WinMM MultiPort VSTi is a unique MIDI port router designed for Windows, primarily intended for use with Soundfont MIDI Player 6.1+ but compatible with various VST hosts. Unlike traditional VST instruments, this plugin does not generate audio samples itself. Instead, it routes MIDI data to selected Windows MIDI output ports, which can include both internal and external synthesizers. This approach allows the plugin to leverage the sound generation capabilities of connected devices rather than relying on built-in sound rendering.

The plugin supports multi-port MIDI playback with up to four ports, enabling control of up to 64 separate MIDI channels. Users can assign MIDI channels to multiple ports simultaneously, such as playing channels 1-16 on two different ports while routing other channels to additional ports. It includes features like Even/Odd channel segmentation to distribute channels between two synths, useful for devices with limited polyphony. Custom channel configuration allows flexible channel assignment across ports, enhancing MIDI routing possibilities. Additionally, delay settings help synchronize devices with different latencies, and volume adjustments enable balancing output levels.

WinMM MultiPort VSTi offers two Windows MIDI APIs for each port: the Real Time API, which provides widespread compatibility and low overhead, and the Stream API, which supports future event scheduling for precise timing and latency compensation but requires newer Windows versions. The plugin includes tools for managing ports easily, such as removing synths and resynchronizing streams to maintain timing accuracy. Version 1.1 introduced SysEx reset options and enhanced channel configuration, making it a flexible tool for complex MIDI routing setups across various hardware and software environments.