⚙ Mcaster1AudioPipe

Mcaster1AudioPipe is a CoreAudio HAL AudioServerPlugin that creates virtual audio devices on macOS. It provides the critical audio routing layer in the Mcaster1 broadcast stack, enabling multiple Mcaster1AMP player instances to output to numbered pipe devices (Mcaster1AudioPipe 1, Mcaster1AudioPipe 2, Mcaster1AudioPipe 3, etc.) which the Mcaster1DSPEncoder captures as input devices for broadcast encoding.

Mcaster1AudioPipe is implemented as a CoreAudio HAL AudioServerPlugin — a userspace driver that runs inside the coreaudiod daemon process. Unlike legacy kernel extensions (kexts), AudioServerPlugins do not require SIP disabling, are compatible with Apple Silicon, and survive macOS updates gracefully.

Object Hierarchy

Ring Buffer Design

Each virtual device contains a lock-free single-producer single-consumer (SPSC) ring buffer that bridges the output and input streams. The output stream’s IO callback writes audio frames into the buffer, and the input stream’s IO callback reads them out. Because CoreAudio IO callbacks execute on real-time threads, the ring buffer uses atomic load/store operations with acquire/release memory ordering — no mutexes, no system calls, no priority inversion.

Mcaster1AudioPipe sits at the center of the Mcaster1 broadcast chain, providing the zero-latency bridge between media playback and broadcast encoding. Each AMP instance selects a numbered pipe device as its audio output; the DSP Encoder captures from the corresponding pipe device as an input source.

Full Broadcast Signal Path

Key Source Files

Architecture — ARM64 vs Intel

The HAL driver is loaded in-process by coreaudiod, which runs natively — arm64 on Apple Silicon, x86_64 on Intel. Rosetta 2 does not apply. The driver binary must contain a matching architecture slice or coreaudiod will silently fail to load it.

# Dev build — native host arch (auto-detected via uname -m, fast)
cmake -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build

# Force a specific arch
cmake -B build -DCMAKE_OSX_ARCHITECTURES="arm64"    # Apple Silicon only
cmake -B build -DCMAKE_OSX_ARCHITECTURES="x86_64"   # Intel only

# Universal binary — arm64 + x86_64 in one bundle (required for distribution)
cmake -B build -DMACOS_UNIVERSAL=ON
cmake --build build

# Verify what was built
lipo -archs build/Mcaster1AudioPipe.driver/Contents/MacOS/Mcaster1AudioPipe
# → arm64          (native dev build on Apple Silicon)
# → x86_64         (native dev build on Intel)
# → arm64 x86_64   (universal / distribution build)

Build the App & Driver

CMake builds both the Qt6 GUI app and the CoreAudio HAL .driver bundle in a single step.

# Configure + build (GUI app + HAL driver)
cmake -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build -- -j$(sysctl -n hw.ncpu)

# Output:
#   build/Mcaster1AudioPipe.app               (Qt6 GUI)
#   build/Mcaster1AudioPipe.driver            (CoreAudio HAL plugin)
#     Contents/MacOS/Mcaster1AudioPipe        (Mach-O — arm64, x86_64, or universal)
#     Contents/Info.plist

Install

# Copy to HAL plugins directory and restart coreaudiod
sudo bash scripts/install.sh

# What it does:
#   1. Detects host architecture (uname -m) and validates driver binary (lipo -archs)
#   2. Errors with rebuild instructions if arch mismatch detected
#   3. Copies Mcaster1AudioPipe.driver to /Library/Audio/Plug-Ins/HAL/
#   4. Sets ownership to root:wheel
#   5. Restarts coreaudiod (sudo launchctl kickstart -kp system/com.apple.audio.coreaudiod)
#   6. Devices appear in Audio MIDI Setup within seconds

Uninstall

# Remove the driver and restart coreaudiod
sudo bash scripts/uninstall.sh

# What it does:
#   1. Removes /Library/Audio/Plug-Ins/HAL/Mcaster1AudioPipe.driver
#   2. Restarts coreaudiod to unload the plugin
#   3. Pipe devices disappear from the system

Audio Driver Management

The Mcaster1AudioPipe control interface provides a tabbed view for managing drivers and audio services:

Verify Installation

# List HAL plugins
ls /Library/Audio/Plug-Ins/HAL/

# Check that devices are registered
system_profiler SPAudioDataType | grep -i "Mcaster1"

# Or open Audio MIDI Setup.app and look for Mcaster1AudioPipe devices

Mcaster1AudioPipe is primarily a macOS project targeting the CoreAudio HAL AudioServerPlugin API. Future cross-platform support is planned for Windows and Linux.

Mcaster1AudioPipe is one component in the Mcaster1 broadcast stack. Each component is a standalone project that integrates with the others to form a complete internet radio and broadcast automation platform.

Integration Flow

AudioServerPlugInDriverInterface Callbacks

The plugin implements the full AudioServerPlugInDriverInterface vtable. These are the 26 callbacks that CoreAudio invokes on the driver, grouped by function.

Ring Buffer Specification

Supported Audio Formats

Object ID Allocation

CoreAudio requires each object (plugin, device, stream, control) to have a unique AudioObjectID. Mcaster1AudioPipe uses a static allocation scheme:

// Object ID layout per device (N = device index, starting at 0)
//
// Plugin object:     kAudioObjectPlugInObject (always 1)
// Device N:          base + (N * 4)
// Output stream N:   base + (N * 4) + 1
// Input stream N:    base + (N * 4) + 2
// Volume control N:  base + (N * 4) + 3
//
// Example for 3 devices (base = 100):
//   Device 1:  100, 101 (out), 102 (in), 103 (vol)
//   Device 2:  104, 105 (out), 106 (in), 107 (vol)
//   Device 3:  108, 109 (out), 110 (in), 111 (vol)

Mcaster1AudioPipe has undergone a full SAST (Static Application Security Testing) audit with 38 findings identified and 28 patched. Security hardening spans the driver, IPC layer, GUI, and build system.

Development phases for Mcaster1AudioPipe: