[BUG] Encoder Crash (Helper1, exitCode: -1073741819)

[Shiguri]

I wasn't sure if this should be categorized as a Bug or just Help, so I'm leaving the title without a prefix until I know which it is.

Hello everyone, I'm new here but been using Hybrid for a few years now.
I wanted to see if I could finally get a bug I've had for ages with Hybrid potentially figured out.
I'll be quite comprehensive with this in hopes to eliminate the need for too many follow-up questions.

First, let's cover the things asked for in the sticky.

• Q: What Hybrid version did you use? 
  
• A: Windows 11 Enterprise Version 10.0.22000 Build 22000 (though this issue existed on Windows 10 as well).
  
• Q: What did happen? If an error message came up what did it exactly say?
  
• A: "CRASHED Helper1, exitCode: -1073741819" in No XSynth or "Crashed with exit status 0" in Vapoursynth
  
• Q: What did you do to get to the bug?
  
• A: I think the rest of this post pretty well covers that I guess.
  
• Q: Debug output would be helpful.
  
• A: I've included debug logs for NoXSynth, VapourSynth, and AviSynth scenarios (AviSynth works but with the caveat of fonts sometimes being the wrong face or spacing).
  
• Q: A small sample, which allows the reprocudtion of the problem would be nice.
  
• A: I'll find a way to include these if you really need one, we're dealing with large files here so unless you're fine with downloading a 5GB+ file I host on GDrive or something I'd need to figure out a way to get the size down by altering the file in such a way that the problem is preserved.
  

Next, I guess I'll give you a rundown of what I'm running and what my primary use for Hybrid is and how I do things.

My Hardware:
Intel i7 8700k (8th Gen)
NVIDIA GTX 1060 6GB
16GB 3200MHz Memory
Samsung 980 NVMe 1TB
OS: Windows 11 Enterprise

Tools I use:

• Hybrid
  
• MKVToolNix
  
• MP4Box
  

I typically encode anime, and I usually do this from raw BluRay rips (M2TS files).
However, I sometimes have to use files that have already been re-encoded as well (MKV files) due to availability issues.

I'm usually re-encoding videos for use in a VR game (VRChat) where video playback within the game has some requirements. The video players are designed to syncronize the video playback to everyone in the room and people can be located all over the world and have varying bandwidths, ping, and hardware, not to mention there are limitations in how Unity (the engine the game uses) handles videos, and the player can break for everyone if it detects issues with syncing it properly to everyone in the instance simultaneously... so these videos need to be encoded to some strict limitations and requirements to ensure that they can be delivered without playback issues arising.

I've learned that worldwide synchronized video playback is a huge pain in the ass where you need to encode to strict low-bandwidth & decode standards and use a good CDN that has good replication across servers hosted globally. But getting to watch things with my internet friends is worth it.

Anyway, the first step of the process is finding acceptable subtitles and remuxing them into the file. Sometimes the included (usually PGS) file that comes with the BluRay is decent enough in which case I'll just slap the M2TS directly into Hybrid. However I typically prefer well-stylized and better translated versions which I have to get from elsewhere, in which case I have to use MKVToolNix to remux them together properly (sometimes I need to add positive or negative delays to the subtitles to get them to sync properly with the video).

Point being, what I slap into Hybrid is either the raw M2TS from BDRip, or an MKV I've created because I wanted a different subtitle than the included subs.

Audio I typically encode from PCM or in rarer cases DTSHD-MA or FLAC. I would only encode from anything else if I couldn't find something lossless.


How I Do Things

When watching movies in VRChat, resolutions above 720p aren't terribly meaningful unless you're using a gigantic screen or standing way too close to the screen, so the first thing I do to push required bitrate down is scale my (usually 1920x1080) source down 1280x720 (or an equivalent if aspect isn't 16:9). This saves on both bitrate and encode/decode immensely.

I assume Bicubic Spline is the best downscale filter available without getting into Avisynth/Vapoursnyth madness, although I've tried to flip between Lanczos and Bicubic Spline and it looks to me like Lanczos might be ever so slightly sharper but BSpline doesn't have as much ringing maybe? Idk, at this point I've just stuck with BSpline because it's default (default is usually default for a reason, so that's what I do if I can't really tell the difference between two things).

