The synthesizer is responsible for converting MIDI messages to waveform data and streaming this to the wave output device. Although DirectMusic comes with its own software synthesizer, it allows the implementation of custom synthesizers. It also allows output to be directed to different devices. Information on these topics in contained in the DirectX Device Driver Kit (DDK).
A synthesizer implemented by an application is represented by the IDirectMusicSynth interface. Most applications do not use this interface, and it is not documented in the DirectX Programmer's Reference.
The wave stream to which the synthesizer is sending data—for example, DirectSound or Win32 waveform audio—is represented by an IDirectMusicSynthSink interface. Most applications do not use this interface, and it is not documented in the DirectX Programmer's Reference.