So after blacklisting and manually loading modules I confirm that there is an issue with the sound.
The snd_audio_usb is the module that is working with my USB DAC, and only this module works. But contrary to the previous version on Wheezy kernel, I can notice a very clear degradation in sound quality. I believe that there is some sort of resource cross contamination with another device on the USB bus. Likely the WiFi module. I have tried unloading the WiFi module and will need some further tests.
I have also noticed that in many cases, although I get the snd_usb_audio module to load properly and address the good device (the USB DAC) on squeezelite startup, I get no sound output. Logs show that the device is prperlly initialized, that the audio stream is properly processed, but I get no sound. To solve this I had to load the kernel without any sound module (blacklisting bcm2835, hiface and usb_audio ones) then load manually only snd_usb_audio by modprobe, in order to restore the sound output. This is quite weird. There seems to be an issue with the order in which modules are loaded, one other module restricting or grabbing the sound output.
I have also noticed that on the new distribution, the /etc/modprobe.d/alsa-base.conf has only one line, whereas on the previous version based on wheezy I had many lines.
New version
options snd-rpi-iqaudio-dac index=-2
Old version
# autoloader aliases
install sound-slot-0 /sbin/modprobe snd-card-0
install sound-slot-1 /sbin/modprobe snd-card-1
install sound-slot-2 /sbin/modprobe snd-card-2
install sound-slot-3 /sbin/modprobe snd-card-3
install sound-slot-4 /sbin/modprobe snd-card-4
install sound-slot-5 /sbin/modprobe snd-card-5
install sound-slot-6 /sbin/modprobe snd-card-6
install sound-slot-7 /sbin/modprobe snd-card-7
# Cause optional modules to be loaded above generic modules
install snd /sbin/modprobe –ignore-install snd && { /sbin/modprobe –quiet snd-ioctl32 ; /sbin/modprobe –quiet snd-seq ; : ; }
install snd-rawmidi /sbin/modprobe –ignore-install snd-rawmidi && { /sbin/modprobe –quiet snd-seq-midi ; : ; }
install snd-emu10k1 /sbin/modprobe –ignore-install snd-emu10k1 && { /sbin/modprobe –quiet snd-emu10k1-synth ; : ; }
# Keep snd-pcsp from beeing loaded as first soundcard
options snd-pcsp index=-2
# Keep snd-usb-audio from beeing loaded as first soundcard
options snd-usb-audio index=-2
# Prevent abnormal drivers from grabbing index 0
options bt87x index=-2
options cx88_alsa index=-2
options snd-atiixp-modem index=-2
options snd-intel8x0m index=-2
options snd-via82xx-modem index=-2
options snd-rpi-iqaudio-dac index=-2
I will start again using a fresh image to check more precisely what is happening (and avoid upgrading the kernel silly me!).
The good thing is that I get to learn a lot on Linux base behavior 😉 This is all very interesting… And I’ve also ordered some I2S sound cards as it’s true that USB is not the best way to utilize the Rpi for sound.