For encoding I use CRF 23 because I want consistent quality-level moreso than a strict bitrate (although I monitor it to ensure the bitrate stays below 2000kbps on average... usually CRF23 pushes it down closer to 1000-1500 when dealing with anime. I really just want to tell the encoder to "encode to this quality level, keep the bitrate as low as you can within this quality level, and you're allowed to use up to this much bitrate maximum", so I think a capped CRF where I could specify a hard maximum bitrate (I'd say 2000kbps) is what I'd actually like to use but I don't see that as an option in Hybrid. But yeah, CRF23 and most of my encodes come out in the 1000kbps to 2000kbps average bitrate range.


Video Encode Settings
Codec: x264

AVC Profile: Base (Unity seems to prefer Base)
Level: 3.1 (I have "Minimize AVC level for mbps and bitrate" enabled)
Entropy Coding: CAVLC
Preset: Veryslow
Tune: fastdecode

Crop/Resize Settings
Resize: 1280x720 (or equivalent)
Resize Method: Bicubic Spline

Audio Encode Settings
Codec: AAC
Bitrate: 320kbps
Channels: Stereo (downmixed from surround if need be)
Sample Rate: 48KHz (VRC doesn't behave with anything else)

Subtitle Settings:
Embed Subtitles Into Frame (required)

Container Settings:
Type: MP4 or MKV (usually MP4, MKV is only for if I need to do some more edits after encoding)
Special Settings: Interleave 500ms

Filter Support:
No XSynth

Config View:

x264 --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 18 --partitions i4x4,p4x4,p8x8,b8x8 --merange 24 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --qpfile GENERATED_QP_FILE --no-deblock --non-deterministic --range tv --colormatrix bt709 --demuxer raw --input-res 1280x720 --input-csp i420 --input-range tv --input-depth 8 --fps 24000/1001 --output-depth 8 --output "C:\Users\Shiguri\Videos\Hybrid\Temp\01.264" -

Post Work:
After output, typically either the job is done (movies), or if its a series I'll use MP4Box to concatenate several outputs into a single large MP4 file (a single monolithic file instead of multiple separate files is better for the player as well as the viewing experience). As far as I can tell Hybrid doesn't have support for concatenation so I use MP4Box and MKVToolNix for this sort of thing. So usually just something like "MP4Box -add 01.mp4 -cat 02.mp4 -cat 03.mp4 -cat 04.mp4 -cat 05.mp4 -cat 06.mp4 Output.mp4" in CMD and I'm done.




My Issue

The issue arises during encoding, where when it gets around to the point where it needs to encode, it just crashes with the following error message:

CRASHED Helper1, exitCode: -1073741819

Now here's the strange part -- simply changing the filter support to Avisynth appears to fix the issue, although with some caveats.
Typically I prefer to use the "no XSynth" support mode because I'm not really using anything in Vapoursynth or Avisynth.

The crash behavior in no XSynth mode is that it'll try to encode video stream before crashing partway through.

Switching to Vapoursnyth will fix it sometimes, but if it doesn't then the crash behavior is that it'll simply look at the video stream and crash immediately, like it won't even bother trying.
It will typically throw the error:

Crashed with exit status 0

Switching to AviSynth, it almost never crashes, and will seemingly encode the video just fine... with the caveat that some subtitles won't be in the right typeface, I guess some part of the subtitle extraction process goes wrong with AviSynth.  It also sometimes will throw alert dialogues about fonts when it starts to encode the video... but I think I've confirmed that subtitles aren't the issue and that this occurs whether or not subtitles are even present, Avisynth either not properly extracting or supporting subtitles seems to be a separate issue of its own.

To make matters more wonky, the crashes are also inconsistent across a batch of files. What I mean by that is, within one set of M2TS or MKV files all extracted from the same disc of the same BluRay, or a batch of files all encoded the same way, only some will crash the encoder while others won't... even though as far as I can tell all the files are all untouched direct rips or all encoded the same way, which makes no sense to me. So I just kinda roll the dice every time I feed a file into Hybrid, hoping it doesn't crash or complain, which if I didn't know better I'd say it does it at random, but I know there must be a reason.

