How to set up and operate mpd on Linux for input from one USB port via alsa and output to another USB port?

[111MilesToGo]

Please, I need the community's help to achieve what the title says. Very unfortunately, the mpd documentation did not help me enough. Neither did searching this forum for similar topics.

Preface: Running mpd as a user system service on openSUSE Tumbleweed, and also on Ubuntu Server. For Tumbleweed:

$ mpd --version
Music Player Daemon 0.23.17 (0.23.17)
Copyright 2003-2007 Warren Dukes <warren.dukes@gmail.com>
Copyright 2008-2021 Max Kellermann <max.kellermann@gmail.com>
This is free software; see the source for copying conditions.  There is NO
warranty; not even MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Database plugins:
 simple proxy upnp

Storage plugins:
 local smbclient udisks nfs curl

Neighbor plugins:
 smbclient upnp udisks

Decoders plugins:
 [mad] mp3 mp2
 [mpg123] mp3
 [vorbis] ogg oga
 [oggflac] ogg oga
 [flac] flac
 [opus] opus ogg oga
 [dsdiff] dff
 [dsf] dsf
 [hybrid_dsd] m4a
 [wavpack] wv
 [openmpt] mptm mod s3m xm it 669 amf ams c67 dbm digi dmf dsm dtm far imf ice j2b m15 mdl med mms mt2 mtm nst okt plm psm pt36 ptm sfx sfx2 st26 stk stm stp ult wow gdm mo3 oxm umx xpk ppm mmcmp
 [modplug] 669 amf ams dbm dfm dsm far it med mdl mod mtm mt2 okt s3m stm ult umx xm
 [mikmod] amf dsm far gdm imf it med mod mtm s3m stm stx ult uni xm
 [fluidsynth] mid
 [gme] ay gbs gym hes kss nsf nsfe rsn sap spc vgm vgz
 [ffmpeg] 16sv 3g2 3gp 4xm 8svx aa3 aac ac3 adx afc aif aifc aiff al alaw amr anim apc ape asf atrac au aud avi avm2 avs bap bfi c93 cak cin cmv cpk daud dct divx dts dv dvd dxa eac3 film flac flc fli fll flx flv g726 gsm gxf iss m1v m2v m2t m2ts m4a m4b m4v mad mj2 mjpeg mjpg mka mkv mlp mm mmf mov mp+ mp1 mp2 mp3 mp4 mpc mpeg mpg mpga mpp mpu mve mvi mxf nc nsv nut nuv oga ogm ogv ogx oma ogg omg opus psp pva qcp qt r3d ra ram rl2 rm rmvb roq rpl rvc shn smk snd sol son spx str swf tak tgi tgq tgv thp ts tsp tta xa xvid uv uv2 vb vid vob voc vp6 vmd wav webm wma wmv wsaud wsvga wv wve rtp:// rtsp:// rtsps://
 [sndfile] wav aiff aif au snd paf iff svx sf voc w64 pvf xi htk caf sd2
 [audiofile] wav au aiff aif
 [pcm]

Filters:
 libsamplerate soxr

Tag plugins:
 id3tag

Output plugins:
 shout null fifo pipe alsa ao openal solaris pipewire pulse jack httpd snapcast recorder

Encoder plugins:
 null vorbis opus lame twolame wave flac shine

Archive plugins:
 [bz2] bz2
 [zzip] zip
 [iso] iso

Input plugins:
 file io_uring archive alsa qobuz curl ffmpeg smbclient nfs mms cdio_paranoia

Playlist plugins:
 extm3u m3u pls xspf asx rss flac cue embcue

Protocols:
 file:// alsa:// cdda:// ftp:// ftps:// gopher:// hls+http:// hls+https:// http:// https:// mms:// mmsh:// mmst:// mmsu:// nfs:// qobuz:// rtmp:// rtmps:// rtmpt:// rtmpts:// rtp:// rtsp:// rtsps:// scp:// sftp:// smb:// srtp://

Other features:
 avahi dbus udisks epoll icu inotify ipv6 systemd tcp un

Sorry, I don't know why Tumbleweed as updated today still is on version 0.23.17; they usually are very quickly on the latest one upon its release. Hope that doesn't matter ...

