So that they are consistent with NES expansions.
For X16, the VERA is more integral to the system and has lower I/O address than YM2151
and YM2151 was once an expansion chip during development.
2 Pulse channels:
- 8 level pulse duty cycle, DAC mode(just ignores duty cycle)/pulse wave mode, 4 bit volume.
- Furnace support PCM playback in pulse channels with duty cycle ignore mode.
Sawtooth:
- nothing but 6 bit volume (8 bit accumulator in technically) and 12 bit frequency (periodic).
VRC6 instrument:
- 6 bit Volume macro for finer sawtooth volume handling, also 3 bit Duty cycle macro for pulse channels.
Duty, PCM mode command and Duty macro affects for pulse channel only.
Standalone OPLs
Sound Blaster Pro 1.0
Earliest Sound Blaster Pro has 2 OPL2s, it supports stereo like successor but with hardpanned OPL2: one for Left output and one for Right output.
PC-FXGA
PC-FX, SuperGrafx on steroids, Released at December 1994 in NEC.
PC-FXGA is PC add-on card version of PC-FX for IBM PC and PC-98. It's released at 1995 with 3D acceleration - it lacks at original PC-FX console.
SAAYM
Modern CMS/Game Blaster Compatible PC ISA sound card with YM2151 addon. Commander X16 will be use its configuration but SAA in X16 is finally replaced to VERA.
Seta 1 + FM Addon
X1-010 and YM3438, Both driven by Z80. Ultra Toukon Densetsu used this configuration.
Natt Akuma