Dreamcast 15khz question

[jamesboner]

Hi!
I'm new here. Very intresting forum this 

I have seen the guide for Dreamcast to do the lovely 15khz

But i wonder how to just activate it (pin 6 & pin 7) with a  switch. In fact i have the Sega RGB cable already with the three capacitators inside it. Can i use it as it is if i connect a switch to the mother board?

Or does i also have to mess upp with the cable?

Thank's!

[jamesboner]

I think i have this cable:


Utilises a proprietary connector for A/V out. Supports CVBS, RGB (TV), RGB (VGA, req. adapter) and luma/chroma. Pin 7 must be connected to ground to enable RGB. Pinouts: 1 ground, 2 audio right, 3 audio left, 4 +12V DC, 5 +5V DC, 6 RGB (VGA) select, 7 RGB (TV) select, 8 vertical sync (for VGA), 9 horizontal sync (for VGA), 10 composite sync, 11 chrominance, 12 luminance, 13 composite video, 14 blue video, 15 green video, 16 red video. RGB output circuit: proprietary DAC.

-----------------------

And this is the scedual for 15khz RGB

[skforty]

The dreamcast supports 3 modes of RGB apparently.  2 types of 15khz RGB and 1 for 31khz RGB (vga).  

Connecting just pin 6 to GROUND will give you the 1st 15khz RGB mode.  
Connecting just pin 7 to GROUND will give you the 2nd 15khz RGB mode.

Connecting both pin 6 and 7 to GROUND will give you the 31khz RGB (VGA) mode.  

What are the differences between the 2 different 15khz modes?  Good question.  I dont know.  I've seen all types of speculation, but most people dont seem to understand what the question is.  I've heard a couple games like fighting vipers are washed out in mode 1 but not 2, or don't boot at all in one of the other modes.  I've experimented myself quite a bit and have never seen a difference personally.

Also, there are quite a few games that dont support RGB modes at all, for games that are not compatible with RGB modes, you can "trick" it sometimes by booting up in composite, and switching at the right time.  This is much easier if you have a VGA box with a "TV" switch, or you fit the dreamcast with a switch yourself.  

If you do have the cable that is based on the diagram above, that only connects pin 7 to GROUND, then you already have a 15khz RGB scart cable and it would seem you are ready to enjoy 15khz rgb  If you would like to have that switch between mode 1 and mode 2 15khz RGB then you need a SPDT switch, where the middle pin is ground and the one side is pin 6 and the other pin 7.  
However, if you would like to switch between your existing 15khz RGB and 31khz RGB, then i would leave the cable as is, open up your dreamcast and use a spst switch for pin 6 and GROUND.  When that switch is on, it would basically make your cable a VGA cable, when its off, you would have normal 15khz RGB.


Actually, one edit:

I use VGA boxes that i modify, and i just bridge H and V sync together for my Csync.  I dont know much about SCART tv's as i am over here in the united states, but my statement about bridging 6&7 may not work with that cable since it uses composite video for sync.  I dont know if  VGA mode can use composite video as sync,or if that pin is even still active when VGA is activated.  I also dont know if many tv's support 31khz through scart at all...So take that into consideration please  

Someone else would probably know better with that specifically. 

[jamesboner]

Aha i think i get it now. pin 6 or 7 is only activating 15kzh.  

So there is no fix to force it to 240p?? I think only 1 / 20 games is in 15khz 240p (the arcade look with scan lines)

I have only two games in 240p (Last hope and Street Fighter 3)

[skforty]

Exactly.  The pin 6 or 7 is just for 15khz. 

Some games are interlaced, some arent.  Some like 3rd strike have methods of forcing progressive/interlaced.

[jamesboner]

How many Low-res games are there for the Dreamcast? 

I only found these:

Megaman X Legends
Street Fighter 3
Fast Striker
Last Hope

[NFG]

Bangaioh is a low-res game too.

[jamesboner]

Ahaa wierd and cool game!

I wonder how these 240p-games looks through a VGA-box to a hi-res CRT-monitor  

[NFG]

In the case of Bangaioh, it won't work on the VGA box.  I was able to trick it into VGA once (it's very tricky to do!) and the game was presented in the top left corner, the other three quarters contained only scattered garbage tiles.  IIRC the game wasn't playable.