This is an issue I've dealt with for years now, kinda just hoping an eventual update to Hybrid would fix it but it never has, so I guess it's time for me to just finally say something and either figure out what I'm doing wrong, or draw attention to the issue if there really is a bug somewhere. All this time I've just kinda lived with it, and told Hybrid to encode all files without pausing for crashes and at the end of it I retry all failed files with AviSynth and just live with whatever fuck-ups it does to the subtitles in those files.

I figured reporting an issue that is intermittent and inconsistent like this probably won't yield any concrete answers... but worth a shot anyway.


Conclusion
Anyway, just figured I'd introduce myself and what the context for what I wanna do is, as well as present this age-old problem of mine.

If anyone has any other advice about things I could do better or you see anything I'm clearly doing wrong here etc, do let me know. I think I may eventually post to another part of the forum to ask about things I could maybe do with AviSynth or Vapoursynth to improve my end results (as long as they don't make the files bigger or more difficult to decode, or increase encode time by too much). Things that can be done to improve downscaling anime would be especially helpful. It's worth noting that I'm a GUI-dweller who doesn't really do great with purely commandline based programs (why I'm using Hybrid in the first place), but fiddling with this stuff has kinda forced me down that road regardless.

Oh and Selur, I'm not sure if you'd remember but I was the guy who was e-mailing you about SVP support a few years back, I believe you actually added Overwrite option to the interframe interpolation in AviSynth because of me wanting to use my specific SVP settings in Hybrid back then, I used it quite a bit so thanks a lot for that.

[Selur]

Looked at the debug output 'Vapoursynth'.
The script used by Hybrid:

# Imports
import vapoursynth as vs
# getting Vapoursynth core
core = vs.core
# Loading Plugins
core.std.LoadPlugin(path="C:/Program Files/Hybrid/64bit/vsfilters/Support/fmtconv.dll")
core.std.LoadPlugin(path="C:/Program Files/Hybrid/64bit/vsfilters/SourceFilter/LSmashSource/vslsmashsource.dll")
# source: 'C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv'
# current color space: YUV420P8, bit depth: 8, resolution: 1920x1080, fps: 23.976, color matrix: 709, yuv luminance scale: limited, scanorder: progressive
# Loading C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv using LWLibavSource
clip = core.lsmas.LWLibavSource(source="C:/Users/Alex/Videos/Hybrid/Input/TESTVIDEO.mkv", format="YUV420P8", cache=0, prefer_hw=0)
# Setting color matrix to 709.
clip = core.std.SetFrameProps(clip, _Matrix=1)
clip = clip if not core.text.FrameProps(clip,'_Transfer') else core.std.SetFrameProps(clip, _Transfer=1)
clip = clip if not core.text.FrameProps(clip,'_Primaries') else core.std.SetFrameProps(clip, _Primaries=1)
# Setting color range to TV (limited) range.
clip = core.std.SetFrameProp(clip=clip, prop="_ColorRange", intval=1)
# making sure frame rate is set to 23.976
clip = core.std.AssumeFPS(clip=clip, fpsnum=24000, fpsden=1001)
clip = core.fmtc.resample(clip=clip, kernel="spline16", w=1280, h=720, interlaced=False, interlacedd=False)
# Loading C:\Users\Alex\Videos\Hybrid\Output\TESTVIDEO_id_2_lang_en_default.ass using SubText
clip = core.sub.TextFile(clip=clip, file="C:/Users/Alex/Videos/Hybrid/Output/TESTVIDEO_id_2_lang_en_default.ass", fontdir="C:/Users/Alex/AppData/Roaming/hybrid/fonts")
# adjusting output color from: YUV420P16 to YUV420P8 for x264Model
clip = core.resize.Bicubic(clip=clip, format=vs.YUV420P8, range_s="limited")
# set output frame rate to 23.976fps
clip = core.std.AssumeFPS(clip=clip, fpsnum=24000, fpsden=1001)
# Output
clip.set_output()

looks fine, but the encoder call:

