wip: vst3 support and drift correction

CMakeLists.txt

@@ -27,6 +27,11 @@ list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_LIST_DIR}/cmake)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 set(CMAKE_C_STANDARD 11)
+option(SL_BUILD_VST3 "Build the VST3 plug-in" OFF)
+
+if(SL_BUILD_VST3)
+  set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+endif()
 
 if(MSVC)
   add_compile_options("/W3")
@@ -87,22 +92,21 @@ set(STATIC
     CACHE BOOL "")
 
 set(MODULES
-    alsa
     auconv
     auresamp
     vp8
-    coreaudio
     dtls_srtp
     g711
     ice
-    jack
     netroam
     opus
-    portaudio
-    pulse
     turn
     CACHE STRING "")
 
+if(NOT SL_BUILD_VST3)
+    list(APPEND MODULES alsa coreaudio jack pulse portaudio)
+endif()
+
 # Baresip dependencies
 add_subdirectory(external/re EXCLUDE_FROM_ALL)
 add_subdirectory(external/baresip EXCLUDE_FROM_ALL)
@@ -112,6 +116,11 @@ find_package(re CONFIG REQUIRED)
 
 add_compile_definitions(SL_VERSION="${PROJECT_VERSION}")
 
+
 add_subdirectory(libsl)
 add_subdirectory(test)
 add_subdirectory(app/cli)
+
+if(SL_BUILD_VST3)
+  add_subdirectory(app/vst3)
+endif()

Makefile

@@ -159,7 +159,7 @@ third_party/portaudio:
 	$(HIDE)cd third_party && \
 		git clone ${PORTAUDIO_MIRROR}/portaudio.git && \
 	    cd portaudio && \
-		cmake -B build -DBUILD_SHARED_LIBS=0 \
+		cmake -B build -DBUILD_SHARED_LIBS=0 -DPA_USE_SNDIO=OFF \
 		-DPA_USE_WMME=OFF -DPA_USE_DS=OFF -DPA_USE_WDMKS=OFF && \
 		cmake --build build -j && \
 		cp -a build/libportaudio.a ../lib/ && \
@@ -205,6 +205,25 @@ external/baresip:
 		patch -p1 < ../../patches/baresip_portaudio.patch
 
 
+.PHONY: vst3sdk
+vst3sdk: external/vst3sdk
+
+# Only the submodules the plug-in actually needs. VSTGUI is skipped, the
+# plug-in has no editor of its own.
+external/vst3sdk:
+	$(HIDE) [ ! -d external/vst3sdk ] && \
+		git -C external clone -b $(VST3SDK_VERSION) --depth=1 \
+		$(VST3SDK_MIRROR) vst3sdk && \
+		cd external/vst3sdk && \
+		git submodule update --init --depth=1 \
+		base cmake pluginterfaces public.sdk
+
+.PHONY: vst3
+vst3: third_party external external/vst3sdk
+	$(HIDE)[ -d build ] || cmake -B build -GNinja \
+		-DCMAKE_BUILD_TYPE=Release -DSL_BUILD_VST3=ON
+	$(HIDE)cmake --build build -j $(CMAKE_VERBOSE) --target studiolink_vst3
+
 ##############################################################################
 #
 # Tools & Cleanup

