Technical Insights: Hardware Negotiation Failure with Multi-channel DACs (Audient iD4 MKII) in Bit-Perfect Mode

[Gordhen]

Hello! I am a user of Sone (0.12.0-1 AUR) on CachyOS and I’ve been investigating some hardware-specific hurdles when using professional audio interfaces (specifically the Audient iD4 MKII) in Bit-Perfect/Exclusive mode.

I’ve spent some time debugging this behavior with the assistance of Gemini (AI), and I wanted to share these findings to provide more context for future developments regarding ALSA backend stability, as it seems closely related to Issue #49 and Issue #27.

While Sone correctly identifies the supported sample rates and formats (detecting $S32_LE$ correctly in the logs), the playback fails during the hardware parameter negotiation phase.The Audient iD4 MKII (and similar "Class Compliant" pro-interfaces) is strictly a 4-channel device at the hardware level in Linux. When an application attempts to open a direct hardware stream (hw:X,Y) requesting only 2 channels (Stereo), the ALSA driver rejects the request with the following error:snd_pcm_hw_params_set_channels error (Invalid argument).

Observed behavior in logs:

[alsa-writer] DAC supported GStreamer formats: ["S32LE"]
[alsa-writer] DAC supported rates: [44100, 48000, 88200, 96000]
[audio] DAC supported formats: [S32LE (32-bit)]
...
(Playback fails to initialize or remains silent if the channel count doesn't match the hardware's 4-channel requirement)

Connection to Existing Issues

Issue #49: The "ALSA writer" seems to struggle when the physical device expects a specific channel layout that differs from the source stream.

Issue #27: This likely stems from the same "strict handshake" where the backend doesn't account for interfaces that require channel padding (sending 2 channels of audio + 2 channels of silence to satisfy a 4-channel DAC).

To bypass this without modifying the application code, I had to manually "tunnel" the audio through a virtual ALSA device using ~/.asoundrc. This configuration performs the necessary channel mapping and format conversion to satisfy the Audient’s hardware requirements:

pcm.audient_exclusive {
    type plug
    slave {
        pcm "hw:iD4,0"   # Direct hardware access
        channels 4       # Forces the 4 channels the DAC requires
        format S32_LE    # Forces the bit-depth the DAC requires
    }
    # Maps Stereo (0,1) to the interface's main outputs (0,1)
    ttable.0.0 1
    ttable.1.1 1
}

By pointing the system's default ALSA output to this "route/plug" device, Sone is able to play Hi-Res audio (verified at 96kHz via hw_params) because ALSA handles the "padding" that the hardware demands.

I truly appreciate all the work put into this project. Unfortunately, I am not a particularly skilled user with these types of technical audio configurations, so I relied on Gemini (AI) to help orient me and condense hours of trial and error into this issue. My hope is that this information might be helpful or at least shed some light for others in a similar situation.

The AI suggests that the ALSA/GStreamer sink might need to handle channel-count discrepancies more elastically (e.g., via automatic channel padding to match the device's native channel count) to support this type of hardware natively in the future.

Best regards!

[Gordhen]

In addition to the ALSA mapping, I discovered that PipeWire often locks the hardware clock to a fixed rate (usually 48kHz). For Sone to successfully switch the DAC to higher sample rates (e.g., 96kHz) in Exclusive Mode without interference, I had to allow multiple rates in the PipeWire configuration:

# ~/.config/pipewire/pipewire.conf.d/allowed-rates.conf
context.properties = {
    default.clock.allowed-rates = [ 44100 48000 88200 96000 ]
}

[lullabyX]

Thanks for taking the time to go through it. I'll go through the edge cases once I get some bandwidth.

[maximbaier]

I wanted to add that i am encountering the same issue on my Scarlett 18i20 as it likely affects all DACs with >2 output channels and would like to express my interest in PR #70 to be merged (:

[lullabyX]

v0.19.0 release adds support for multi channel. It should in theory work, I couldn't test it tho. Would be nice if you guys could test it out and let me know.
@Gordhen @maximbaier

[maximbaier]

As far as i can tell it works on my hardware.

Thank you so much for taking the time and implementing a fix (:

[Marko-M]

v0.19.0 works perfectly for my Neumann MT 48. Previously I had to patch Sone manually to detect ALSA exposing my MT48 as a fixed multichannel endpoint, to then open the hardware at its native multichannel count, and explicitly map stereo L/R into the first two hardware channels while zero-filling the rest. Great job, thanks!

[iliverez]

@lullabyX thanks for the fix! Also works for me with Focusrite Scarlet 2i4 2nd Gen, both in simple and bit perfect mode. There is a small strange behavior with the channel selection, e.g. I choose output channels pair 1-2 and then both pairs 1-2 and 3-4 look selected. If I open the output selection menu again, I see both pairs highlighted in purple and I can only select both pairs. If I change selected sound card, e.g. laptop internal, I then can select an individual pair on the scarlet, but after that it turns again to double pair in the selected output and in the selection menu.
I guess my pull request for #60 is not needed anymore.

[Gordhen]

I was driving myself crazy assuming that since I had the app via AUR, I would have the latest version, until I decided to check and realized I was still on v0.18.0.

Flathub has version v0.19.0 available, and I can confirm that both exclusive mode and bit-perfect mode are working. Thank you very much for all the effort and work you've put into SONE, lullabyX!