What I want to achieve:

• mpd is configured to have several alsa outputs through USB ports of the laptop. Amongst them, one USB goes directly into my Chord Hugo 2 DAC, another one goes into the Matrix X-SPDIF 3 USB-to-SPDIF (TOSLINK and coaxial) converter. These outputs work fine when playing - as the most basic use case - local audio files from hard disk.
• I want to be able to input the audio stream from my CD transport (an old AVM CD1 plus) into mpd and play it via one of the above outputs. I am taking the TOSLINK signal from the CD transport into the Optical In of a miniDSP USBStreamer B box, whose USB output in turn sends that stream into the laptop via a third USB port.

I did a test whether this audio path works using Audacity on Tumbleweed. It works perfectly well. It records the audio stream from the USB input described in the second bullet point, and plays it to either of the USB outputs described in the first bullet point.
Plus a far-fetched side remark: I did achieve the same goal with the same hardware on Ubuntu Server running HQPlayer Embedded.

Here are the relevant snippets from my ~/.config/mpd/mpd.conf:

## Two alsa outputs:

audio_output {
	type                "alsa"
	name                "Chord Hugo2 - USB Audio"
	device              "hw:CARD=Hugo2,DEV=0"
	auto_resample       "no"
	auto_format         "no"
	auto_channels       "no"
	dop                 "yes"
	replay_gain_handler "none"
	mixer_type          "none"
	filters             "HDCD"
}

audio_output {
	type                "alsa"
	name                "Matrix X-SPDIF3 Opt (Hugo) - USB Audio"
	device              "hw:CARD=XSPDIF3,DEV=0"
	auto_resample       "no"
	auto_format         "no"
	auto_channels       "no"
	allowed_formats     "44100:24:2 48000:24:2 88200:24:2 96000:24:2 176400:24:2 192000:24:2 dsd64:2=dop"
	dop                 "yes"
	replay_gain_handler "none"
	mixer_type          "none"
	filters             "HDCD"
}

#### general settings for all output devices
resampler {
	plugin              "libsamplerate"
	type                "0"
}
replaygain              "off"

### # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

## One alsa input:

input {
	plugin              "alsa"
	default_device      "hw:CARD=USBStreamer,DEV=0"
	enabled             "yes"
	default_format      "44100:16:2"
	auto_resample       "no"
	auto_format         "no"
	auto_channels       "no"
}

### # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

## Additional settings:

#### filter plugins
filter {
	plugin              "hdcd"
	name                "HDCD"
}

### # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

## The standard stuff:

music_directory         "~/AVs/I/Music"
db_file                 "~/.config/mpd/database"
log_file                "~/.config/mpd/mpd.log"
playlist_directory      "~/.config/mpd/playlists"
pid_file                "~/.config/mpd/pid"
state_file              "~/.config/mpd/state"
sticker_file            "~/.config/mpd/sticker.sql"
save_absolute_paths_in_playlists  "no"

auto_update             "yes"
auto_update_depth       "4096"
filesystem_charset      "UTF-8"
## id3v1_encoding          "UTF-8"
# Suppress all messages below the given threshold.  Use "verbose" for
# troubleshooting. Available setting arguments are
# "default", "notice", "info", "verbose", "warning" and "error":
## log_level               "default"
log_level               "verbose"
## log_level               "info"

metadata_to_use         "+comment"

connection_timeout      "60"
max_connections         "10"
max_playlist_length     "16384"
max_command_list_size   "2048"
max_output_buffer_size  "8192"

### # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

bind_to_address         "0.0.0.0"

zeroconf_enabled        "yes"

I hope this mpd.conf is correct. Otherwise, please comment!

Unfortunately, I did not find a way yet to operate mpd so as to input the CD transport audio stream and output it to the external DAC. Amongst other things, I tried either one of the commands mentioned in the mpd documentation:

$ mpc add alsa://hw:2,0
$ mpc add alsa://hw:2,0?format=44100:16:2

I also tried to add a stream URL in Cantata, which is my standard mpd controller on Tumbleweed.

Nothings works as of now.

