OPL emulation

OPL emulation is a feature of some source ports used for playback of MUS and MIDI music.

MIDI music can be seen as sheet music for the computers, contrarily to formats such as  which render the same on any compliant platform, MIDI renditions depend on their interpretation by the synthesizer and two different platforms will produce very different outputs.

In the nineties, most home PCs were equipped with  cards, which used the Yamaha "FM Operator Type-L" chips ( or ) for MIDI synthesis. For many gamers, this chip's sound remains associated to the games of the era, including Doom and its derivatives, however modern computers generally use instead their operating system's, typically relying on a. The differences in rendering will make it sound "wrong" for people accustomed to the Sound Blaster output.

Note that this is a very subjective domain; OPL rendition of the Doom sound track is not more authentic or accurate. The Doom tracks themselves were composed on a, but id Software tried to make sure they would sound as good as possible on the two most popular gaming sound cards of the time, the Sound Blaster and families, through the GENMIDI and DMXGUS lumps. The variety of available MIDI hardware in the era meant that it was common for games to be issued with several different versions of their soundtracks (Adlib/Sound Blaster, GUS, MT-32, MPU-401, and so on), something which Doom mostly avoided by instead providing these control lumps that would adapt the hardware to the songs. There is, however, one song that has an OPL-specific variant, D_INTRO(A).

OPL emulators
Two different OPL emulators are used in Doom source ports, originally developed for the and  projects respectively. ZDoom uses the MAME-derived emulator, and the DOSBox one is used by Chocolate Doom.