First post, by Great Hierophant
- Rank
- l33t
The OPL Passthrough patch serves a very useful purpose. DOSBox's emulation of the Yamaha OPL series is very good but not perfect. The produced sound is also very much idealized, lacking the noise and warmth of the original chip as output through a Sound Blaster's mixer and amplifier.
Thus we have the OPL Passthrough patch, which passes port writes to I/O registers 2x0-2x3, 2x8/9 and 388-391 to the real hardware connected to an ISA bus. These registers correspond to the location of the CMS-301/SAA-1099, FM-1312/YM-3812 OPL2 or YMF-262 OPL3. Well this is all fine for a Sound Blaster 1.0, 1.5 or 2.0, it is insufficient for a Sound Blaster Pro or more advanced cards. These cards passed the analog OPL3 output through the CT-1345 or CT-1745 mixer chip. These mixer chips could filter each output (CD, PC Speaker, Voice and FM) and control the volume of each source. I believe it would be best if all sound would come out of one device, rather than have the card output only bare FM while the emulator handled the digitzed sound and CD sound.
