[PROBLEM] ALSA Support for Soundcards with multiple output interfaces

[Stampede]

Hi Tim,

First of all, thanks for the great software and the support that is provided here in the forum. This is some sort of bug / problem report and some request how to improve the situation.

The hardware setup is a RPI Compute Module 4 on a custom board with an integrated USB soundcard (DIY, i can share more details in case of interest). This USB soundcard features multiple input and output interfaces:

Output:
8 channel speaker interface (directly routed to a set of amplifiers) 
2 channel headphone interface
2 channel SPDIF interface

Input:
8 channel input ADC (line in)
2 channel input ADC (microphone / line)
2 channel SPDIF

Code:

picpl@moOde-Picpl:~ $ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: Audio [USB2.0 PICPLAYER Audio], device 0: USB Audio [USB Audio]
 Subdevices: 0/1
 Subdevice #0: subdevice #0
card 0: Audio [USB2.0 PICPLAYER Audio], device 1: USB Audio [USB Audio #1]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 0: Audio [USB2.0 PICPLAYER Audio], device 2: USB Audio [USB Audio #2]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 0: Audio [USB2.0 PICPLAYER Audio], device 3: USB Audio [USB Audio #3]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 1: vc4hdmi0 [vc4-hdmi-0], device 0: MAI PCM i2s-hifi-0 [MAI PCM i2s-hifi-0]
 Subdevices: 1/1
 Subdevice #0: subdevice #0
card 2: vc4hdmi1 [vc4-hdmi-1], device 0: MAI PCM i2s-hifi-0 [MAI PCM i2s-hifi-0]
 Subdevices: 1/1
 Subdevice #0: subdevice #0

The USB soundcard is correctly detected in MoOde as card 0: USB2.0 Audio Interface. MoOde selects the first interface to playback the music, which works perfect.
I think it would make a lot of sense if the user was able to select the USB output interface that should be used. Would it be possible to add this option to MoOde ? I guess this is interesting for all those user who may use an USB based DAC or sound card.

Most USB soundcards support an hardware-based volume control, which I would like to use. In the case of my card, each input or output has it's own volume control.
I dont know what, but amixer returns the available controle in an arbitrary manner. In consquence, when I switch to "hardware volume control" in MoOde, the headphone volume control is used, as it's the first one to appear in the list. Would it be possible to specifically add an option to choose the mixer that it to be used?

Code:

picpl@moOde-Picpl:~ $ amixer scontrols
Simple mixer control 'Headphone',0
Simple mixer control 'Speaker',0
Simple mixer control '2Ch-Line In1',0
Simple mixer control '2Ch-Mic In',0
Simple mixer control 'ASRC-Mux Capture Source',0
Simple mixer control 'SPDIF Out',0
Simple mixer control 'SPDIF-Mux Capture Source',0

Cheers,
Stampede

[Tim Curtis]

The vast majority of USB DAC's used with moode are stereo and have only one hardware volume controller. The amixer scontrol output for these DAC's lists the hardware mixer on the first line of output and so this is what moode uses.

In your case with a custom multi I-O audio device you could prolly use CamillaDSP to achieve what you want but making a custom WebUI on top of the Camilla configs for ease of use would be a significant amount of work

[Stampede]

Hi Tim,

thanks for the reply. I was not thinking about anything complicated. Just one drop down menu in the settings that can select the mixer, so it doesn't choose the first entry returned from amixer.

br, Stampede

[Tim Curtis]

On the surface it sounds simple but there are a lot of moving parts in the audio configuration mechanism. I can't see investigating a change like this unless there were lots of devices that needed it.

Your best bet if you are a dev yourself is to make a patch that provides what you need.