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What leg of the SMS VDP supplies color subcarrier? I'd like to try lifting it away it for testing purposes.

See... This signal, while important with composite/etc, causes significant interference to RGB output. Visible as vertical bars. In particular affecting large blue areas of the screen (most often skies).

It's a problem inherited by and more commonly associated with Megadrive. Due to their shared hardware origins. Thanks. :)

http://cgfm2.emuviews.com/txt/genvdp.txt

Maybe this could be useful to you? Not sure.

That gives us the general idea. We're likely looking at a 315-5124 chip. Probably easier task once found. Half as many pins, and they're larger.

The 315-5124 pinout at the bottom here:

http://cgfm2.emuviews.com/txt/m3tech.txt

The 3.58 MHz clock output from the VDP is fed into the both the Z80 clock and video encoder chip (the Rohm part in that text file, or the Sony CX-whatever in the SMS). So there's no dedicated color subcarrier output.

The I/O chip in PAL machines generates the 4.48 MHz color subcarrier output for the encoder, and the VDP's 3.58 MHz output is used for the VDP only in those consoles.

There are some video related outputs of the 315-5124 that are unique to the Rohm chip and are used to tell it where to insert the color subcarrier signal. The Sony chips do this internally and don't need to be told when to do this, so the pins are no-connects in that case. It looks like the 315-5124 was built around the Rohm part and then the Sony chip came out later and made a better option, so Sega used it in everything else after that.

EDIT:

I'm curious, how does the color subcarrier relate to RGB? In arcade systems that have a SMS VDP, they just connect the RGB outputs to a monitor after buffering it and there is no interference in the display.

IIRC the color subcarrier is only used when you do RGB to composite or S-Video conversion, otherwise it can be ignored and the RGB output is unaffected.

Charles MacDonald wrote

I'm curious, how does the color subcarrier relate to RGB? In arcade systems that have a SMS VDP, they just connect the RGB outputs to a monitor after buffering it and there is no interference in the display.

IIRC the color subcarrier is only used when you do RGB to composite or S-Video conversion, otherwise it can be ignored and the RGB output is unaffected.

I'm not exactly certain. I don't have tech knowledge or equipment to go into detail. Circumstances suggest at some point the color subcarrier and RGB output clash when both connected.

Even if not a miracle fix, RGB without vertical bars is more benefit to the game experience for hardcore users than correct composite or s-video.

I suppose there's ways to reattach the subcarrier later. You'd have to if selling the console and want American buyers, too little RGB use here.

Author	Message
lumclaw Joined: 13 Aug 2010 Posts: 65	SMS VDP subcarrier pin Posted: Sat Jun 16, 2012 11:29 pm
lumclaw Joined: 13 Aug 2010 Posts: 65	What leg of the SMS VDP supplies color subcarrier? I'd like to try lifting it away it for testing purposes. See... This signal, while important with composite/etc, causes significant interference to RGB output. Visible as vertical bars. In particular affecting large blue areas of the screen (most often skies). It's a problem inherited by and more commonly associated with Megadrive. Due to their shared hardware origins. Thanks. :)

Maraakate Joined: 05 Jun 2010 Posts: 545 Location: Pennsylvania, USA	Posted: Sun Jun 17, 2012 2:51 am
	http://cgfm2.emuviews.com/txt/genvdp.txt Maybe this could be useful to you? Not sure.

lumclaw Joined: 13 Aug 2010 Posts: 65	Posted: Sun Jun 17, 2012 3:31 am
lumclaw Joined: 13 Aug 2010 Posts: 65	That gives us the general idea. We're likely looking at a 315-5124 chip. Probably easier task once found. Half as many pins, and they're larger.

Charles MacDonald Joined: 28 Sep 1999 Posts: 1012	Posted: Sun Jun 17, 2012 6:28 am
Charles MacDonald Joined: 28 Sep 1999 Posts: 1012	The 315-5124 pinout at the bottom here: http://cgfm2.emuviews.com/txt/m3tech.txt The 3.58 MHz clock output from the VDP is fed into the both the Z80 clock and video encoder chip (the Rohm part in that text file, or the Sony CX-whatever in the SMS). So there's no dedicated color subcarrier output. The I/O chip in PAL machines generates the 4.48 MHz color subcarrier output for the encoder, and the VDP's 3.58 MHz output is used for the VDP only in those consoles. There are some video related outputs of the 315-5124 that are unique to the Rohm chip and are used to tell it where to insert the color subcarrier signal. The Sony chips do this internally and don't need to be told when to do this, so the pins are no-connects in that case. It looks like the 315-5124 was built around the Rohm part and then the Sony chip came out later and made a better option, so Sega used it in everything else after that. EDIT: I'm curious, how does the color subcarrier relate to RGB? In arcade systems that have a SMS VDP, they just connect the RGB outputs to a monitor after buffering it and there is no interference in the display. IIRC the color subcarrier is only used when you do RGB to composite or S-Video conversion, otherwise it can be ignored and the RGB output is unaffected.

lumclaw Joined: 13 Aug 2010 Posts: 65	Posted: Sun Jun 17, 2012 7:49 am
lumclaw Joined: 13 Aug 2010 Posts: 65	Charles MacDonald wrote I'm curious, how does the color subcarrier relate to RGB? In arcade systems that have a SMS VDP, they just connect the RGB outputs to a monitor after buffering it and there is no interference in the display. IIRC the color subcarrier is only used when you do RGB to composite or S-Video conversion, otherwise it can be ignored and the RGB output is unaffected. I'm not exactly certain. I don't have tech knowledge or equipment to go into detail. Circumstances suggest at some point the color subcarrier and RGB output clash when both connected. Even if not a miracle fix, RGB without vertical bars is more benefit to the game experience for hardcore users than correct composite or s-video. I suppose there's ways to reattach the subcarrier later. You'd have to if selling the console and want American buyers, too little RGB use here.

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