Querying MIDI Output Devices

Before playing a MIDI file, you should use the midiOutGetDevCaps function to determine the capabilities of the MIDI output device that is present in the system. This function takes an address of a MIDIOUTCAPS structure, which it fills with information about the capabilities of the given device. This information includes the manufacturer and product identifiers, a product name for the device, and the version number of the device driver (specified in the wMid, wPid, szPname, and vDriverVersion members, respectively).

MIDI output devices can be either internal synthesizers or external MIDI output ports. The wTechnology member of the MIDIOUTCAPS structure specifies the technology of the device.

If the device is an internal synthesizer, additional device information is available in the wVoices, wNotes, and wChannelMask members. The wVoices member specifies the number of voices that the device supports. Each voice can have a different sound or timbre. Voices are organized into MIDI channels. The wNotes member specifies the polyphony of the device — that is, the maximum number of notes that can be played simultaneously. The wChannelMask member is a bit representation of the MIDI channels that the device responds to. For example, if the device responds to the first eight MIDI channels, wChannelMask is 0x00FF. If the device is an external output port, wVoices and wNotes are unused, and wChannelMask is set to 0xFFFF.

The dwSupport member of the MIDIOUTCAPS structure indicates whether the device driver supports volume changes, patch caching, and streaming. Volume changes are supported only by internal synthesizer devices. External MIDI output ports do not support volume changes. For information about changing volume, see Changing Internal MIDI Synthesizer Volume.