevice #7
card 1: ICUSBAUDIO7D [ICUSBAUDIO7D], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

aplay -L
null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=Headphones
    bcm2835 Headphones, bcm2835 Headphones
    Default Audio Device
sysdefault:CARD=Headphones
    bcm2835 Headphones, bcm2835 Headphones
    Default Audio Device
dmix:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Direct sample mixing device
dsnoop:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Direct sample snooping device
hw:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Direct hardware device without any conversions
plughw:CARD=Headphones,DEV=0
    bcm2835 Headphones, bcm2835 Headphones
    Hardware device with all software conversions
default:CARD=ICUSBAUDIO7D
    ICUSBAUDIO7D, USB Audio
    Default Audio Device
sysdefault:CARD=ICUSBAUDIO7D
    ICUSBAUDIO7D, USB Audio
    Default Audio Device
front:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Front speakers
surround21:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    4.0 Surround output to Front and Rear speakers
surround41:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    IEC958 (S/PDIF) Digital Audio Output
dmix:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct sample mixing device
dsnoop:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct sample snooping device
hw:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct hardware device without any conversions
plughw:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Hardware device with all software conversions

arecord -l
**** List of CAPTURE Hardware Devices ****
card 1: ICUSBAUDIO7D [ICUSBAUDIO7D], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

arecord -L
null
    Discard all samples (playback) or generate zero samples (capture)
default:CARD=ICUSBAUDIO7D
    ICUSBAUDIO7D, USB Audio
    Default Audio Device
sysdefault:CARD=ICUSBAUDIO7D
    ICUSBAUDIO7D, USB Audio
    Default Audio Device
front:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Front speakers
surround21:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    2.1 Surround output to Front and Subwoofer speakers
surround40:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    4.0 Surround output to Front and Rear speakers
surround41:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    4.1 Surround output to Front, Rear and Subwoofer speakers
surround50:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    5.0 Surround output to Front, Center and Rear speakers
surround51:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    5.1 Surround output to Front, Center, Rear and Subwoofer speakers
surround71:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    7.1 Surround output to Front, Center, Side, Rear and Woofer speakers
iec958:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    IEC958 (S/PDIF) Digital Audio Output
dmix:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct sample mixing device
dsnoop:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct sample snooping device
hw:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Direct hardware device without any conversions
plughw:CARD=ICUSBAUDIO7D,DEV=0
    ICUSBAUDIO7D, USB Audio
    Hardware device with all software conversions

Answer 1

AFAIK, the LogiLink does not have any capability of internal routing, so in any case you'd have to go through the software layer of the Pi. I am not familiar with possibilities to directly route ALSA inputs to outputs, but you can certainly do that using the Jack audio connection kit (sudo apt install jackd). My go-to front-end is qjackctl which lets you quickly establish a patchbay routing from any inputs to any outputs, and the routing can be persisted and automatically loaded when QJackCtl starts. If you want a headless solution (no UI popping up), there are certainly ways control the Jack interconnections directly with a terminal tool / dbus.

Answer 2

arecord -f dat -D iec958:CARD=ICUSBAUDIO7D,DEV=0 | aplay -D front:CARD=ICUSBAUDIO7D,DEV=0

This is working but it has lots of over and under runs on a raspberry pi, hence the latency is very bad. I also played around with alsaloop but kept getting a buserror. What could be related to: https://github.com/alsa-project/alsa-utils/issues/85

But I found a solution for the logilink card, it has a internal routing from line in to front. This works fine for me. But the general solution would be the one above.