OPL synth emulation
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ZDoom's OPL emulator replicates an 
OPL3 chip. It is the only software synthesizer offered by ZDoom which does not use sound fonts. While the Doom music, for example, was composed on and for a Roland Sound Canvas, most gamers at the time had a more affordable Sound Blaster card and the OPL emulator can seem "more authentic" (or at least more nostalgic) for them because of this. Doom was one of the relatively rare games which included instructions for the OPL registers (contained in the GENMIDI lump), allowing to tweak the OPL's output to make it as good as it could be despite its inherent limits.
Advantages and drawbacks
- Despite the GENMIDI lump's best efforts, OPL synthesis does not sound like real musical instruments.
- Although the emulation of the OPL chip's inner working is bit-accurate, the output can still differ because it has to be resampled to be mixed with the rest of the audio, and OPL chips used a very nonstandard sample rate of 49716 Hz. Further, many of the cheaper sound cards did not use actual Yamaha OPL chips, but compatible clones that differed slightly. Finally, several sound cards applied additional filters to the output of the OPL chip to achieve various effects.
