23.976fps "judder"

[madhatter300871]

Hello, my second post today, I have too much spare time on my hands !

I'll try to keep this direct and to the point.

My LCD TV accepts 24hz, 25hz, 50hz and 60hz signals at a few different resolutions (1920x1080 being native).

If I play a 23.976fps video into my TV with my nvidia card set to output at 50HZ, I get judder on horizontal panning (about every second).

If I play a 23.976fps video into my TV with my nvidia card set to output at 60HZ, I don't get judder on horizontal panning.

If I play a 23.976fps video into my TV with my nvidia card set to output at 24HZ, I don't get judder but the audio goes right out of sync !

I "think" I know all about 24fps film and 25fps PAL(50 fields ps) and 30fps NTSC(60 fields ps) video and how pull down is used for displaying film on video equipment. I also "think" I know all about pulldown removal and the benefits it brings.

However :-

My question is that the maths doesn't seem to add up to me so I am sure I am missing a very simple point here.

Surely 23.976 will judder on both 50HZ and 60HZ displays but should display fine on a 24HZ display ? Why does it play judder free on a 60Hz display ? What is the crucial point here that I am missing ?

Thanks in advance, as usual.

[madhatter300871]

When my LCD TV receives a 23.976 signal, does it perform 2:3 pulldown automatically ? Is that why 23.976 video playback is smooth at 60Hz ?

I have just played a 25fps video with my nvidia card set to output at 60HZ. It played perfectly smoothly ! (Plays perfectly smoothly with nvidia set to output at 50Hz as well).

While the answer seems simple, just leave the nvidia card set to output at 60Hz, it just doesn't seem right. I need to understand or I'll never sleep !!

[Ghitulescu]

Most LCD TVs are driven at high refresh rates, typically 60Hz or multiples thereof. Only a few AFAIK can natively display 24fps. That's so because this way the LCD technology works. AFAIK there's no TV that can display the correct info: 23.976 instead of 24 or 59.94 instead of 60.

[madhatter300871]

So do you know why a 23.976 (or 24p) frame rate movie doesn't show "judder" when played at 60HZ ?

[2Bdecided]

Either the TV detects the 3:2 pulldown and recovers the 24fps, displaying them at, say, 72fps.

Or else you have been watching 3-2 pulldown so much that you don't notice the judder. You have been conditioned to think that that's what real 24p looks like.

Cheers,
David.

[madhatter300871]

No, I know what the judder looks like and have not been conditioned. Living in the UK I have been watching 25fps 50Hz video all my life, judder from NTSC sources is very easy to spot for me.

How can the TV detect 3:2 pulldown when the original is 23.976 fps ? Surely the TV will perform 3:2 pulldown in order to display it on a 60Hz TV not detect it and remove it. (Or the graphics card if not the TV).

[2Bdecided]

The PC converts the 24fps to 60fps using 3-2 pulldown. The TV detects that the 60Hz input consists of a 3-2 pulldown sequence, undoes it to recover the original 24fps, and displays that at an integer multiple of 24fps (e.g. 72fps) - or at a high non-integer multiple (e.g. 200Hz) where the judder from 24>200 conversion is almost unnoticeable.

Pulling clean 24fps out of a 60fps input containing 3-2 pulldown is a very common feature in modern TVs.

Cheers,
David.

[Blue_MiSfit]

Try using MadVR to feed your TV. I just bought a new Vizio LCD, and was UTTERLY astonished at how much it eliminated judder playing 24p content at 59.94 Hz refresh on the LCD. Truly astonishing!

Derek

[SeeMoreDigital]

The PC converts the 24fps to 60fps using 3-2 pulldown. For the sake of clarity...

Unless people elect to speed-up their DVD and Blu-ray disc back-ups, there are very few sources actually encoded at 24fps.

[madhatter300871]

Yes, I didnt think there where many sources encoded at 24fps either.

So, are we saying the PC does the 3:2 pulldown, and not the TV ? OK, that makes sense, I'm ok with that.

So, any ideas why the audio is out of sync if I set the PC to output at 24fps ?

The TV does accept 24hz, 25hz, 50hz and 60hz (as shown in the Nvidia config dialogue). 24hz is judder free but the audio is just way out of sync.

This is just to complete the answer as to be honest, setting the PC to 60Hz does the trick.

I have installed and am now using madVR. Works fine, can't say I see any difference to other renderers though but I'll stick with it and give it a go.

[Blue_MiSfit]

Try pulling your TV refresh down to 59hz as well. You may only see this in the Windows "Screen Resolution" controls, under advanced. This made a big difference for me.

[Ghitulescu]

Most LCD TVs are driven at high refresh rates, typically 60Hz or multiples thereof.

Try using MadVR to feed your TV. I just bought a new Vizio LCD, and was UTTERLY astonished at how much it eliminated judder playing 24p content at 59.94 Hz refresh on the LCD. Truly astonishing

This is just to complete the answer as to be honest, setting the PC to 60Hz does the trick.

As I said, few LCD TV really accept a true 24fps (23.976) source, and even if, this is upconverted to the working frequency of the display panel (usually 60).

So it's better to feed it with the native fps of the LCD (which you can control to be judder free), just to avoid any uncontrollable and nasty surprises.

[lqy]

I think you are going about this backwards. Have you checked your source material is 24 fps progressive ? And have you checked the TV is receiving 24 fps progressive, i.e. your computer is not changing the format ?