Here is my verbose mpd.log:

Jul 02 11:02 : state_file: Loading state file /home/myself/.config/mpd/state
Jul 02 11:02 : inotify: initializing inotify
Jul 02 11:02 : inotify: watching music directory
Jul 02 11:03 : client: [0] opened from 127.0.0.1:40490
Jul 02 11:03 : client: [0] process command "commands"
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [0] process command "status"
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [0] process command "currentsong"
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [1] opened from 127.0.0.1:40492
Jul 02 11:03 : client: [0] process command "playlistinfo "13""
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [0] process command "status"
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [0] process command "currentsong"
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [0] process command "playlistinfo "13""
Jul 02 11:03 : client: [0] command returned 0
Jul 02 11:03 : client: [1] process command "idle playlist player mixer options"
Jul 02 11:03 : client: [1] command returned 1
Jul 02 11:03 : client: [2] opened from 127.0.0.1:60488
Jul 02 11:03 : client: [2] process command list
Jul 02 11:03 : client: process command "status"
Jul 02 11:03 : client: command returned 0
Jul 02 11:03 : client: process command "currentsong"
Jul 02 11:03 : client: command returned 0
Jul 02 11:03 : client: [2] process command list returned 0
Jul 02 11:03 : client: [2] closed

...

Jul 02 11:06 : client: [60] opened from 127.0.0.1:34402
Jul 02 11:06 : client: [60] process command list
Jul 02 11:06 : client: process command "add "alsa://hw:2,0?format=44100:16:2""
Jul 02 11:06 : client: command returned 0
Jul 02 11:06 : client: [60] process command list returned 0
Jul 02 11:06 : client: [60] closed
Jul 02 11:06 : client: [61] opened from 127.0.0.1:34404
Jul 02 11:06 : client: [61] process command "status"
Jul 02 11:06 : client: [61] command returned 0
Jul 02 11:06 : client: [61] process command "currentsong"
Jul 02 11:06 : client: [61] command returned 0
Jul 02 11:06 : client: [61] process command "playlistinfo "13""
Jul 02 11:06 : client: [61] command returned 0
Jul 02 11:06 : client: [61] process command "playlistinfo"
Jul 02 11:06 : client: [61] command returned 0
Jul 02 11:06 : client: [1] process command "idle playlist player mixer options"
Jul 02 11:06 : client: [1] command returned 1
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [62] opened from 127.0.0.1:59222
Jul 02 11:06 : client: [62] process command list
Jul 02 11:06 : client: process command "status"
Jul 02 11:06 : client: command returned 0
Jul 02 11:06 : client: process command "currentsong"
Jul 02 11:06 : client: command returned 0
Jul 02 11:06 : client: [62] process command list returned 0
Jul 02 11:06 : client: [62] closed
Jul 02 11:06 : client: [63] opened from 127.0.0.1:59224
Jul 02 11:06 : client: [63] process command "outputs"
Jul 02 11:06 : client: [63] command returned 0
Jul 02 11:06 : client: [63] closed
Jul 02 11:06 : client: [64] opened from 127.0.0.1:59234
Jul 02 11:06 : client: [64] process command list
Jul 02 11:06 : client: process command "status"
Jul 02 11:06 : client: command returned 0
Jul 02 11:06 : client: process command "currentsong"
Jul 02 11:06 : client: command returned 0
Jul 02 11:06 : client: [64] process command list returned 0
Jul 02 11:06 : client: [64] closed
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "status"
Jul 02 11:06 : client: [5] command returned 0
Jul 02 11:06 : client: [5] process command "currentsong"
Jul 02 11:06 : client: [5] command returned 0

...

