shmups.system11.org

Shmups Forum
 
* FAQ    * Search
 * Register  * Login 
It is currently Tue Jul 16, 2019 8:04 am View unanswered posts
View active topics



Post new topic Reply to topic  [ 5 posts ] 
Author Message
 Post subject: NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out picture
PostPosted: Wed Jul 10, 2019 8:35 am 



Joined: 10 Jul 2019
Posts: 2
Colleagues,

I'm facing the issue connecting NEO-GEO MVS MV-1B RGB modified board with OSSC via automatic scart swtich.
So I have the following setup

-> - means connected to

NEO-GEO -> Automatic SCART switch -> Mechanical SCART switch -> OSSC

RGB cable for NEO-GEO is hand made but good quality and equipped with 460 Ohm resistor on sync line and 220 Ohm resistors and 220uF capacitors on each RGB line in serial connection. 75 Ohms are not enough because picture is washed out and I was using Color Test Tables from Neo-geo HW test to adjust resistors.

Automatic Active SCART switch is like this
Image

Mechanical SCART switch is like this
Image

When I connect NEO-GEO -> Automatic SCART switch -> Mechanical SCART switch -> OSSC the picture is very bright and voltage level (I was using multimeter in DC postition shows 2,5V on each RGB line)

When I remove Automatic SCART switch and connect
NEO-GEO -> Mechanical SCART switch -> OSSC - the picture is awesome and color tables from HW tests I see are correct.

If I increase Resistors to 2Kohm - the colors look correct but picture is smooth and not good.

The rest of consoles I have N64, SNES, NESRGB, Saturn etc all look good with AutoSCART inline or without - no changes.
But MVS is not. Mostly I have 75 Ohm resistors in all RGB cables.

If anyone knows, could You tell me what I'm doing wrong?

Apparently active switch is amplifying the signal but why the rest of consoles show the good picture but MVS not?

Regards


Top
 Offline Profile  
 
 Post subject: Re: NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out pic
PostPosted: Thu Jul 11, 2019 10:29 am 



Joined: 20 Feb 2016
Posts: 272
I think your Automatic SCART switch is not terminating with 75 Ohm to ground and is outputting the signal with 75 Ohm impedance causing it to be terminated with 75 Ohm from the switch and 75 Ohm in the OSSC instead of 220 Ohm+resistor in console and 75 Ohm in OSSC.

You need to output a 1.4v p.p signal with 75 Ohm impedance from the Neo-geo. Probably the easiest way is to install a video amp like THS 7314/7316 with 75 Ohm resistors on the output and use a resistor voltage divider to attenuate the input to the video amp to around 0.7v.

Most likely the switch is doing the same thing to the sync so the sync voltage after 75 Ohm termination is much too high for the OSSC and you need to attenuate that too.


Top
 Offline Profile  
 
 Post subject: Re: NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out pic
PostPosted: Thu Jul 11, 2019 11:15 am 



Joined: 13 Apr 2013
Posts: 253
We designed an impedance matching RGB amp for the MVS if you are interested in. It uses a THS7374 video buffer.
https://www.beharbros.com/product-page/ ... fier-board


Top
 Offline Profile  
 
 Post subject: Re: NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out pic
PostPosted: Sun Jul 14, 2019 11:10 pm 



Joined: 10 Jul 2019
Posts: 2
paulb_nl - thanks a lot for the reply. I think I would go this way. So, as far as I understood one point of divider goes to console signal. The second one goes to GND and middle point attenuated to 0.7 will go to amplifier input. Right?

Seems that I need to power up the scope:) to be able to make proper attenuation

beharius - also, thanks a lot for Your proposal. I have plenty TH7316 so I prefer to go this way, but will have Your proposal in mind.


Top
 Offline Profile  
 
 Post subject: Re: NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out pic
PostPosted: Sun Jul 14, 2019 11:33 pm 



Joined: 20 Feb 2016
Posts: 272
Currently the resistor divider is 220 Ohm in the cable and 75 Ohm to ground in OSSC. Now you need to move that to the amp.

So you connect 220 Ohm in series between signal and amp and 75 Ohm parallel to ground.

Then if you put 75 Ohm on the output of the amp then you shouldn't need any resistors in the SCART cable.


Top
 Offline Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 5 posts ] 

All times are UTC


Who is online

Users browsing this forum: Google [Bot], Link83 and 9 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  
cron
Space Pilot 3K template by Jakob Persson
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group