Here's some more technical information about the OPL2, including some images of the decapped chip: https://yehar.com/blog/?p=665
The 4-operator FM synths that Yamaha produced from the DX100 (OPP/YM2164) onwards get that characteristic 'lo-fi' sound from the length of the log-sin table. The 4-operator chips used 256 samples for the quarter log-sin wave, whereas the DX7 had 1024 samples. As you increase the length of the sine table, the timbre gets much 'smoother'.
The 4-operator FM synths that Yamaha produced from the DX100 (OPP/YM2164) onwards get that characteristic 'lo-fi' sound from the length of the log-sin table. The 4-operator chips used 256 samples for the quarter log-sin wave, whereas the DX7 had 1024 samples. As you increase the length of the sine table, the timbre gets much 'smoother'.
See also: https://www.righto.com/2021/11/reverse-engineering-yamaha-dx...