Sync problems with NTSC Sega Genesis

[coderkind]

Hi all,

received my OSSC yesterday and so far I’ve been impressed; my Sony PSP Go via component works very well and my VideoGamePerfection-modded SNES Mini performs like a champ (credit to you & your guy’s work @BuckoA51).

However… I can’t get my American NTSC Sega Genesis to work. It’s an unmodded, model 1 using a RetroGamingCables CSYNC cable. The console works 100% via a flatscreen SCART socket TV, an old CRT via SCART and via BVM cables to a Sony BVM. However, connecting to the OSSC results in no picture and the resolution display suggests the OSSC is having trouble getting a lock on the resolution the console is outputting.

The OSSC seems to be struggling while this is occurring as well; selecting menu options via the remote becomes VERY sluggish/unresponsive. I don’t know which sub-menu to go into to tweak LPF sync stuff or whether I should be doing this BEFORE switching the Sega Genesis on, or afterwards.

Video of what’s going on (which results in no picture on my TV) here: https://youtu.be/6cmc9MpO6Mc.

The unit seems to have the latest firmware on it too (0.71).

Help my Genesis please!!! Only things I can think of to help it are buying some sort of sync-splitter device (although the OSSC *should* deal with this I thought) or wait for an OSSC firmware update?

[afangus]

I’ve had the same problem with the same cable, going av3 (rgbhv with sync strike) solve this

[dead_screem]

Open up the scart plug, see if there is an in-built sync booster/cleaner. That may be the problem.

[coderkind]

Thanks for the suggestions guys. @afangus; what’s entailed for that with a Genesis? Getting some sort of VGA cable?

@dead_screem; I’m reluctant to open up the scart plug as the cable works for a wide variety of other purposes (old CRT, modern widescreen with a scart socket).

I guess I’m trying to work out why it’s happening initially as my understanding is that the OSSC should be able to tackle the Genesis with a good quality cable like this. Ideally I don’t want to pay for more cables/adapters if I can. If anything, I was expecting the SNES to present problems, which is why I tested the Genesis first as I thought it was going to be a given to work.

[dead_screem]

IIRC the OSSC has a built in sync filter and reccommends that you don’t use any external sync cleaner/boosters (in the cable for instance) as they can cause issues with the OSSC.

If the retro gaming cables cable is like the cable I got from retro_console_accessories on ebay it might have the in built sync cleaner/booster. Opening the cable and seeing what’s inside won’t hurt anything you know…

And AV3 RGBHV requires TTL level sync, AV1 RGB SCART does not it requires 1Vp-p 75Ohm terminated, so if it works in AV3 then the cable might be boosting the MD’s Csync output to TTL level which is why it cant lock to it on AV1.

[marqs]

I’d be interested to know how that cable is wired. The challenge with MD/Genesis is that C-sync available from DIN connector comes from VDP and is more or less weak (apparently depending on console model) if connected to normal 75ohm terminated input (TV, AV1 on OSSC etc.) without buffering circuitry. The cable could have LM1881 sync stripper generating c-sync from composite video, it could be directly wired to DIN c-sync via a capacitor, it might have a buffer on this c-sync line, or some combination of previous. They should all work fine with OSSC AV1 if the console works on TV, but depending on the cable implementation the sync signal may actually be quite bad and needs some options tweaked on OSSC side (try “analog sync LPF” first”).

There is actually an additional setting on the digitizer chip which *might* improve syncing of problematic systems – it’d help if I had such system to try and debug, but I can release a test firmware with that option exposed if no other solution is found.

[BuckoA51]

That is the second report of that particular cable not working with OSSC. Indeed we really need someone to open it and see what’s going on.

Composite video for sync MD cables work, as do the “Pack a punch” ones from retrogamingcabiles.co.uk (that uses the capacitor/resistor method from the consoles csync output).

[coderkind]

The “Pack a punch” cables haven’t been in-stock for months. I’ve seen notice that they’re due to stock again soon, but I think they’ll be going for near £50-a-pop. I’d be willing to open my existing SCART cable but there’s no obvious seal or way to crack it open, and I don’t want to be left without any method to play the Genesis should I open it up and inadvertently break it.

It probably doesn’t help much, but in my original emails with Robert from Retro Gaming Cables I was asking about my SCART-to-BNC cable not being a sync-stripped version, and he replied

…your megadrive RGB SCART cable is wired for CSYNC therefore you don’t need a sync cleaner inside of the BNC adapter cable

That sounds like the Genesis/Megadrive cable is doing some sort of cleaning at the very least. Considering it works fine with a non sync-stripped SCART-to-BNC cable to connect to a Sony BVM, I’d imagine that should mean the sync should be ok? I realise I’m being very vague here as I’m making some assumptions while not fully appreciating the sync situation.

[BuckoA51]

As far as CSYNC goes they should be wired the same, there’s no reason for one to work and the other not.

To open a SCART cable you unscrew the bit that grips the cable near the SCART plug, slide that down then carefully pry the casing apart.

You can tweak settings like LPF etc without the console being turned on.

[coderkind]

Video of the internals of the cable now posted: https://youtu.be/vutSJYi6gjc

[coderkind]

@BuckoA51; which menu/remote button do I need to go into to tweak that? I saw the Video LPF settings (95MHz, 35MHz, etc), but couldn’t spot where the “Analog sync LPF” options are (Off, 33MHz, 10MHz, 2.5MHz).

[BuckoA51]

It’s under “Sync opt”.

@marqs – Worth us getting one of these cables in for testing?

[marqs]

It looks like the cable has just a cap on the sync line – I wouldn’t expect very good quality sync from such passive solution, but I can make a similar connection next time I get MD for testing and scope the sync to verify that assumption. BTW, the cable also seems to lack capacitors and individual shielding on R/G/B lines – are those near-£50 “pack-a-punch” cables any better quality?

[dead_screem]

actually it looks like the resistor is there on the sync line in the gray shrink tube before the cap.

And yeah I see only resistors on the RGB lines, and like you said, according to this page it needs resistors and caps. And maybe for pal mega drives only? As my cable from retro_console_accessories on ebay which I use on my NTSC Genesis has only caps on the RGB lines, no resistors. dont have an ossc so who knows if it works with it or not.

[coderkind]

@marqs; I don’t know if the “pack-a-punch” cables will be better quality. Where did you get your cables from when testing out the Genesis/Megadrive with the OSSC?

How realistic is a firmware workaround in time?

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