http://wiki.superfamicom.org/snes/show/BackgroundsWhen bit 6 of $2133 is set, you get a related mode known as Mode 7 EXTBG. In this mode, you get a BG2 with 128 colors, which uses the same tilemap and character data as BG1 but interprets the high bit of the pixel as a priority bit. The priority map is:
Sprites with priority 3
Sprites with priority 2
BG2 pixels with priority 1
Sprites with priority 1
Sprites with priority 0
BG2 pixels with priority 0
Note that the BG1 pixels (if BG1 is enabled) will usually completely obscure the low-priority BG2 pixels.
BG2 uses the Mode 7 scrolling registers ($210d-e) rather than the 'normal' BG2 ones ($210f-10). Subscreen, pseudo-hires, math, and clip windows work as normal; keep in mind OBJ and that you can do things like enable BG1 on main and BG2 on sub if you so desire. Mosaic is somewhat weird, see the section on Mosaic below.
Note that BG1, being a 256-color BG, can do Direct Color mode (in this case, of course, there is no palette value so you're limited to 256 colors instead of 2048). BG2 does not do direct color mode, since it is only 7-bit.
Oh . . . why???Honestly, I think it's trying to play PCM through a FM channel... That's why it's a bad idea to even think about doing PCM through FM =P
That would also explain the crappy PC8801 examples, though you'd think developers would be smart enough to realize the SSG channels (a la YM2149) would work far better for sample playback (even without the 2 or 3 channel look-up output method).
Likewise, using the MD PSG for samples would have been more realistic than the FM channels. (which was basically stated a few posts back)
Too bad Yahama FM chips can't do what Atari's AMY chip could. (actually more primitive than FM in a number of ways -limited to additive synthesis via 64 sine wave oscillators, but only 8 frequency ramps -the rest being amplitude ramps, but Amy could effectively be used as a lossy digital audio decoder with extremely high compression ratio -intelligible speech at 2.4 kbps)
Edit: wait, or could FM chips be used for a sort of sub-band coded audio format by using multiple FM channels (similar to what Amy can do)?
Well, there's also video compression, software rendering (various), impact of various design choices of the system (sacrificing VDP die space for the SMS logic, leaving the VDP pixel bus off the cart and expansion ports, etc), use of FM (or sound engines in general -apart from PCM), among other things. (not to mention what-could-have-happened-if type discussions -especially on the, SVP, 32x, CD, and Saturn- )Didn't you know? It's illegal to have a technical discussion on MD hardware unless it's about how to do sample mixing and compression on the Z80, especially once Kool Kitty is involved.
PCM has been coming up a lot recently though.