Playing NHL94 on a Retrotink 5X & Real Hardware -- resolving stuttering

[bcrt2000]

Hey folks, I just wanted to make a note of this, I think it may help some people playing the game, and also maybe guide some folks making rom hacks.

So I had noticed playing NHL 94 on my real Sega Genesis model 2 hooked up to a Retrotink 5X (dedicated video game scaler) via SCART that at times the game would start stuttering like crazy occasionally. This would last until the game was paused or the play ended.

I asked on the forums, and other people had not seen this on real hardware, so I thought that maybe this was related to the Retrotink 5X. Today I had some time to do some controlled testing to figure out what was going on.

So the Retrotink 5X has different V-sync modes:

- Triple Buffer: Buffers multiple frames to eliminate the "Chrono Cross" problem (HDMI resync when resolution switching) and ensure high display compatibility, but introduces increased lag and minor judder.
- Frame Lock: Matches the console's output frame rate directly to minimize lag, but may cause sync issues with certain displays or resolutions like 1440p+.
- Gen Lock: Available on units sold after July 2021 with firmware 2.73+, it matches the input frame rate using a regenerated clock for improved stability over Frame Lock, reducing judder and artifacts, especially at 1440p+ with composite/S-video sources.
- VRR (Freesync): Adds a "Freesync" VRR flag to the HDMI output, signaling non-standard frame rates to reduce screen tearing without triple buffering, though true VRR passthrough depends on the HDMI transmitter chip.
- TBC (Time Base Correction): Stabilizes video output by processing the source at its native frame rate while rejecting glitches, ideal for noisy sources like VHS or older consoles, though it’s experimental and may be temperamental with some capture cards.

I tested stock NHL94. I was using Gen Lock-- its great for games that do resolution switching, there's pretty much no glitch/resync noticeable. However, seems like what happens is when NHL94 stresses the system and drops a few frames, it introduces a stutter in Gen Lock until the the game is paused/play ends, then it fixes itself. Note that this only happens like once or twice per game.

So I switched to Frame Lock. Worked fine. I noticed there were moments the game would drop a handful of frames, but they were very short/not noticeable. I'm sensitive to this, but a normal person might not even notice. I'm assuming this is how the game is normally, even on a CRT. And at the end of the game when the resolution switched (displaying the stats page), the picture dropped for a few seconds, but it wasn't a big deal.

I tried TBC mode, and that had the same stuttering issue as Gen Lock.

Then I switched to AdamCatalyst's NHL94 2024-25 ROM which has widescreen mode, and immediately what became a problem on Frame Lock was that when the resolutions switched (whether when pausing/unpausing the game, going in/out of intermission, or a penalty happened) the screen would go black and have to do an HDMI re-sync, and this would be long enough that gameplay could start happening before the picture came up.

So I moved to Triple Buffering-- that eliminates the frame drop out when switching resolutions. It's just unfortunate it adds a few frames of input lag.

I'm guessing VRR/Freesync mode might resolve this issue while also keeping low input lag. But the way I had the retrotink hooked up to the TV (Auxilary port on my receiver) I couldn't take advantage of VRR/Freesync.

One thing that would help is if the Widescreen Roms just stayed in 320 mode in the menus as well... that will be something I look to try to do once I get more into messing around with the '94 code. This would allow Frame Lock to be a viable mode to use.

I assume the issues seen here may apply to the Retrotink 4K as well, but I don't have one yet to test. I do plan on getting one to finish my retro/home theatre set up at some point in the next 18 months.

edit: I also should note this was all tested in 1440p mode. I probably should do another round of testing in 1080p mode.

Edited May 25 by bcrt2000