The YM2413, a.k.a. OPLL, is a cost-reduced FM synthesis sound chip manufactured by Yamaha Corporation and based on their YM3812 (OPL2).
Yamaha YM2413
The OPL series are a family of sound chips developed by Yamaha. The OPL series are low-cost sound chips providing FM synthesis for use in computing, music and video game applications.
Yamaha YM3812 (OPL2 chip)
Creative Labs and other companies often hid the original Yamaha labels
Decapsulated YM3812, showing the die surface
OPL3 waveforms