Feature Request: Integrate snd-aloop ALSA loopback virtual sound card for system-wide HRIR/CTC virtual surround DSP | Target: CoreELEC 22 NO Latest Nightly Build

Development Feature Requests
1

Hi CoreELEC Development Team and fellow community members,

I submit this feature request to ask for native support of the ALSA loopback kernel module snd-aloop in the official kernel of CoreELEC 22 NO latest nightly builds. This optional module will unlock stable system-level audio routing to run customized HRIR & 2×2 matrix CTC spatial audio processing, namely virtual surround sound on Amlogic playback devices.

Existing Limitations on CoreELEC 22 NO Nightly

The current nightly releases of CoreELEC 22 NO do not compile and ship the snd-aloop kernel module, bringing critical obstacles for advanced audio DSP implementation:

  1. No persistent system-wide virtual audio endpoint is available to capture full PCM audio streams before signals are sent to physical audio hardware.
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  2. Once audio bitstream passthrough is enabled, compressed formats including AC3, DTS and Dolby Atmos are transmitted directly to HDMI hardware, skipping all software DSP pipelines entirely, which renders virtual surround functions useless.
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  3. Current makeshift workarounds like Entware-based custom ALSA configuration and temporary PulseAudio loopback modules are extremely fragile. All custom settings get discarded after system reboot or nightly version upgrade, failing to support long-term stable DSP operation.
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  4. FIFO buffering can only ease minor playback jitter, yet it cannot implement complete audio routing required by fixed-block low-latency DSP workflows.

My Practical DSP Use Case on CoreELEC

I have built a dedicated HRIR-CTC spatial audio solution with fixed standardized parameters:

  • Sample rate: 48kHz, float32 precision
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  • Processing block size: 64 samples
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  • 2048-point FFT for overlap-add convolution
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  • 1024-tap HRIR filter bank
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  • 2×2 matrix CTC (Crosstalk Cancellation) algorithm for loudspeaker virtual surround rendering

This architecture relies strictly on a stable ALSA loopback device: raw PCM audio is routed to the virtual capture endpoint, processed with matrix inversion and frequency-domain convolution, then output to physical HDMI or USB DAC devices. Without native snd-aloop support, this mature virtual surround solution cannot run properly on the latest CoreELEC 22 NO nightly builds.

Exact Technical Requirements

  1. Enable the kernel flag CONFIG_SND_ALOOP in the build configuration for CoreELEC 22 NO nightly releases.
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  2. Compile the snd-aloop.ko module and bundle it into the official system image, allowing users to load the module manually via the modprobe snd-aloop command.
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  3. Preserve all default native functions completely: HDMI bitstream passthrough, hardware video decoding and original audio output will stay unchanged.
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  4. The loopback virtual sound card acts only as an optional feature for advanced users, with zero interference to the out-of-box playback experience for regular users.

Community Benefits

  1. Empower Amlogic CoreELEC devices to support stable, system-wide HRIR/CTC spatial audio and virtual surround solutions.
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  2. Accommodate professional fixed-block low-latency DSP pipelines for audio enthusiasts, replacing unstable third-party hacks.
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  3. Expand CoreELEC’s audio capability beyond pure media playback, supporting audio capture, multi-room streaming and customized audio processing scenarios.
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  4. This is a non-breaking supplementary feature that adds merely one optional kernel module, with no negative impact on system stability, resource overhead or default playback performance.

I fully understand that CoreELEC prioritizes lightweight, reliable media playback. This feature is purely optional and targeted at advanced users without breaking the core design philosophy of the project.

Thanks very much for your time and consideration.

Best regards

2

Currently on CoreELEC 22 NO latest nightly builds, third-party audio loopback workarounds are unable to support professional spatial DSP processing. My fixed low-latency audio algorithm uses 48kHz float32, Block 64, FFT 2048, HRIR 1024, and 2×2 Matrix CTC virtual surround.

Only the native snd-aloop kernel module can provide stable full-system PCM audio routing. FIFO buffer adjustment cannot replace a real virtual sound card, nor can it solve the fundamental audio routing problem.

Enabling snd-aloop is a purely optional, non-breaking feature. It does not affect default HDMI passthrough, hardware decoding or ordinary user experience, but greatly expands CoreELEC’s professional audio processing capability for advanced users.

3

CONFIG_SND_ALOOP doesn’t exist in the 5.15 kernel, which likely means neither does this module.

4

Thanks for the note about the removal of CONFIG_SND_ALOOP in Linux 5.15.
I’ve checked the official linux.aarch64.conf for CoreELEC 22 NO, and the replacement option CONFIG_SND_PCM_LOOPBACK is also missing now.

I sincerely request to enable CONFIG_SND_PCM_LOOPBACK=m as an optional kernel module. It provides necessary ALSA loopback capability for my HRIR/CTC virtual surround DSP workflow. The module will not be loaded by default and will not break any existing core functions for regular users. @YadaYada

5

You’d need to find the module source and compile it with the CE-NO kernel so that you can test and verify it before proposing it be added to CE.

6

Unfortunately I can’t compile it myself on my low-spec PC, and I don’t have the module source code for independent building either. I’m just requesting this feature to be enabled upstream by the project maintainers.

7

What you need? Image with this module snd-aloop?

Or just download this one and insert it.

wget -O snd-aloop.ko https://test.coreelec.org/vpeter/snd-aloop.ko
insmod snd-aloop.ko
lsmod | grep aloop
8

I’ll perform manual testing to adapt it for my in-progress plugin development. If testing confirms full functionality, please merge it into the next nightly build. Thanks.

9

Try first and then we will see.

11 
[virtualsurround] [AUDIO] Failed to configure audiooutput.mode
[virtualsurround] [AUDIO] Failed to configure audiooutput.passthrough
[virtualsurround] [GUARD] DSP exited abnormally, will perform the 1st restart after a 2000ms delay
[virtualsurround] [DSP] stopping (pid=1423)...
[virtualsurround] [DSP] stopped (exit code: 0)
12

Is it mandatory to use the system-provided alsa-lib ? Can I compile a standalone custom version of alsa-lib myself and bundle it within my own plugin package? Has anyone encountered this scenario or knows the answer?