[zapco]

OP, my money is on frame interpolation, i.e. some kind of "smooth motion" function. These "features" seem to be enabled by default on most LCDs these days, and they take 24/60 Hz up to 120 Hz (the usual native processing rate of most LCDs now, if not 240) by creating new intermediate frames.

That stuff drives me right up the f*cking wall - I can literally walk past a display in a store, see a grand total of one second of video and know if frame interpolation is being used or not...makes everything look like a damned video game.

Note that some (many?) LCDs can properly display 23.976 by just doing frame replication: 24 * 5 = 120. Of course, you may run into issues (occasional glitches) with 23.976 vs. 24, depending on the TV, but the only TV I've had long-term experience with in displaying 23.976 is my Pioneer KRP-500M plasma, which can do perfect frame replication up to 72 Hz and never glitches.

Regarding A/V sync issues @ 23.976...that's not a bug, it's a feature! of your Nvidia card. Both my 8400 and 9500 do the exact same thing: perfect A/V sync @ 59.94/60 Hz, wildly varying sync @ 23.976/24. Drivers didn't make any difference when I was messing around with them several months ago, so I just gave up and went ATI. I've since ditched that and gone to a Dune media player, so I have no idea if Nvidia has addressed that or not.

[Ghitulescu]

And now I come again with the conspiracy theories :p

There will always be trouble with the interface PC-A/V chain, I haven't seen in my whole life a single "expansion card" (as it was called since the PS/2 model of IBM) that would allowed perfect digital output for regular consumers, be it audio or video. I mean for purists - there will always be a recornversion somewhere in the chain, or a slight desynch or a missing bit, or or or or ...

Don't believe me, am I exagerating? Just scroll into the archive of doom9, videohelp, hydrogenaudio & co and see ...

The only way of getting out is to buy and use professional tools (not even prosumers - these are the most perverse toys). It would cost, but, hey, so does a 20000€ HiFi-chain. Perfection costs ...

[madhatter300871]

Source material is 23.976fps progressive, its a TS from a blu-ray disc (A disc I own I hasten to add !)

Wether the TV is receiving 24fps I cannot tell, all I can do is have blind faith that when I ask my Nvidia card to output at 24fps it really is outputting at 24fps.

I don't think I'm going about this backwards, I've been quite methodical in my approach and like I say I do actually get a good result just by setting the Nvidia to output at 60Hz, but I'm enjoying engaging myself in this question so please do keep answering if you can shed any light.

As a side question, some people are referring to 23.976 as 24, they write in their text "24fps(23.976)". Im getting a bit confused with this, I thought "fim" was filmed at 24fps then slowed to 23.976 then 3:2 pulldown applied to get it to 59.94fps for display on a 60Hz TV. So when I talk about frame rates can I interchange the term 23.976 and 24, also can I interchange 59.94 and 60 or should I really be thinking of these four different frame rates independently.

Also, I thought 3:2 pulldown was only needed when converting 24fps frame based film to NTSC field based video, once a video is frame based...well a frame is a frame and you can just display them at the required rate.

OK, I'm getting off the beaten track perhaps.

I take on board the point about Nvidia a/v sync, thanks.

[2Bdecided]

So when I talk about frame rates can I interchange the term 23.976 and 24, also can I interchange 59.94 and 60 or should I really be thinking of these four different frame rates independently.Given that there's no real commercial 24fps or 60fps content available to consumers, I always say 24 and 60 when I always mean 24/1.001 and 60/1.001. Other people are more careful. ;) Some cameras claim to shoot 24fps exactly, but that's hardly conducive to keeping the audio in-sync without resampling, given most potential uses actually require 24/1.001.

Also, I thought 3:2 pulldown was only needed when converting 24fps frame based film to NTSC field based video, once a video is frame based...well a frame is a frame and you can just display them at the required rate.No, 60fps progressive video was output by in NTSC "progressive" DVD players over component video long before true 24fps was available via HDMI or component. 60fps progressive needs the same 3-2 pulldown pattern as 60 interlaced fields per second - just using frames instead of fields.

Cheers,
David.

[madhatter300871]

David, thanks for the reply.

OK, interesting. So the general thinking is that we can say "24" and "60" as long as we understand that under the bonnet we really mean "23.976" and "59.94". OK, no problem with that.

So do you think, then, that when I set my nvidia card to output at 24Hz it really is outputting at 24Hz, not 23.976, and that could be why the audio is all out of whack !

Setting my nvidia to output at 60Hz gives me smooth playback with in-synch audio, I'm just trying to get to the bottom of why I cant play back at 24Hz when both my TV and Nvidia card do "claim" to work in 24Hz. Could it be the difference between 24 and 23.976 ?

I thnik I'm pretty close to giving up on this question, It's not overly important but I am so intruiged as to what the underlying cause of the problem is.

Incidently, do you know if "film" material is really filmed at 24fps or is it actually filmed at 23.976fps. I'm just curious as I was lead to beleive it was shot in 24 then either sped up to 25fps OR slowed to 23.976 then 3:2 pulldown applied.

Thanks.

[SeeMoreDigital]

OK, interesting. So the general thinking is that we can say "24" and "60" as long as we understand that under the bonnet we really mean "23.976" and "59.94".No not really.

Given the Blu-ray standard fully supports video encoded at 24.000fps and we now have display devices that fully support 24Hz frequencies, it's important to provide accurate information.


Cheers