"C:\Program Files\Hybrid\64bit\Vapoursynth\vspipe.exe" "C:\Users\Alex\Videos\Hybrid\Temp\encodingTempSynthSkript_2022-04-11@04_32_38_7610.vpy" - -c y4m | "C:\Program Files\Hybrid\64bit\x264.exe" --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 18 --partitions i4x4,p4x4,p8x8,b8x8 --merange 24 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --no-deblock --non-deterministic --range tv --colormatrix bt709 --demuxer y4m --input-range tv --fps 24000/1001 --output-depth 8 --output "C:\Users\Alex\Videos\Hybrid\Temp\2022-04-11@04_32_38_7610_06.264" -

crashes with:

y4m [error]: bad sequence header magic

This usually means:

• something is amiss with the Vapoursynth script (source or subtitle filter having problems with the input), or
  
• the input is broken in some way, or
  
• some anti virus tool (or similar) is interfering, or
  
• there is another Vapoursynth installation on the system that is causing the problem
  

-> Does the Vapoursynth Preview work without a problem or does is show an error message?

Looking at the AviSynth script:

ClearAutoloadDirs()
SetFilterMTMode("DEFAULT_MT_MODE", MT_MULTI_INSTANCE)
LoadPlugin("C:\PROGRA~1\Hybrid\32bit\AVISYN~1\LSMASHSource.dll")
LoadPlugin("C:\PROGRA~1\Hybrid\32bit\AVISYN~1\VSFilter.dll")
# loading source: C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv
# color sampling YV12@8, matrix: bt709, scantyp: progressive, luminance scale: limited
LWLibavVideoSource("C:\Users\Alex\Videos\Hybrid\Input\TESTVI~1.MKV",cache=false,format="YUV420P8", prefer_hw=0,repeat=true)
# current resolution: 1920x1080
# filtering
# scaling to 1280x720
Spline36Resize(1280,720)
# embedding subtitles
TextSub("C:\Users\Alex\Videos\Hybrid\Output\TESTVI~1.ASS")
PreFetch(6)
# setting output fps to 23.976fps
AssumeFPS(24000,1001)
#  output: color sampling YV12@8, matrix: bt709, scantyp: progressive, luminance scale: limited
return last

the only thing that I not is that TextSub is used.

Looking at the 'NoSynth' debug output, using:

"C:\Program Files\Hybrid\64bit\mencoder.exe" -lavdopts threads=8 -really-quiet -subcp UTF-8 -sub "C:\Users\Alex\Videos\Hybrid\Output\TESTVI~1.ASS" -sid 6 -ass -of rawvideo -o - -ovc raw -noskip -vf scale,format=i420,scale=1280:720,ass,scale,format=i420 -sws 10 -nosound -mc 0 "C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv" | "C:\Program Files\Hybrid\64bit\x264.exe" --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 18 --partitions i4x4,p4x4,p8x8,b8x8 --merange 24 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --no-deblock --non-deterministic --range tv --colormatrix bt709 --demuxer raw --input-res 1280x720 --input-csp i420 --input-range tv --input-depth 8 --fps 24000/1001 --output-depth 8 --output "C:\Users\Alex\Videos\Hybrid\Temp\2022-04-11@04_19_36_1610_05.264" -

the runs for 26589 (of 35037) frames and then the decoder aborts with '-1073741819' which is an access violation.

-> So my guess is that either:

• something is interfering with the communication between decoder and encoder
  
• hardware issues (system is overclocked or general heat/memory issues)
  
• some issue with the source or subtitles around frame 26589
  

Do you use any other anti virus software than Windows Defender?
(if you do try whether, setting an exception for the Hybrid folder and it subfolders or better temporarily uninstalling the software fixes the issue.)


Cu Selur

[Shiguri]

• something is amiss with the Vapoursynth script (source or subtitle filter having problems with the input), or
  
• the input is broken in some way, or
  
• some anti virus tool (or similar) is interfering, or
  
• there is another Vapoursynth installation on the system that is causing the problem
  

      -> Does the Vapoursynth Preview work without a problem or does is show an error message?

• I'm just using the default values in Vapoursynth so... hmm.
  
• Considering this is an issue that happens with many different files and many of them being raw BD files, I can't imagine all of those being broken.
  
• I'm only using Windows Defender (in today's world paid Antivirus is just a scam and are often spyware themselves imho, not to mention none of them integrate with Windows as well as Microsoft's own solution of course).
  
