Help! OSSC + Neo Geo MVS – No Video

[willbot2000]

I just got an OSSC to hook up to my new 4K monitor and… I’m having trouble.

I get the grey test pattern, but as soon as switch to my MVS (AV1: RGBS) I see nothing. I’ve tried it on two monitors and a TV (4K Samsung, Ultrawide AOC and 1080p Samsung TV). I changed my sync settings (“Sync OPT. -> H-PLL Post-Coast – 2 lines” and “Sync OPT. -> Hsync Tolerance – 8.30us”) to get sync and see 59.4Hz. I checked the 4K Samsung spec sheet and it supports 56 – 61Hz, so notionally it’s not that problem.

But it appears that the sync rate is jumping on the OSSC screen. And some of the displays seem to detect a signal then lose it.

Any ideas? Some things I think it could be:

– Sync not locking.
– Color space?
– Monitor incompatibility (all three…)

Equipment: MV-1A (consolised), Samsung 27UK850 (4k monitor) and Samsung 40f6300 (1080p TV).

[willbot2000]

Cracked it! Changed “Sync > Analog sync Vth” to 90mV. I’m guessing this is the voltage threshold for the sync pulse?

I was checking the sync on my scope earlier and it was peaking at 1.3v.

[nmalinoski]

Sync fed into AV1 should be limited to consumer-video-level, 75Ohm, 1Vp-p. Anything beyond that should be attenuated, or perhaps fed into AV3, which is designed to take 5Vp-p TTL sync.

[willbot2000]

I can attenuate it down to 1v peak but then I also have to reduce the “Analog sync Vth” further.

[marqs]

Did you measure the sync level with 75ohm load?

[willbot2000]

I absolutely did not – my mistake. I also have my PVM in parallel.

For the next revision of my board I plan to add a THS7274 or THS7376 which I believe will give matched impedance. Right now I’m running straight from the JAMMA edge, but attenuated through 500ohm pots (RGB and sync).

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