DMSynth

A lightweight MIDI synthesizer proxy for Windows 10/11 that drives the native DirectMusic software synthesizer (Microsoft Synthesizer) via COM.

An upgraded Microsoft GS Wavetable Synth, in your hands.

Features

• Native DirectMusic — COM interfaces redeclared from the original GUIDs and vtable layouts; builds with stock MSVC.

• DLS instrument loading — Loads GM instruments from gm.dls (system default or user-specified) at startup.

• GS compatibility — Translates incoming system messages:

  Incoming	Translated to
  GM System On (F0 7E 7F 09 01 F7) or GM2 System On (F0 7E 7F 09 03 F7)	GS Reset
  GM System Off	GS Reset
  XG System On (F0 43 10 4C ...)	GS Reset
  XG Reverb / Chorus type changes	Similar GS Reverb / Chorus macro

• Capital Tone Fallback — When a requested bank/patch is missing from the DLS, the closest available instrument is substituted by walking up the GS tone-map hierarchy (sub-capital → capital → any bank → first available). Drum kits fall back along the program number instead of the bank MSB.

• Drum Note Off workaround — Tracks GS "Use for Rhythm Part" SysEx; ignores Note Off on drum channels to prevent voices from being cut off prematurely if a Note Off message follows a Note On too closely.

• 32-channel MIDI — Two MIDI inputs map to Channel Group 1 (CH 1 - 16) and Group 2 (CH 17 - 32).

• Scheduled playback — Outgoing MIDI is timestamped against DirectMusic's latency clock to absorb callback jitter.

• Low-latency audio — DirectSound output with MMCSS thread boosting on MIDI input.

Comparison

Feature	MSGS	DMSynth	VirtualMIDISynth
Core Engine	DirectMusic	DirectMusic (Native COM)	BASSMIDI
Sound Format	DLS (gm.dls only)	DLS	SF2 / SFZ
Sample Rate	22,050 Hz	44,100 Hz (*)	48,000 Hz (*)
Voices	32	128 (1–1000 *)	16–100,000 (*)
Latency	150 ms	Dynamic: Avg. 46 ms (Min. 30 ms)	0 ms (*)
Capital Tone Fallback	No	Yes	Yes
Channels	16	32 (2 groups, shared engine)	64 (4 independent groups)

* Configurable

Requirements

• Windows 10 / 11 (x64 or x86)
• CMake 3.15+
• MSVC with C++17 support (Visual Studio 2019 or later recommended)

Microsoft DirectX SDK (August 2007) is not required to build this project.

Building

mkdir build && cd build
cmake ..
cmake --build . --config Release

The binary is produced at build/Release/dm_synth.exe.

A build.bat script is also provided that builds both x64 and x86 in one step.

Usage

dm_synth.exe [options]

Connect a MIDI device, launch the executable, and select your devices from the on-screen prompts. All prompts can be bypassed using command-line arguments for headless operation.

To use DMSynth as a drop-in replacement for the Microsoft GS Wavetable Synth, a virtual MIDI port is required to route MIDI from your application to DMSynth's input. loopMIDI works on Windows 10. On the latest versions of Windows 11, MIDI 2.0 changes may affect virtual port behavior — check compatibility before use.

Options

Option	Description
--midi1 <idx|name>	MIDI IN 1 device (CH 1 - 16). Index number or partial device name (case-insensitive)
--midi2 <idx|name>	MIDI IN 2 device (CH 17 - 32). -1 to disable
--port, -p <idx>	Synthesizer port index
--rate, -r <hz>	Sample rate in Hz (default: 44100). Supported: 11025, 22050, 44100
--voices <n>	Maximum polyphony (default: 128)
--dls <path>	Path to a DLS file (default: system gm.dls)
--verbose, -v	Enable MIDI event logging on startup
--immediate	Bypass timestamp scheduling and send events for immediate playback. Use when running alongside software like DirectMusic Producer, which can throttle DirectSound and cause scheduled playback to run slow
--no-reverb	Disable port-level reverb effect
--no-chorus	Disable port-level chorus effect
--no-delay	Disable port-level delay effect
--gap-ns <ns>	Minimum gap between events in nanoseconds (default: 100). Higher values roughly simulate MIDI cable bandwidth (e.g., 960,000 ns for a Note On at 31250 bps).
--list	List available devices/ports and exit
--help, -h	Show help and exit

Runtime keys

Key	Action
R	Reset all MIDI state (All Sound Off + controller reset on every channel)
V	Toggle MIDI event log display
Ctrl+C	Quit

Examples

# List available devices
dm_synth.exe --list

# Single MIDI input, no second port
dm_synth.exe --midi1 0 --midi2 -1 --rate 44100

# Select by partial device name
dm_synth.exe --midi1 "Loopback (A)" --midi2 nanoKEY -v

# Custom DLS sound set
dm_synth.exe --dls "C:\soundfonts\my_gm.dls"

Architecture

graph TD
    subgraph Input ["MIDI Input (MidiInput)"]
        M1[MIDI IN 1<br/>Group 1] --> MI
        M2[MIDI IN 2<br/>Group 2] --> MI
        MI["WinMM Wrapper<br/>(MMCSS Priority Boost)"]
    end

    subgraph Logic ["Processing (DmSynth)"]
        MI --> DS["Message Dispatcher"]
        DS -->|SysEx| GS["GS/XG Translation"]
        DS -->|Program Change| FB["Capital Tone Fallback"]
        DS -->|Note Off, Drum Ch| NF["Note Off Filter<br/>(dropped)"]
        DS -->|Other| TS
        GS --> TS
        FB --> TS
        TS["Timestamping<br/>(scheduled / immediate)"] --> Buf["IDirectMusicBuffer"]
    end

    subgraph Output ["Synthesis (DirectMusic)"]
        Buf --> Synth["Microsoft Synthesizer<br/>(IDirectMusicPort)"]
        LC["Latency Clock"] -.->|reference time| TS
        Synth --> DirectSound["DirectSound"]
        DirectSound --> AudioOutput((Audio))
    end

    subgraph UI ["Logging (Main Loop)"]
        Logic -- "SPSC Ring Buffer" --> Log["Screen Display"]
    end

Loading

• MidiInput — WinMM wrapper with MMCSS priority boost and lock-free SysEx buffering.
• DmSynth — Processes MIDI messages including GS/XG translation, Capital Tone Fallback, and Note Off filtering. In scheduled mode each event is timestamped against the port's latency clock (auto-tuned at runtime) to absorb callback jitter; --immediate bypasses timestamping and sends events for immediate playback.
• Main loop — Manages log event draining via an SPSC ring buffer to ensure the high-priority MIDI thread remains non-blocking for real-time performance.

License

Licensed under the MIT License.