Jul 02 11:07 : client: [83] opened from 127.0.0.1:47640
Jul 02 11:07 : client: [83] process command "play"
Jul 02 11:07 : playlist: play 12:"WR/Evans, Bill/Another Time - The Hilversum Concert [HiRes 2017]/DSF64/01 - You're Gonna Hear From Me.dsf"
Jul 02 11:07 : client: [83] command returned 0
Jul 02 11:07 : playlist: queue song 13:"WR/Evans, Bill/Another Time - The Hilversum Concert [HiRes 2017]/DSF64/02 - Very Early.dsf"
Jul 02 11:07 : client: [83] process command list
Jul 02 11:07 : client: process command "status"
Jul 02 11:07 : client: command returned 0
Jul 02 11:07 : client: process command "currentsong"
Jul 02 11:07 : client: command returned 0
Jul 02 11:07 : client: [83] process command list returned 0
Jul 02 11:07 : decoder_thread: probing plugin dsf
Jul 02 11:07 : client: [83] closed
Jul 02 11:07 : decoder: audio_format=dsd64:2, seekable=true
Jul 02 11:07 : client: [84] opened from 127.0.0.1:47648
Jul 02 11:07 : client: [84] process command "status"
Jul 02 11:07 : alsa_output: opened hw:CARD=XSPDIF3,DEV=0 type=HW
Jul 02 11:07 : alsa_output: buffer: size=46..352800 time=260..2000000
Jul 02 11:07 : alsa_output: period: size=23..176400 time=130..1000000
Jul 02 11:07 : alsa_output: default period_time = buffer_time/4 = 500000/4 = 125000
Jul 02 11:07 : alsa_output: format=S32_LE (Signed 32 bit Little Endian)
Jul 02 11:07 : alsa_output: buffer_size=88200 period_size=22050
Jul 02 11:07 : alsa_output: DoP (DSD over PCM) enabled
Jul 02 11:07 : output: opened "Matrix X-SPDIF3 Opt (Hugo) - USB Audio" (alsa) audio_format=dsd64:2
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [84] process command "currentsong"
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [84] process command "playlistinfo "13""
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [84] process command "status"
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [84] process command "currentsong"
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [84] process command "playlistinfo "13""
Jul 02 11:07 : client: [84] command returned 0
Jul 02 11:07 : client: [1] process command "idle playlist player mixer options"
Jul 02 11:07 : client: [1] command returned 1
Jul 02 11:07 : client: [84] process command "status"
Jul 02 11:07 : client: [84] command returned 0
...
Jul 02 11:07 : client: [85] opened from 127.0.0.1:47654
Jul 02 11:07 : client: [85] process command "stop"
Jul 02 11:07 : playlist: stop
Jul 02 11:07 : player: played "WR/Evans, Bill/Another Time - The Hilversum Concert [HiRes 2017]/DSF64/01 - You're Gonna Hear From Me.dsf"
Jul 02 11:07 : output: closed "Matrix X-SPDIF3 Opt (Hugo) - USB Audio" (alsa)

(... denotes cutting repetitive stuff)

In this log, you see the startup sequence first. Then an mpc add alsa://... command is being issued, but no effect. Then, an mpc play
command is being issued, which however lets mpd play from the playlist I had before this sequence. This replay is being ended with an mpc stop.

In summary: I am in need of some education and close guidance. Please help me by providing your hints with respect to the mpd.conf and with respect to the question of how to operate mpd to get the desired audio through.

Thanks a lot in advance.

[111MilesToGo]

I will add the outputs from the aplay and arecord commands for reference:

$ aplay -l
**** Liste der Hardware-Geräte (PLAYBACK) ****
Karte 0: PCH [HDA Intel PCH], Gerät 0: 92HD81B1X5 Analog [92HD81B1X5 Analog]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 1: NVidia [HDA NVidia], Gerät 3: HDMI 0 [DELL E2225HSM]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 1: NVidia [HDA NVidia], Gerät 7: HDMI 1 [HDMI 1]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 1: NVidia [HDA NVidia], Gerät 8: HDMI 2 [HDMI 2]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 1: NVidia [HDA NVidia], Gerät 9: HDMI 3 [HDMI 3]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 2: USBStreamer [USBStreamer], Gerät 0: USB Audio [USB Audio]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 3: Hugo2 [Hugo2], Gerät 0: USB Audio [USB Audio]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 4: XSPDIF3 [X-SPDIF3], Gerät 0: USB Audio [USB Audio]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0

$ arecord -l
**** Liste der Hardware-Geräte (CAPTURE) ****
Karte 0: PCH [HDA Intel PCH], Gerät 0: 92HD81B1X5 Analog [92HD81B1X5 Analog]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0
Karte 2: USBStreamer [USBStreamer], Gerät 0: USB Audio [USB Audio]
  Sub-Geräte: 1/1
  Sub-Gerät #0: subdevice #0

$ arecord -L
null
    Discard all samples (playback) or generate zero samples (capture)
speexrate
    Rate Converter Plugin Using Speex Resampler
pipewire
    PipeWire Sound Server
upmix
    Plugin for channel upmix (4,6,8)
default
    Default ALSA Output (currently PipeWire Media Server)
sysdefault:CARD=PCH
    HDA Intel PCH, 92HD81B1X5 Analog
    Default Audio Device
front:CARD=PCH,DEV=0
    HDA Intel PCH, 92HD81B1X5 Analog
    Front output / input
sysdefault:CARD=USBStreamer
    USBStreamer, USB Audio
    Default Audio Device
front:CARD=USBStreamer,DEV=0
    USBStreamer, USB Audio
    Front output / input

This is in addition to my reporting above that I could prove the audio path by using Audacity and HQPlayer Embedded.

