alsa-project/alsa-ucm-conf issue #588 was opened from mkuhl:
## Summary The ALSA UCM configuration for Focusrite Scarlett 18i20 incorrectly defines 20 capture channels (`Define.DirectCaptureChannels 20`), but the 1st generation hardware only supports 18 channels. This causes the device to fail initialization with UCM profile errors.
## Device Information - **Device**: Focusrite Scarlett 18i20 (1st Generation) - **USB ID**: 1235:800c - **USB Name**: Focusrite Scarlett 18i20 USB - **Driver**: snd_usb_audio
## Channel Configuration (Gen 1) - 8 analog inputs - 8 ADAT inputs - 2 S/PDIF inputs - **Total**: 18 input channels (not 20)
## Error Message ``` spa.alsa: Error in ALSA UCM profile for _ucm0007.hw:USB,0 (HiFi: Direct 2: source): CaptureChannels=20 > avail 18 spa.alsa: Errors in ALSA UCM profile for card Scarlett 18i20 ```
## Affected Files The channel definition is in the main configuration file: `/usr/share/alsa/ucm2/USB-Audio/Focusrite/Scarlett-18i20.conf`
Current (incorrect): ``` Define.DirectCaptureChannels 20 ```
Should be: ``` Define.DirectCaptureChannels 18 ```
This file then includes `Scarlett-18i20-HiFi.conf` which uses the defined value.
## System Information - **Distribution**: Fedora 42 / Aurora-dx - **Kernel**: 6.15.3-200.fc42.x86_64 - **PipeWire**: 1.4.5-1.fc42 - **WirePlumber**: 0.5.8-1.fc42 - **ALSA UCM**: 1.2.14-2.fc42 - **ALSA lib**: 1.2.14-2.fc42
## Steps to Reproduce 1. Connect Focusrite Scarlett 18i20 Gen 1 (USB ID 1235:800c) 2. Start PipeWire/WirePlumber 3. Check device status: `wpctl status | grep Scarlett` 4. Device shows "[ALSA UCM error]" 5. Journal shows channel count mismatch error
## Workaround Users can create a local override: ```bash mkdir -p ~/.local/share/alsa/ucm2/USB-Audio/Focusrite/ cp /usr/share/alsa/ucm2/USB-Audio/Focusrite/Scarlett-18i20.conf \ ~/.local/share/alsa/ucm2/USB-Audio/Focusrite/ cp /usr/share/alsa/ucm2/USB-Audio/Focusrite/Scarlett-18i20-HiFi.conf \ ~/.local/share/alsa/ucm2/USB-Audio/Focusrite/ sed -i 's/Define.DirectCaptureChannels 20/Define.DirectCaptureChannels 18/g' \ ~/.local/share/alsa/ucm2/USB-Audio/Focusrite/Scarlett-18i20.conf ```
## Root Cause Analysis The UCM configuration appears to be written for newer Scarlett 18i20 generations (2nd/3rd/4th Gen) which may have 20 channels due to additional loopback channels. The configuration includes generation detection logic but doesn't properly identify USB ID 1235:800c as a 1st Gen device, causing it to use the default 20-channel configuration.
## Proposed Solution The UCM configuration should detect Gen 1 devices and set the appropriate channel count:
1. Add detection for USB ID 1235:800c as Gen 1 2. Set `DirectCaptureChannels` to 18 for Gen 1 devices 3. Keep 20 channels for newer generations if applicable
## Impact - Device fails to initialize properly - Users cannot use the audio interface without manual workarounds - Affects all Linux distributions using this UCM configuration
## Additional Information - This is a regression - older ALSA versions worked correctly - The issue affects PipeWire/WirePlumber but not JACK when using Pro Audio profile (which bypasses UCM) - Other Gen 1 Scarlett devices (6i6, 8i8) may have similar issues
## References - Device specifications: https://support.focusrite.com/hc/en-gb/articles/206849839-Scarlett-1st-Gen-T...
Issue URL : https://github.com/alsa-project/alsa-ucm-conf/issues/588 Repository URL: https://github.com/alsa-project/alsa-ucm-conf