app/vst3/CMakeLists.txt

@@ -0,0 +1,137 @@
+#
+# CMakeLists.txt
+#
+# StudioLink VST3 plug-in
+#
+# Copyright (C) 2026 Sebastian Reimers
+#
+
+##############################################################################
+#
+# Project and Versioning
+#
+project(studiolink_vst3 VERSION 26.01.0 LANGUAGES C CXX)
+
+set(SL_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/../..)
+set(vst3sdk_SOURCE_DIR ${SL_ROOT}/external/vst3sdk)
+
+if(NOT EXISTS ${vst3sdk_SOURCE_DIR}/CMakeLists.txt)
+  message(STATUS "VST3 SDK not found in external/vst3sdk"
+                 " - skipping VST3 plug-in (run: make vst3sdk)")
+  return()
+endif()
+
+##############################################################################
+#
+# Compile options
+#
+# The top level CMakeLists.txt adds a set of C only warning flags. They would
+# be rejected when compiling the C++ sources of the SDK, so start from a
+# clean slate here and re-apply them per language below.
+#
+
+set_directory_properties(PROPERTIES COMPILE_OPTIONS "")
+
+set(SL_VST_C_FLAGS
+    -Wall
+    -Wextra
+    -Wmissing-declarations
+    -Wmissing-prototypes
+    -Wold-style-definition
+    -Wshadow
+    -Wstrict-prototypes
+    -Wuninitialized
+    -Wvla)
+
+if(MSVC)
+  set(SL_VST_C_FLAGS "/W3")
+endif()
+
+##############################################################################
+#
+# VST3 SDK
+#
+# Examples, hosting tools and VSTGUI are not needed: StudioLink is operated
+# through its web frontend, so the plug-in ships without an editor.
+#
+
+set(SMTG_ENABLE_VST3_PLUGIN_EXAMPLES OFF CACHE BOOL "" FORCE)
+set(SMTG_ENABLE_VST3_HOSTING_EXAMPLES OFF CACHE BOOL "" FORCE)
+set(SMTG_ENABLE_VSTGUI_SUPPORT OFF CACHE BOOL "" FORCE)
+set(SMTG_CREATE_PLUGIN_LINK OFF CACHE BOOL "" FORCE)
+set(SMTG_RUN_VST_VALIDATOR OFF CACHE BOOL "" FORCE)
+
+list(APPEND CMAKE_MODULE_PATH ${vst3sdk_SOURCE_DIR}/cmake/modules)
+
+add_subdirectory(${vst3sdk_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR}/vst3sdk
+                 EXCLUDE_FROM_ALL)
+
+smtg_enable_vst3_sdk()
+
+##############################################################################
+#
+# Source section
+#
+
+find_package(SampleRate REQUIRED)
+
+# vstsinglecomponenteffect.cpp is not part of the "sdk" library target and
+# has to be compiled into the plug-in, like the SDK samples do.
+set(SRCS
+  ${vst3sdk_SOURCE_DIR}/public.sdk/source/vst/vstsinglecomponenteffect.cpp
+  src/driver.c
+  src/engine.c
+  src/io.c
+  src/entry.cpp
+  src/plugin.cpp
+)
+
+set(HEADERS
+  include/sl_vst.h
+  src/cids.h
+  src/plugin.h
+  src/version.h
+)
+
+##############################################################################
+#
+# Main target
+#
+
+set(target studiolink_vst3)
+
+smtg_add_vst3plugin(${target} PACKAGE_NAME "StudioLink" SOURCES_LIST ${SRCS}
+                    ${HEADERS})
+
+target_include_directories(${target}
+  PRIVATE
+    include
+    ${SAMPLERATE_INCLUDE_DIR}
+)
+
+target_link_libraries(${target}
+  PRIVATE
+    sdk
+    sl
+    baresip
+    re
+    ${SAMPLERATE_LIBRARIES}
+)
+
+target_compile_definitions(${target} PRIVATE ${RE_DEFINITIONS})
+
+target_compile_features(${target} PRIVATE cxx_std_17)
+
+target_compile_options(${target}
+  PRIVATE
+    $<$<COMPILE_LANGUAGE:C>:${SL_VST_C_FLAGS}>
+)
+
+if(SMTG_MAC)
+  smtg_target_set_bundle(${target}
+    BUNDLE_IDENTIFIER "de.studio-link.vst3.studiolink"
+    COMPANY_NAME "Studio Link"
+  )
+elseif(SMTG_WIN)
+  target_sources(${target} PRIVATE resource/studiolink.rc)
+endif()