• Only other things I've installed that are anything encode/decode related are MKVToolNix, GPAC (for MP4Box), and MPC-HC... so I don't think so. I think to this point it's also worth noting this issue has persisted across multiple Windows clean installs over the years, so I believe the likelihood of this is pretty low considering that.
  

-> Yes, Vapoursynth preview works without issue. Also did it with NoXSynth and AviSynth out of curiosity, no issues.

-> So my guess is that either:

• something is interfering with the communication between decoder and encoder
  
• hardware issues (system is overclocked or general heat/memory issues)
  
• some issue with the source or subtitles around frame 26589
  

• Unsure of what the violating program would be or how to check.
  
• My system is overclocked (both CPU, GPU, and Memory frequency and timings).
  
• This last one I figure I could only get assessed if I give you the source files. If this is the case I should be educated on what it is so I can know how to either avoid or fix the issue in the future.
  

From this I'd say my first order of business try pulling my clocks and timings back to default and see what happens. My system is well cooled (liquid) and these settings have been stable for a long time, but... who knows, maybe my system is degrading or something.

However I think it may be the last point because I believe it crashes around the same point every time, and I think if it were an OC issue it'd be less consistent.

Gonna revert OC, then test and report back.

[Selur]

Fingers crossed. Overclocking the memory easily can cause such issues.
Also it seems like enabling the root kit detection in Microsoft Defender can cause problems, so you might want to check that too.

Cu Selur

[Shiguri]

It appears that neither OC nor Windows Defender is the issue.

I ran through a series of tests, the final and most aggressive one I did the following:

• All OC disabled. Voltages, clocks, and timings all returned to Default/Auto in the UEFI (meaning even my 3200MHz rated memory was now running at 2133MHz).
  
• Disabled all Windows protections and Windows Defender. This included turning off Realtime Protection, Core Isolation (HVCI) and all Exploit Protections.
  
• Disconnected from the internet, ensured there's no startup programs enabled.
  
• Rebooted.
  
• Made a fresh Mux from the source files.
  
• Deleted everything in Hybrid's Temp folder to force it to rebuild that.
  

Yet still, crashed again, with the same error. Attaching debug log for whatever it's worth, it's No XSynth.

Far as I'm concerned that must mean something is wrong with my sources, so I'm uploading quite a large file to GDrive as I speak, I hope you won't mind downloading a fairly large (4.7GB) file because I think altering the video in any way will only "break" the issue and throw-off the troubleshooting endeavor. I just really just want to learn what this issue is so I can know how to deal with it in the future, it's ruined enough of my encodes now.

Edit: Sent you the file link via private message.

[Selur]

Okay, error did accur at another point, but in it's vicinity:

x264 output: 26569 frames: 74.79 fps, 1556.88 kb/s
Helper(1) finished with exitCode: -1073741819 and exitStatus: 1

"C:\Program Files\Hybrid\64bit\mencoder.exe" -lavdopts threads=8 -really-quiet -subcp UTF-8 -sub "C:\Users\Alex\Videos\Hybrid\Output\TESTVI~1.ASS" -sid 6 -ass -of rawvideo -o - -ovc raw -noskip -vf scale,format=i420,scale=1280:720,ass,scale,format=i420 -sws 10 -nosound -mc 0 "C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv" | "C:\Program Files\Hybrid\64bit\x264.exe" --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 18 --partitions i4x4,p4x4,p8x8,b8x8 --merange 24 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --no-deblock --non-deterministic --range tv --colormatrix bt709 --demuxer raw --input-res 1280x720 --input-csp i420 --input-range tv --input-depth 8 --fps 24000/1001 --output-depth 8 --output "C:\Users\Alex\Videos\Hybrid\Temp\2022-04-11@08_19_17_3710_05.264"

Do you see any errors when calling:

"C:\Program Files\Hybrid\64bit\mencoder.exe" -lavdopts threads=8 -subcp UTF-8 -sub "C:\Users\Alex\Videos\Hybrid\Output\TESTVI~1.ASS" -sid 6 -ass -of rawvideo -o NUL -ovc raw -noskip -vf scale,format=i420,scale=1280:720,ass,scale,format=i420 -sws 10 -nosound -mc 0 "C:\Users\Alex\Videos\Hybrid\Input\TESTVIDEO.mkv"

?

Cu Selur

Ps.: I'm downloading the file, will do some testing an report back.

