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As it turns out, the video overlay feature described in the article also survived in the Genesis/Mega Drive VDP which is derived from the SMS one, but drops the

As it turns out, the video overlay feature described in the article also survived in the Genesis/Mega Drive VDP which is derived from the SMS one, but drops the TMS9918A modes. This feature was used in the System 18 arcade board which combines System 16B video hardware with the Gen/MD VDP. It may have also been used in the LaserActive for combining VDP output with LaserDisc video, but I'm less sure there.

glhaynes 4y ago
Kinda reminds me of how the Genesis/Mega Drive’s audio incorporated both the new Yamaha FM synth chip and the Master System’s TI programmable sound generator. Covered backwards compatibility but they also could both be used at the same time and mixed. See the “Audio” section here: https://www.copetti.org/writings/consoles/mega-drive-genesis/ https://www.copetti.org/writings/consoles/mega-drive-genesis...
- djmips 4y ago
  It's not quite the same thing because the Yamaha FM synth chip is a completely new architecture whereas you could say the Mega Drive graphics are more evolved from the SMS / 9918.
msk-lywenn 4y ago
Any idea why they didn't use the feature for the 32X ?
- toast0 4y ago
  Without knowing the details, I'd guess that in order to link the VDP, you'd need to connect via pins that weren't available on the cartridge slot or the expansion slot. It would have to have been planned (or at least considered) during the design of the genesis/megadrive.
- mikepavone 4y ago
  It's a bit unclear. Between the cartridge port and the A/V connector you should have access to the signals you need to make it work. If I had to guess, the fact that CSYNC reverses direction is kind of awkward for a device that needs to act as a transparent pass-through for non-32X games.
fredoralive 4y ago
Are you sure, I can’t seem to see any inputs on the pin outs given at for example at https://md.railgun.works/index.php?title=VDP#Pinout https://md.railgun.works/index.php?title=VDP#Pinout - there’s definitely an ROUT, GOUT and BOUT, but no RIN etc. They could be using the digital pixel bus that the Model C uses to get more colours from the VDP than a Megadrive, and merging that with other data streams from other video chips ala Supergrafx??? Its interesting though that the Megadrive VDP has features like the pixel bus and extra VRAM support to let them use the chip in arcade machines without some of the limitations the home system has because of cost.
- mikepavone 4y ago
  To be precise, the SMS and Gen/MD VDP versions of this feature work slightly differently from the TMS9918A version of it. The CSYNC output pin becomes an input and is used to synchronize video generation, but you need external hardware to mix the video together. This is also true of TMS9918A variants that output separate luminance and color difference signals like the TMS9928A. Generally this mixing is done with the analog video signals (RGB in the case of the SMS and Gen/MD, luminance/color difference for the TMS9928A), but you could use the digital pixel bus. The downside to that is you need external CRAM and video DAC. edit: Technically, the TMS9928A doesn't have the CSYNC as input part and just relies on the two VDPs running in lockstep.

glhaynes 4y ago

Kinda reminds me of how the Genesis/Mega Drive’s audio incorporated both the new Yamaha FM synth chip and the Master System’s TI programmable sound generator. Covered backwards compatibility but they also could both be used at the same time and mixed. See the “Audio” section here: https://www.copetti.org/writings/consoles/mega-drive-genesis/ https://www.copetti.org/writings/consoles/mega-drive-genesis...

djmips 4y ago
It's not quite the same thing because the Yamaha FM synth chip is a completely new architecture whereas you could say the Mega Drive graphics are more evolved from the SMS / 9918.

msk-lywenn 4y ago

Any idea why they didn't use the feature for the 32X ?

toast0 4y ago
Without knowing the details, I'd guess that in order to link the VDP, you'd need to connect via pins that weren't available on the cartridge slot or the expansion slot. It would have to have been planned (or at least considered) during the design of the genesis/megadrive.
mikepavone 4y ago
It's a bit unclear. Between the cartridge port and the A/V connector you should have access to the signals you need to make it work. If I had to guess, the fact that CSYNC reverses direction is kind of awkward for a device that needs to act as a transparent pass-through for non-32X games.

fredoralive 4y ago

Are you sure, I can’t seem to see any inputs on the pin outs given at for example at https://md.railgun.works/index.php?title=VDP#Pinout https://md.railgun.works/index.php?title=VDP#Pinout - there’s definitely an ROUT, GOUT and BOUT, but no RIN etc. They could be using the digital pixel bus that the Model C uses to get more colours from the VDP than a Megadrive, and merging that with other data streams from other video chips ala Supergrafx??? Its interesting though that the Megadrive VDP has features like the pixel bus and extra VRAM support to let them use the chip in arcade machines without some of the limitations the home system has because of cost.

mikepavone 4y ago
To be precise, the SMS and Gen/MD VDP versions of this feature work slightly differently from the TMS9918A version of it. The CSYNC output pin becomes an input and is used to synchronize video generation, but you need external hardware to mix the video together. This is also true of TMS9918A variants that output separate luminance and color difference signals like the TMS9928A. Generally this mixing is done with the analog video signals (RGB in the case of the SMS and Gen/MD, luminance/color difference for the TMS9928A), but you could use the digital pixel bus. The downside to that is you need external CRAM and video DAC. edit: Technically, the TMS9928A doesn't have the CSYNC as input part and just relies on the two VDPs running in lockstep.