[111MilesToGo]

I did correct the one mistake I could find and removed all local files from the playlist. Then I tried again and ran the following commands:

[myuser ~]$ mpc add alsa://hw:2,0?format=44100:16:2

[myuser ~]$ mpc play
alsa://hw:2,0?format=44100:16:2
[playing] #1/1   0:00/0:00 (0%)
volume: n/a   repeat: off   random: off   single: off   consume: off

[myuser ~]$ ll /proc/asound
insgesamt 0
dr-xr-xr-x  6 root root 0  2. Jul 15:14 card0
dr-xr-xr-x 16 root root 0  2. Jul 15:14 card1
dr-xr-xr-x  9 root root 0  2. Jul 15:14 card2
dr-xr-xr-x  8 root root 0  2. Jul 15:14 card3
dr-xr-xr-x  8 root root 0  2. Jul 15:14 card4
-r--r--r--  1 root root 0  2. Jul 15:16 cards
-r--r--r--  1 root root 0  2. Jul 15:16 devices
lrwxrwxrwx  1 root root 5  2. Jul 15:16 Hugo2 -> card3
-r--r--r--  1 root root 0  2. Jul 15:16 hwdep
-r--r--r--  1 root root 0  2. Jul 15:16 modules
lrwxrwxrwx  1 root root 5  2. Jul 15:16 NVidia -> card1
dr-xr-xr-x  4 root root 0  2. Jul 15:14 oss
lrwxrwxrwx  1 root root 5  2. Jul 15:16 PCH -> card0
-r--r--r--  1 root root 0  2. Jul 15:16 pcm
dr-xr-xr-x  6 root root 0  2. Jul 15:14 seq
-r--r--r--  1 root root 0  2. Jul 15:16 timers
lrwxrwxrwx  1 root root 5  2. Jul 15:16 USBStreamer -> card2
-r--r--r--  1 root root 0  2. Jul 15:16 version
lrwxrwxrwx  1 root root 5  2. Jul 15:16 XSPDIF3 -> card4

[myuser ~]$ cd /proc/asound/card2/pcm0c

[myuser /proc/asound/card2/pcm0c]$ ll
insgesamt 0
-r--r--r-- 1 root root 0  2. Jul 15:16 info
dr-xr-xr-x 7 root root 0  2. Jul 15:14 sub0
-rw-r--r-- 1 root root 0  2. Jul 15:16 xrun_debug

[myuser /proc/asound/card2/pcm0c]$ cd sub0

[myuser /proc/asound/card2/pcm0c/sub0]$ ll
insgesamt 0
-r--r--r-- 1 root root 0  2. Jul 15:16 hw_params
-r--r--r-- 1 root root 0  2. Jul 15:16 info
-r--r--r-- 1 root root 0  2. Jul 15:14 status
-r--r--r-- 1 root root 0  2. Jul 15:16 sw_params
--w------- 1 root root 0  2. Jul 15:16 xrun_injection

[myuser /proc/asound/card2/pcm0c/sub0]$ cat status
closed

[myuser /proc/asound/card2/pcm0c/sub0]$ cat info
card: 2
device: 0
subdevice: 0
stream: CAPTURE
id: USB Audio
name: USB Audio
subname: subdevice #0
class: 0
subclass: 0
subdevices_count: 1
subdevices_avail: 1

[myuser /proc/asound/card2/pcm0c/sub0]$ mpc
volume: n/a   repeat: off   random: off   single: off   consume: off
ERROR: Failed to decode alsa://hw:2,0?format=44100:16:2; Cannot set sample format: Invalid argument

[myuser /proc/asound/card2/pcm0c/sub0]$ 

As can be seen, this failed again.

The latter error message lets me ask what the digital format out of the miniDSP USBStreamer Box at its USB output actually is. The optical TOSLINK input is directly from my CD transport, thus it should be 44100:16:2. How could this be elaborated further? But again referring to my positive HQPlayer Embedded experience, that digital format cannot be far off from the original CD format.

[111MilesToGo]

The latest post above indeed leads to the solution here. The miniDSP USBStreamer B box has only S32_LE as its output format into USB. Therefore, all of the above works now when 44100:32:2 is used as the relevant format string in mpd and mpc, resp.