app/vst3/README.md

@@ -0,0 +1,111 @@
+# StudioLink VST3 Plug-in
+
+Runs StudioLink inside a DAW. The DAW replaces the sound card: the plug-in
+input is sent to the remote peers, the plug-in output carries the mix of all
+remote peers.
+
+```
+  DAW track input  ──►  local StudioLink track  ──►  remote peers (SIP/Opus)
+  DAW track output ◄──  aumix of all remote tracks
+```
+
+The plug-in has no editor. StudioLink is operated through its existing web
+frontend, which the "Open Web UI" parameter launches.
+
+## Build
+
+```bash
+make vst3sdk   # fetch external/vst3sdk (once)
+make vst3
+```
+
+The result is `build/VST3/<config>/StudioLink.vst3`. Copy it to your VST3
+folder (`~/.vst3` on Linux, `~/Library/Audio/Plug-Ins/VST3` on macOS,
+`%COMMONPROGRAMFILES%\VST3` on Windows).
+
+The plug-in is off by default in the normal build; `-DSL_BUILD_VST3=ON`
+enables it.
+
+## How it hooks into libsl
+
+`libsl` picks the audio device driver for the local track from
+`sl_conf()->play.mod` / `src.mod`, which defaults to `portaudio`. The plug-in
+registers a baresip `ausrc`/`auplay` pair named `vst` and points the config at
+it *before* `sl_init()` runs, because the local track chooses its driver
+during `sl_tracks_init()`. From there on everything else in libsl is
+unchanged.
+
+| File | Role |
+| --- | --- |
+| `src/engine.c` | Shared, reference counted StudioLink instance running `re_main()` on its own thread |
+| `src/driver.c` | The `vst` baresip driver and the two ring buffers to the audio thread |
+| `src/io.c` | Per instance channel mapping, format and samplerate conversion |
+| `src/resample.c` | libsamplerate wrapper for non 48 kHz hosts |
+| `src/plugin.cpp` | VST3 `SingleComponentEffect` |
+| `src/entry.cpp` | VST3 module factory |
+
+### Threading
+
+Three threads are involved:
+
+- **DAW audio thread** — `process()` converts, pushes into `ab_in`, drives the
+  20 ms frame exchange with baresip, pulls from `ab_out`. It never allocates
+  and never blocks: the driver lock is taken with `mtx_trylock()` and simply
+  skipped when the StudioLink thread is restarting the driver, in which case
+  `aubuf` zero fills for a few blocks.
+- **StudioLink thread** — `re_main()`, same as the CLI app.
+- **DAW GUI thread** — parameter edits. These are marshalled onto the
+  StudioLink thread through an `mqueue`, since libre objects must only be
+  touched from there. Commands are only sent on actual value changes, so the
+  audio thread stays free of syscalls in the steady state.
+
+This is the same arrangement baresip's own `portaudio` module uses, where the
+read/write handlers are likewise called from a foreign callback thread into
+lock protected `aumix`/`aubuf` objects.
+
+### Clock drift
+
+The DAW clock and the aumix thread's OS clock drift apart just like a sound
+card and the OS clock do. The correction lives in libsl (`libsl/src/drift.c`,
+applied in `driver_write_handler`), because the buffer on that clock boundary
+is `ab_mix` inside libsl either way, so the plug-in inherits it for free.
+
+### Samplerate
+
+StudioLink is fixed at 48 kHz stereo S16LE with 20 ms frames. libre's
+`auresamp` only handles integer ratios, so 44.1 kHz hosts would be rejected —
+`libsamplerate` is used instead (`libsl/src/resample.c`, shared with the
+drift correction), with
+`SRC_SINC_FASTEST` and preallocated state so `src_process()` does not allocate
+on the audio thread. At 48 kHz the resampler degenerates to a copy.
+
+## Known limitations
+
+- **One active instance per host process.** libsl currently supports a single
+  local track (`tracks.c`: *TODO: refactor allow multiple local tracks*) and
+  keeps one global libre loop, HTTP server and track list. The first instance
+  claims the audio bridge; any further instance loads but stays silent. Lifting
+  this needs multi local track support in libsl first.
+- **Stereo in / stereo out only.** Mono input is duplicated to both sides.
+- **32 bit float processing only.** Double precision would only add
+  conversions on the way to 16 bit.
+- **Latency is not reported.** The delay is a network path, so the host cannot
+  compensate it; `getLatencySamples()` returns 0 and the plug-in declares an
+  infinite tail so hosts keep calling `process()` while remote peers are
+  talking.
+- `sl_open_webui()` launches Chromium with fixed flags on non-Windows hosts. A
+  `sl_http_port()` getter in libsl would let the plug-in open the URL with the
+  platform default browser instead.
+- `sl_baresip_init()` calls `setbuf(stdout, NULL)` and `sys_coredump_set(true)`
+  process wide, which is fine for a standalone app but intrusive inside a host
+  process.
+
+## Licensing
+
+This repository is MIT. The Steinberg VST3 SDK is **not** — it is dual
+licensed under GPLv3 or Steinberg's proprietary licence, and it is fetched
+into `external/` rather than vendored. Distributing binaries of this plug-in
+means accepting one of those two, and a GPLv3 build would relicense the
+combined work. Steinberg also requires registering as a VST3 licensee before
+shipping. This is a maintainer decision, not something the build system can
+settle.