https://github.com/ku6dra/dmsynth

A lightweight MIDI synthesizer for Windows using DirectMusic
https://github.com/ku6dra/dmsynth

directmusic midi windows

Last synced: about 2 months ago
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A lightweight MIDI synthesizer for Windows using DirectMusic

Host: GitHub
URL: https://github.com/ku6dra/dmsynth
Owner: ku6dra
License: mit
Created: 2026-04-15T15:29:43.000Z (2 months ago)
Default Branch: master
Last Pushed: 2026-04-26T16:03:43.000Z (about 2 months ago)
Last Synced: 2026-04-26T17:26:29.793Z (about 2 months ago)
Topics: directmusic, midi, windows
Language: C++
Homepage:
Size: 48.8 KB
Stars: 2
Watchers: 0
Forks: 0
Open Issues: 0
Metadata Files:
- Readme: README.md
- License: LICENSE

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README

          # 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

```bash

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](https://www.tobias-erichsen.de/software/loopmidi.html) works on Windows 10.

On the latest versions of Windows 11, [MIDI 2.0](https://github.com/microsoft/midi) changes may affect virtual port behavior — check compatibility before use.

### Options

| Option | Description |

|---|---|

| `--midi1 ` | MIDI IN 1 device (CH 1 - 16). Index number or partial device name (case-insensitive) |

| `--midi2 ` | MIDI IN 2 device (CH 17 - 32). `-1` to disable |

| `--port, -p ` | Synthesizer port index |

| `--rate, -r ` | Sample rate in Hz (default: 44100). Supported: 11025, 22050, 44100 |

| `--voices ` | Maximum polyphony (default: 128) |

| `--dls ` | 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 ` | 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

```bash

# 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

```mermaid

graph TD

    subgraph Input ["MIDI Input (MidiInput)"]

        M1[MIDI IN 1
Group 1] --> MI

        M2[MIDI IN 2
Group 2] --> MI

        MI["WinMM Wrapper
(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
(dropped)"]

        DS -->|Other| TS

        GS --> TS

        FB --> TS

        TS["Timestamping
(scheduled / immediate)"] --> Buf["IDirectMusicBuffer"]

    end

    subgraph Output ["Synthesis (DirectMusic)"]

        Buf --> Synth["Microsoft Synthesizer
(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

```

- **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](LICENSE).

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