Periodic jerkiness to encoded video?

[Tyler.Durden]

Hi,

I've often come to this forum looking for info & tips over many years, so finally decided to sign up.

I have a question about a 1080p source I'm trying to work with and I'm not sure where to post my Q - or what to do with the source.

I've got a 25fps progressive video source and am looking to recompress/re-encode (not sure what the correct term is) to reduce the overall size as it's almost 8.5GB with a high bitrate.

When I check the source with MediaInfo, it tells me that the original frame rate was 24fps, but it is now 25fps? (see photo). I am assuming this is contributing to my current problem which is...



When I run the source through my program (I'm currently using Staxrip as I found MeGUI was a bit of a learning curve) and apply all of my x264 settings to the source, the resulting file (now a 4GB .mkv file and not an 8.5GB .mkv) looks perfectly fine & the audio is also fine EXCEPT - every few seconds, there is a periodic jerkiness to the video like it's skipping a few frames?

I'm not quite sure why this is occurring as I didn't attempt to change the fps when reducing the file size and also, the original (large file) doesn't have this jerkiness at all, so it's not the source? Obviously something within my process is causing this to happen, but I don't know what it is? I've run thousands of sources through Staxrip over the years, but I've never encountered a problem like this with a resulting file?

Has anyone come across this type of thing before and is there a way to mitigate this effect? Or would trying to restore it to the original 24fps help in any way? If so, how would I do that? I have limited knowledge with .avs files - I just know the basics as I found Avisynth harder to understand than MeGUI.

Would appreciate any insight from anyone.

[manono]

You should probably supply a 10-second or so clip, one with steady movement, so we can have a look.

[Tyler.Durden]

It is quite a dark movie so hard to find something with good movement. See attached.

The jerkiness I refer to is very subtle, but I'm sure if you strip it down you guys will pick up on it. Seems to happen every couple of seconds, almost like it gets stuck for a fraction and then jerks forward - but it is subtle and happens throughout the movie.

I tried it on my PC, a laptop, a spare WDTV I had lying around, a Minix Neo 8 and a Popcorn Hour, got the same results with them all. Very slight, but still there.

[manono]

I don't see anything out of the ordinary. Sorry. Maybe someone else will spot something.

[Tyler.Durden]

I don't see anything out of the ordinary. Sorry. Maybe someone else will spot something.

No problem. I haven't had any other replies, so perhaps no one else spotted it either. Rather than focusing on the short clip I uploaded then, can anyone suggest anything based on the information I provided? i.e. Would the initial work to change it from 24fps to 25fps have caused anything? Would trying to restore it to the original 24fps help in any way? If so, how would I do that with the limited knowledge I have with .avs files?

[jagabo]

I see normal 25 fps to 59.94 fps frame duplication (3:2:3:2:2) judder. But that's not much different than 23.976 fps to 59.94 fps (3:2) judder inherent to all 24 fps video viewed on 60 Hz displays. Stepping through the video frame by frame shows no duplicate or missing frames. The camerawork might not be perfectly smooth. There might be an issue where motion prediction has introduced a bit of jerkiness. For example a vertical edge might have moved 2.5 pixels to the left between two frames then another 2.5 pixels at the next frame, but the motion prediction moved it 3 pixels and 2 pixels. So that edge is slightly displaced in the middle frame.

Conversion from 24 fps to 25 fps could cause problems. For example, one way to do it is to duplicate every 24th frame, turning 24 frames into 25. That creates a very obvious jerk every second. I've attached an example that shows this poor method. But that's not the case in your video. What's usually done with film sources (24 fps) in PAL releases (25 fps) is the film is sped up to 25 fps. That reduces the running time by about 4 percent so the audio has to be adjusted too. That doesn't cause additional jerkiness. In fact, it's the least jerky method for PAL TV. That appears to be what was done with your video.

[Tyler.Durden]

Conversion from 24 fps to 25 fps could cause problems. For example, one way to do it is to duplicate every 24th frame, turning 24 frames into 25. That creates a very obvious jerk every second. I've attached an example that shows this poor method. But that's not the case in your video.

Wow, you really know your stuff - thanks for your help. Your example was a very good way to demonstrate what you meant - I could see the judder every second and it's identical to what I am seeing in my recompress/re-encode (not sure what the correct term is) except mine's only happening every 4-5 seconds which must be due to the 3:2:3:2:2 you mentioned.

It's much more noticeable on my TV than it is on my PC (the downside to having a 95" LED), but I went into my TV settings and played around with the LED motion settings and got things to a point where it's now undetectable. I never came across this before in anything I've done - so I was worried I screwed up somewhere without realising it. Must be just a funny source to some effect - I've no idea why the encoder changed the frame rate?

Thanks for your help jagabo .

[jagabo]

It's much more noticeable on my TV than it is on my PC (the downside to having a 95" LED), but I went into my TV settings and played around with the LED motion settings and got things to a point where it's now undetectable.

Something like the attached file? That's 25 fps motion interpolated to 60 fps.

I've no idea why the encoder changed the frame rate?

Someone probably took a 24 fps source and converted it to 25 fps for smooth playback on PAL TV.