By the way, when I say I wouldn’t support specific features I don’t mean to discourage. The end user, the prospective buyer, is the most important for any hardware, not the developer. Developer support can be a necessary evil to satisfy the end user 
, but if you can already add value to existing hardware even without requiring development of new software for it, I think that’s one of the best scenarios.
For V9990, a superimpose feature has value to the end user even without developer support, and the same applies to an OPL4 with MSX-AUDIO BIOS. (Maybe even better: one with 4 MB RAM and a BIOS which auto-initialises it to the ROM tones on boot.)
Mmm there has been a video9000 (superimpose msx vdp over v9990) where a modification was required in the gfx9000. So I highly doubt that you need to modify msx computers. I dont know what the modification was though, it may be very complex or not, dont know.
When @hit9918 said it was best to build something that could be activated on an emulator, I immediately thought about Video9000... maybe just the superimposition section (unless someone asks), because it's easier (and faster) nowadays to just use a PC to convert graphics for the MSX. So, you coders could start coding right away. And I (or someone else) could start doing GFX9000 and Powergraph upgrade kits that meet Video9000 standards... OR, we can make something different based on your needs.
Btw have you seen maxis' procyon? the final product should be a fpga v9958 with superimpose from your msx (although through HDMI, you may need some converter/upscaler/whatsnot ). It looks somewhat similar to the ideas you have been posting about (although your ideas are a bit more higher spects with the v9990 and all)
I don't think I ever saw this. But sure it's interesting. if he manages to synthesize a V9958 with HDMI output maybe we should adopt it and take it as starting point for anything we wan't on graphics side. Our 15kHz TVs and Monitors won't last as long as our MSXs!
Existing GFX9000 users (and Powergraph ones too) might have their V9990 carts modded in order to have superimposed graphics (in this case, the possibility of using just one monitor to get both images without any switching)
As a software developer, I can’t count on everyone to modify their hardware, or for such modifications to be easily available 5 years from now, and I want to maximise my audience. For the same reason I can not support the MSX-AUDIO BIOS (regardless of whether I personally want to use an abstraction layer).
Consider for example the 256K sample RAM expansion for the Philips Music Module, it used to be relatively popular but I’m sure the majority of Music Modules still has the original 32K (myself included). Also last I heard the SuperSoniqs kit is sold out, besides it requires some soldering skill… (Still, I do support it in VGMPlay because 1. it required no extra effort, and 2. there are VGMs which use it.)
Don't the Music Module 256k sample RAM expansion comes with MSX-Audio BIOS upgrade? I'm sure it does! The layout of the board Supersoniqs sell is almost identical to the one FRS (a.k.a. @sd_snatcher) and MSXPRO made, which included both BIOS and Sample RAM upgrade. I'm not sure about the HX-MU900 (Toshiba's Y8950 cartridge) tough.
p.s. I edited a new paragraph to my last post in case you missed it:
Maybe I wasn’t clear what I meant by multi-VDP engine: I was referring to a game engine which will work on both V9958, V9990 and possibly the V9938 as well, but not simultaneously. For example if your system has a V9938 you get an 8x8 scroll, if your system has a V9958 you get a smooth scroll and if your system has a V9990 you get a dual-layer scroll and animated background tiles. This is my idea for keeping compatibility while offering benefits to owners of new hardware.
No, I've missed it. And I can't see it happening on a cartridge game, only on a disk game (that you can have an different binary for V9990, like PC games do). But I'm no coder, (not a good one on MSX ASM pehaps) and maybe it is possible with a MegaROM cartridge.
The thing is that you got the desire to do something crazy 
Then you didn't understand, because your thoughts always go in another direction.
Probably...
When I talk questions of compatible performance, of turbo cpu port transfer performance,
that is meeeh, but the idea of VDP overclocking you immedeately found interesting
Well.. I found interesting because I never thought seriously about. About the Turbo I already have some drafts... What do you think about a buffer for I/O writes with programmable delay between writes, (I/O reads will wait until buffer is empty tough) and uncontended RAM access? Oh, by the way, FAST SRAM with FAST mapper registers. It won't be faster because you can't turn off Z80 refresh cycle like you do on Z180 and HD64180. And what about copying machine's ROM to some fast SRAM before boot? External bus will preserve original timmings, but CPU and internal memory will be lightning fast. I can't see how it can go faster without the need to redraw the entire machine. (airwires are one of the worse villains to any turbo kit)
That's some ideas I have that I must build a prototype before going on.
grauw my bad I thought the video9000 was like what tvalenca was trying to do 
my mistake
No, that's MY bad because I wasn't clear enough!
@syn Isn’t that what tvalenca said? He also considered the other option, but discarded it because it would require modifications of the MSX.
Yes, that's exactly what I said. But, I have a theory that might work at the cost of a more complex device. At least on computers that don't prevent internal I/O accesses to reach external bus, like on Turbo R.
I think a 4MB opl4 card without YRW801 ROM would be ideal. I mean, a ROM that occupies 384 of the 512 available wave instruments / "tone space" whether you use them or not, feels pretty restrictive. 128 instruments (or 64 in MBwave) may sound like a lot, and while I haven't run into problems yet in MS2, I can see scenarios where that is not enough (like a videogame with many opl4 sound effects, or some electronic dance track with tons of percussion and multiple samples of the same instrument with different filter or adsr settings.. or a combination of both )
And for backwards compatibility, just load the old rom sounds manually. You could even load in a higher quality soundbank (since now you have 4MB) to "improve" the sound in current Moonsound/opl4 software!
I had this same idea a month ago or so, I'm not sure on which post. But I'm glad someone also thinks this way. I'm not sure how YRW801 should be uploaded on the 4MB SRAM, but If it can be done through YMF278B, It's a matter of replace one (or two on Tecnobytes's Shockwave) EPROM by a SRAM (they have almost the same pinout!) And a smaller, with better analog audio section board can be produced.
Don't the Music Module 256k sample RAM expansion comes with MSX-Audio BIOS upgrade? I'm sure it does! The layout of the board Supersoniqs sell is almost identical to the one FRS (a.k.a. @sd_snatcher) and MSXPRO made, which included both BIOS and Sample RAM upgrade. I'm not sure about the HX-MU900 (Toshiba's Y8950 cartridge) tough.
Yes it does, V1.2 though (so it’s already outdated ). Supersoniqs did still have limited supply of HX-MU900 upgrade kits in stock, I got one of those but haven’t installed it yet.
Well, if you have the upgrade board, you just need someone to burn another EPROM and replace the one inside your Music Module.
And I heard something about a second version of this upgrade, using a FlashROM instead an EPROM. That was on Brazilian discussion board, we might have some news eventually...
No, I've missed it. And I can't see it happening on a cartridge game, only on a disk game (that you can have an different binary for V9990, like PC games do). But I'm no coder, (not a good one on MSX ASM pehaps) and maybe it is possible with a MegaROM cartridge.
No reason why there would be a difference between ROM and disk… Regardless of storage medium the concept stays the same. Of course it must be a MegaROM, 32K is not enough for most MSX2 titles, it can barely fit one screen 5 bitmap.
Isn't V9990 code INcompatible with V9958? you need two (or three) graphic "drivers" in your code, one to deal with each videochip. (it's different than having one graphic "driver" that deals with two chips at the same time)
And I didn't said differences between disk and ROM programs. I said you'll probably need to have different graphic codes to V9990 (even pattern modes are stored different on VRAM due do different capabilities)