[Selur]

Okay, I managed to setup the encoding the same way you did:

mencoder  -lavdopts threads=8 -really-quiet -subcp UTF-8 -sub "E:\Output\ONEPUN~1.ASS" -sid 6  -ass -of rawvideo -o - -ovc raw -noskip -vf scale,format=i420,scale=1280:720,ass,scale,format=i420 -sws 10 -nosound -mc 0 "C:\Users\Selur\Desktop\One Punch Man S01E07.mkv" | x264 --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 48 --partitions i4x4,p4x4,p8x8,b8x8 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --non-deterministic --range tv --colormatrix bt709 --demuxer raw --input-res 1280x720 --input-csp i420 --input-range tv --input-depth 8 --fps 24000/1001 --output-depth 8 --output "E:\Temp\2022-04-11@16_35_20_0910_02.264" -

One thing I noticed, which is probably not related is that you are using 'baseline' profile. May I ask: Why? (It's ages since I saw someone using it, which is why I'm curious. )

1st result: It crashed with:

x264 output: 26542 frames: 103.91 fps, 1511.37 kb/s
exitCode: -1073741819 and exitStatus: 1

(so around the same position)
-> testing more

Cu Selur

[Selur]

Now I tried what would happen if I did not embed the subtitles:

ffmpeg -y -loglevel fatal -noautorotate -nostdin -threads 8 -i "C:\Users\Selur\Desktop\One Punch Man S01E07.mkv" -map 0:0 -an -sn -vf scale=1280:720,zscale=rangein=tv:range=tv -pix_fmt yuv420p -vsync 0  -sws_flags spline -f rawvideo - | x264 --preset slower --profile baseline --level 3.1 --ref 5 --sync-lookahead 48 --partitions i4x4,p4x4,p8x8,b8x8 --vbv-maxrate 42000 --vbv-bufsize 14000 --sar 1:1 --non-deterministic --range tv --colormatrix bt709 --demuxer raw --input-res 1280x720 --input-csp i420 --input-range tv --input-depth 8 --fps 24000/1001 --output-depth 8 --output "E:\Temp\2022-04-11@16_41_54_0310_02.264" -

(this time Hybrid uses FFmpeg as decoder.)
-> encoding worked fine.

I also noticed that using Vapoursynth the 'Vapoursynth Preview' does not work here and should not work for you either, since the '# Loading Plugins'-section of the script is not loading 'SubText'. (fixed that here, running an encode now)

Cu Selur

[Shiguri]

One thing I noticed, which is probably not related is that you are using 'baseline' profile. May I ask: Why? (It's ages since I saw someone using it, which is why I'm curious. )

As detailed in the original post where I quite needlessly detailed what my usual reasons and such are, it's because for my specific case I need to prioritize bitrate and easy decode for streaming video, and it also appears that Unity prefers the Baseline profile as well. You'll also notice that I'm also using the fastdecode tune and have interleave enabled.

It's because I'm doing movie nights in VRChat and the player is designed to synchronize all watchers, which isn't an easy undertaking when you're dealing with 10+ people with varying bandwidths and hardware located all over the world. The player will sometimes struggle and break for everyone if it can't synchronize all viewers. It requires the encode to be at an acceptably low bitrate, decode to be fast, comply with Unity and VRC's standards, and for the videos to be uploaded to a good CDN with worldwide file replication (IE: Linode, maybe BunnyCDN).

I've even got friends who have been making their own video players (the default has been Merlin's) to try to do a better job of handling things (we currently use this one).

I've considered learning more about HLS and DASH but I'll get into all of that another time.

TL;DR: Because I'm coordinating mass watching sessions in a Unity-based VR game and that requires settings geared far more towards streaming video rather than quality.

Okay I'll start poking around with the stuff you're mentioning.

[Selur]

When opening the Avisynth Filter Preview, I noticed that aroung 26726 the left view (original without subtitles), and the right view (where the subtitle filter is applied) seem to lose sync.
(this is happening in Avisynth and Vapoursynth)
Disabling the subtitle filter this does not happen.
So the issue seems to be cause thy these subtitles in combination with the subtitle renderers.
(Also happens when using DGDecNV it's not a bug in LWLibavSource)