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Hello, I've been modding SMS2 (PAL) units for RGB SCART output by tapping the Sony CXA video amp chip for RGB and using composite video as sync. The picture looks great on my CRT.

Recently I thought I'd try a different method, an "RGB bypass", where you tap the RGB signal from the VDP chip and then use a THS7316 as a video amp. I then tap csync (not composite) from the CXA and use that as sync. Technically a full bypass would tap csync from the VDP, but then I'd have to use another THS7316 to amp the signal which I can't be bothered to do. The picture also looks very nice and clean on my CRT.

When I have more time I want to compare how they look through the OSSC, as that picks up issues with jailbars, sync etc better.

So using my CRT, I'm trying to decide whether the RGB bypass looks any different/better than just tapping RGB from the Sony CXA chip.

I noticed that the Alex Kidd title screen does indeed look slightly different:
The output from the CXA has plain red text for "in miracle world" and plain blue text for "sega". Using the RGB bypass method, both of these bits of text now have small bits of black inside and round some of the edges. I have attached pictures showing the differences.

It seems that the RGB bypass is showing more detail? Although it could be interference?! Any thoughts please on which is the definitive output?

bypass_miracle.jpg (2.79 MB)

bypass_sega.jpg (2.01 MB)

cxa_miracle.jpg (2.1 MB)

cxa_sega.jpg (1.23 MB)

I think the CXA is better - there's no black pixels there.

Author	Message
dobbo2k Joined: 17 Jun 2019 Posts: 1	SMS2 RGB mod output comparison - which is more accurate? Posted: Mon Jun 17, 2019 9:51 pm
dobbo2k Joined: 17 Jun 2019 Posts: 1	Hello, I've been modding SMS2 (PAL) units for RGB SCART output by tapping the Sony CXA video amp chip for RGB and using composite video as sync. The picture looks great on my CRT. Recently I thought I'd try a different method, an "RGB bypass", where you tap the RGB signal from the VDP chip and then use a THS7316 as a video amp. I then tap csync (not composite) from the CXA and use that as sync. Technically a full bypass would tap csync from the VDP, but then I'd have to use another THS7316 to amp the signal which I can't be bothered to do. The picture also looks very nice and clean on my CRT. When I have more time I want to compare how they look through the OSSC, as that picks up issues with jailbars, sync etc better. So using my CRT, I'm trying to decide whether the RGB bypass looks any different/better than just tapping RGB from the Sony CXA chip. I noticed that the Alex Kidd title screen does indeed look slightly different: The output from the CXA has plain red text for "in miracle world" and plain blue text for "sega". Using the RGB bypass method, both of these bits of text now have small bits of black inside and round some of the edges. I have attached pictures showing the differences. It seems that the RGB bypass is showing more detail? Although it could be interference?! Any thoughts please on which is the definitive output? bypass_miracle.jpg (2.79 MB) bypass_sega.jpg (2.01 MB) cxa_miracle.jpg (2.1 MB) cxa_sega.jpg (1.23 MB)

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14735 Location: London	Posted: Tue Jun 18, 2019 6:08 am
	I think the CXA is better - there's no black pixels there.

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