SNES stutter in Adaptive line mode

[SK123]

<p style=”text-align: left;”>I know there’s a sticky thread about SNES having jittery sync from 2018 has this been resolved or is my issue similar?</p>
<p style=”text-align: left;”></p>
<p style=”text-align: left;”>Using the Mister FPGA and OSSC pro for some reason on the SNES core it stutters when is adaptive line multiplier mode no matter what settings I use. Also, the picture goes black for a few seconds every minute or so. Strangely it doesn’t happen when I use pure line multiplier or scaler. This happened with the original firmware the OSSC pro came with and still with the newest 0.77.</p>

[onthewaveagain]

I remember raising this a few months back. Marqs said : “Regarding SNES core, it doesn’t have de-jitter option so Pro hardware cannot maintain framelock since the number of VCLKs per frame doesn’t remain constant. You thus have to use scaler mode with framelock off (it might remain stable even with framelock option on but under the hood the lock cannot be kept). I’ll need to look into integer scale of 224 height.”

What I found strange is that it does not stutter and go black when I use the MiSTer RGB output with Adaptive Line.

• This reply was modified 5 months, 3 weeks ago by onthewaveagain.

[SK123]

Interesting. It also doesn’t stutter in Pure Line multiplier mode.

 

Marqs, is this something that can be implemented in a firmware update?

[marqs]

The root cause is SNES (clone) and for genuine one a solution (de-jitter mod) has been available for many years. A SNES core with digital output ends up constantly re-triggering frame syncing on Pro (properly enforced on upcoming fw) since its framelock is designed to operate with constant number of VCLKs per frame (that you get from any normal HDMI source or from digitized RGB). Aside from turning framelock off, there are essentially 2 other ways to solve the problem:

* addition of de-jitter option on the SNES core (which could also improve compatibility with some picky CRTs)
* re-implementing Pro’s framelock via general genlock which adjusts timing based on source vsync alone

The latter might also have some other minor benefits on Pro, but it’s currently not high priority since the existing solution works well with vast majority of sources.

[lyrical]

Hello everyone, I signed up to join in the conversation as I was about to buy the OSCC Pro specifically for my SNES setup.

For my purposes I aim to video capture the SNES exactly as it is without mods, and after seeing a lot of successful captures like on RetroRGB’s old OSSC Pro stream, I thought that I would be golden with the OSSC Pro.

A SNES core with digital output ends up constantly re-triggering frame syncing on Pro […] since its framelock is designed to operate with constant number of VCLKs per frame

I did not realize that it would have trouble keeping sync with my system. I currently have a simple RetroTink line doubler which is not having this issue, and I was thinking that whatever magic that does to prevent might be the same in the OSSC Pro’s “adaptive” mode, and certainly the scaling mode.

* re-implementing Pro’s framelock via general genlock which adjusts timing based on source vsync alone

The latter might also have some other minor benefits on Pro, but it’s currently not high priority since the existing solution works well with vast majority of sources.

If it is a matter of priorities, I am at least one person who is interested in the OSSC Pro purely for using it with the SNES and no other systems. I especially like the Reverse LPF feature it has for the SNES.

As it stands without future improvements, would the Frame unlock at source framerate in scaler mode stay in sync enough to not drop frames for video capture? This would fulfill one use case for me, though I really am rooting for it accounting for the SNES in the future.

[marqs]

Real SNES with analog output and modern SNES clones with digital output should not be mixed in this discussion.

I have not heard reports of Pro having sync issues with former since its sync jitter gets damped by the ADC chip and resulting clock should be good enough for any downstream processing. On the other hand, the damping can cause visual jitter on top but this can be mitigated by slightly increasing H-PLL Loop Gain option.

The clones with digital output that accurately replicate video timings have different kind of issue. Each frame in 240p has different amount of pixels compared to previous which is not standard digital video. As mentioned above, the line-locking implementation is unable to keep output synced so the only solution right now is to disable framelock.

[lyrical]

Understood. I had not realized that this thread is not about the real SNES.

[SK123]

marqs, why does the OSSC pro work fine without stutter in Pure line multiplier mode though?

When I connect the Mister FPGA via direct video the only two methods that work with the SNES core are scaler and pure line multiplier. Frame lock is not on in scaler mode.

I actually prefer to use the line multiplier modes for the reduced latency. That is my top priority and the reason I purchased the OSSC pro. I just wish adaptive line multiplier worked without stutter because it scales better on my television than pure. With pure the picture is completely stretched out to fill the entire 16:9 screen.

[marqs]

Pure LM mode also does resyncing every ~5sec (you see this from red LED), but it occurs during vblank of both input and output so perhaps that explains why there are no visible artifacts on some monitors. I just tried a workaround which sets output frame interval to average of SNES frames and that seems to work nicely in A-LM, but I’m not sure how many games output 240p at this quasi-constant timing (at least on NES there is no guaranteed pattern). The best solution would still be having a compatibility mode in the (S)NES core instead of OSSC Pro.

[SK123]

Is there a way I can try that workaround?

Is it via firmware or settings?

[marqs]

It needs a customized firmware, I’ll see if can create a test version on the upcoming week.

[SK123]

That would be awesome. Thanks.

I own a terasic USB blaster if necessary.

[marqs]

A test firmware can be now downloaded here. It’s certainly good to have USB Blaster available as a backup if something goes wrong.

[SK123]

Thank you marqs! I will test it now and let you know.

[SK123]

Marqs thank you so much for this update. I have the Mister connected via direct video and every SNES game I try plays stutter free in adaptive line multiplier mode.

On my LG OLED it shows Freesync is enabled now. Variable frame rate now works as well.

Thank you so much for fixing this!

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