OSSC RGB to VGA would still need LM1881?

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Por gustavobk

Supporter (16)

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04-05-2021, 06:10

so, my OSSC just arrived and while I wait for a rgb to scart cable I would like to start using making a rgb to vga cable (ossc new model accept both inputs plus ybr).
Just I´m not sure if I would still need to use the LM1881 for such cable (which is advised when going to connect to 15hz monitors or the GBS8200), or can be connected direct to the respective positions in each end that ossc will take care of sync?

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Por bsittler

Master (245)

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04-05-2021, 07:18

OSSC can work pretty well even with composite as sync. I use it without a sync separator with several RGB-outputting MSX models and it seems fine with stable sync. RGB MSX 1, MSX 2, MSX 2+, Turbo R, and various OCM flavors all cooperate

Por Metalion

Paragon (1444)

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04-05-2021, 08:51

I have ordered mine a few days ago (a new 1.6 version).
I'm quite impatient to receive it Smile

What is LM1881 ?

Por gustavobk

Supporter (16)

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04-05-2021, 09:14

LM1881 is an ic for video sync (horizontal / vertical) separation. (msx is csync, composite sync)

bsittler, gotcha, so it seems a simple solded RGB->VGA(DE-15) will do the trick. tks.

Por sdsnatcher73

Prophet (2234)

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04-05-2021, 11:45

The question is if it accepts composite as sync on the VGA input, it works fine of course on the SCART input. I think it might not looking at the documentation

AV3 (VGA)
The AV3 input is a VGA/HD-15 connector which supports video in RGBHV, RGBS (pin 13), RGsB and YPbPr formats. RGBHV and RGBS modes require clean TTL-level sync signals and cannot extract sync from composite video or luma. AV3 is best suited for high-quality input sources as video LPF functionality is limited (the AV1 and AV2 inputs are routed through a dedicated LPF chip). Therefore, it is generally recommended to connect older consoles and arcade boards to these other inputs.

Why not build a SCART cable? Either way, let us know your experience.

Por sdsnatcher73

Prophet (2234)

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04-05-2021, 11:52

Please note that CSYNC (or composite sync) is not the same as composite video (the common yellow plug). CSYNC is embedded into composite video, to extract pure CSYNC from composite video you need something called a sync stripper. A sync splitter splits CSYNC into HSYNC and VSYNC. Whether your MSX outputs a clean CSYNC or just composite for use with RGB depends on the model.

Por bsittler

Master (245)

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04-05-2021, 17:52

Correct, it works on the SCART input but likely not on VGA input. Also in general the SCART input seems more flexible about slightly-strange timings than the VGA input is

Por gustavobk

Supporter (16)

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04-05-2021, 18:02

Thanks for the detailed answer guys.
I ordered a scart cable, but it will take take (for some reason) quite some time to arrive here. :/
I just ordered some lm1881, will try with and without it, "for science".

Por Wild_Penguin

Hero (641)

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04-05-2021, 19:04

FWIW my projector can accept composite sync on it's VGA, but it needs to be stripped, and TTL-level! Which is kind of strange, but that's how it seems to work (I know the NMS8255 outputs clean SYNC at the SCART, but at a lower level). It may be there are other devices which have similar limitations.

I've shared my endeavors here before. Cool See: here. It may be difficult to reliably power the sync stripper (especially if you need a regulator as I did, since I couldn't source an LM1881 but needed to use EL4581 instead) without an external power supply (as the control signals, often used for powering the stripper, were not designed for that...).

Por Grauw

Ascended (10063)

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04-05-2021, 21:10

Quoting from the manual:

“You can connect either 15khz or 31khz sources to any input on the OSSC, but remember that the AV3 (VGA or D-Sub15) connector does not have the additional filtering that RGB SCART sources often require.”

Let us know your findings, I’m curious Smile.

Por sdsnatcher73

Prophet (2234)

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05-05-2021, 10:19

So I was also curious, I can inform you that you can connect both OCM as well as Zemmix Neo in 15kHz RGB mode to OSSC with a simple VGA cable. There is slightly more noise in the image compared to using a VGA to SCART cable (passive ones, I have one from RepairBas as well as one from RetroCables). This can be explained by the fact the AV3 input bypasses the low pass filters in the OSSC.

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