Software synthesizer
ZDoom offers a variety of 
software synthesizers for MIDI playback, which make using custom MIDI instruments possible. They can be selected through the MIDI device entry of the sound options menu. In addition to whatever MIDI devices are offered by the operating system used, these choices are available:
MIDI music does not contain raw sound data, instead, it only contains instructions for instruments. The synthesizer creates sound from these instructions, and different synths will produce very different sounds from the same MIDI input. Most synths operate either by using a collection of samples or through frequency modulation; in ZDoom's case, the OPL emulator, the libADLMIDI and the libOPNMIDI are using FM synthesis while the others use collections of samples referred to as sound fonts.
On Windows systems, four synthesizers should work right out of the box: the OPL emulator, the LibADL, the LibOPN, and the Windows-provided wavetable synth. Other synths will require to be set up and configured: ZDoom itself is not provided with any sound font, and furthermore two of the synthesizers are not provided directly with ZDoom and need to be downloaded separately. Please refer to each synth's article for specific instructions on how to install and configure them.
System-wide synthesizers
- Note: information about the Linux or MacOS equivalents would be welcome.
ZDoom can use any synthesizer set up as a MIDI driver. Typically, there will be only one (e.g. "Microsoft GS Wavetable SW Synth" on all Windows systems since DirectMusic was developed), but additional ones can be present. Some global synthesizers that you may try to add to your system as alternatives to the ZDoom synths include:
- BASS MIDI driver, for Windows systems (more information)
- TiMidity++ Windows Synth, for Windows systems (more information)
- Virtual MIDI Synth, for Windows systems
- MIDI Yoke and LoopBe1, for Windows systems, both allow to redirect MIDI output to any MIDI application rather than to a normal driver.