No audio output when target sink is a mono Bluetooth HFP/HSP device (e.g. call profile) #298

Environment

• JDSP4Linux version: 2.7.0-50-gb585084 (PipeWire flavor)
• OS: Linux Mint 22.3 (Cinnamon 6.6.9)
• Audio server: PipeWire 1.0.5 + WirePlumber 0.4.17 (via pipewire-pulse)
• Bluetooth headset: Sennheiser MOMENTUM 4
• Bluetooth profile at time of bug: headset-head-unit-msbc (HFP, mono, 16kHz) — this profile is auto-selected by WirePlumber whenever the headset's microphone is used (e.g. during a VoIP call)

Description

When the configured output device is a Bluetooth sink that only exposes a single mono port (playback_MONO, as produced by the BlueZ HFP/HSP headset-head-unit profiles), JDSP4Linux produces no audible output, even though:

• pw-link -l shows a valid connection from the plugin node to the sink:

  jdsp_@PwJamesDspPlugin_JamesDsp:output_FL
    |-> bluez_output.XX_XX_XX_XX_XX_XX.1:playback_MONO
  

• The upstream app (tested with Teams for Linux / Chromium) is confirmed to be sending a live signal to jamesdsp_sink (verified visually via pavucontrol's peak meter, which moves during playback).
• jamesdsp_sink and the Bluetooth sink both report state RUNNING (not suspended).
• Running --relink does not fix it.
• Disabling all DSP processing via --set master_enable=false (passthrough) does not fix it either — so this is not an effects/processing bug, but appears to be in the plugin's own routing/resampling to a mono-only sink.

Steps to reproduce

1. Connect a Bluetooth headset that supports both A2DP and HFP/HSP (e.g. Sennheiser MOMENTUM 4).
2. Set JDSP4Linux's output device to the Bluetooth sink, with AudioOutputUseDefault=true.
3. Start a call in an app that uses the headset's microphone (e.g. Teams for Linux), forcing WirePlumber to switch the Bluetooth profile to headset-head-unit-msbc (mono, 16kHz).
4. Observe: playback stream is active (visible level in pavucontrol / pactl list sink-inputs), pipeline is linked (pw-link -l shows the connection), but no audio is audible on the headset.
5. Confirm the pipeline itself is fine: quit JDSP4Linux entirely. The same app now connects directly to the Bluetooth sink and audio works correctly.
6. Confirm it's not a DSP/effects issue: with JDSP4Linux running, run jamesdsp --set master_enable=false (passthrough) — still no audio.

Expected behavior

Audio should play correctly regardless of whether the target sink exposes stereo (FL/FR) or a single mono port, the same way it does when the Bluetooth device is in A2DP mode (stereo, works fine).

Actual behavior

Silent output whenever the target device is a mono-only Bluetooth sink (HFP/HSP call profile), both with processing enabled and in passthrough mode.

Workaround

Quit JDSP4Linux before starting a call, relaunch afterwards for normal (A2DP) listening.