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1080P/60 with film sources observation/ question

J

JerryL

Well-known member
Usually I would advise to watch film based material in its native 24fps. But the other day I had the occasion to see a movie played back via a Premiere player at 1080P/60. Given that 24 does not smoothly go into 60- I was expecting the result to be very uneven motion. Indeed, I have seen 24fps sources played back via my now prior Lumagen Radiance processor to 30fps and the result was terrible. (I was just curious). So I was astounded that somehow the Kaleidescape player could take a 24fps source and somehow frame scale it to 60 with the result being very smooth, and yet still very film like, motion. Can anyone lend some insight into this? The result of this experience has me wondering about just using this mode for movie playback all the time because on my large screen (lucky problems!) sometimes 24fps playback can exhibit a but too much judder. I would have worried it would be a smooth motion mode like some of the TV manufacturers use- where film based sources now look like they were shot on video, which is a real pet peeve for me. But this looked completely film like and yet, it was smoother....

Anyone?
 
Conversion of 24fps material to 30fps just plain looks awful, so it's not a surprise that you had bad results there.

Conversion of 24-frame material to 60-frame output rates is called "3:2 pulldown". Since 24 doesn't go evenly into 60, the player will display some film frames twice, and some frames three times, to give you a total of 60 frames per second at the output. This uneven conversion produces what's called "judder" in the output, which is more or less visible depending upon the type of motion in the scene.

The pity is that many displays available today have native refresh rates that are actually even multiples of 24. For example, 24 goes into 120 five times, and into 240 ten times. But, the HDMI connections don't support these high frame rates, so the player can't just convert to a nice even multiple. The best it can do is 60.

However, some display devices are able to detect the "cadence" of the 24 frame per second content that has been converted to 60 frames per second, and then undo that conversion internally, so that they can then do the conversion to their native frame rate and take advantage of the judder-free even division.

Example: My Sony projector's native panel refresh rate is 120Hz. If I set up my player to convert 24fps film content to 60fps content, rather than just displaying each of those frames twice (which would still show the judder), the projector will instead internally recover the original 24fps signal, then frame-quintuple it for display on the 120Hz panels. The result is judder-free display.

Note that this type of conversion is not the same thing as the frame interpolation "smooth motion" features that many displays offer, and that are sometimes criticized as producing "soap opera effect". This is a straight-up display of the original signal, evenly mapped onto the panel's native rate.

So, it's possible that your display is doing this type of conversion, which it could not do with the Lumagen outputting 30fps.
 
I should expand by answering the natural question: if the display is going to have to undo the 60fps conversion to recover 24fps, then re-convert to 120 or 240Hz, why do the conversion in the player at all? Why not just output 24fps from the player for film content?

The simplest answer is that the player's user interface (especially Strato's) is designed to run at 60fps. If you set your player to allow mode switches, so that it will switch to 24fps for film content, the player and the display have to re-synchronize to the new frame rate, and this produces a delay in getting picture onto the screen. Some displays are very fast at this switch, so there's essentially no penalty. Some take quite awhile, and it's beneficial to just keep things at 60 all the time.

In the latter case, if the display can also reverse that 3:2 pulldown and convert to its native panel rate, you eliminate both the 3:2 judder and the re-synchornization delay.
 
MikeKobb said:
I should expand by answering the natural question: if the display is going to have to undo the 60fps conversion to recover 24fps, then re-convert to 120 or 240Hz, why do the conversion in the player at all? Why not just output 24fps from the player for film content?
Click to expand...

My extensions to that question would be whether "Pass through" allows for 480p24, or if 24fps is only available for HD quality, and why not just add the option for the UI to run at 24fps to eliminate the frequency re-sync?
 
Thanks for the excellent discussion!

I actually knew all of this before.

And I know just how awful it can look when you take a 24fps source and try it at 30. I was surprised how could it actually look better at 60.

The thing is the same projector exhibits an image with significantly more judder when displaying images when the Premiere player is outputting 24fps vs 60.

So my question is to wonder- what's going on. I'm not sure what my projector is doing and that could be part of the answer.

I do understand keeping the same output rate would be a way to avoid a display from going blank to re-sync- but I don't mind that. I generally prefer to watch things played back at 24 but it surprised me that this interaction between the Premiere player and my projector produced such a pleasing result....
 
Transepoch: DVDs (and DVD-quality content from the Movie Store) don't support 24fps, so there's not a lot of opportunity for 480p24 to exist. I'm not 100% sure if this is a possible format on Blu-ray disc, but I've never heard of one.

As for running the onscreen display at 24fps, we have tried it. Here's the thing. When you become a cinematographer, one of the things you learn is where the limits are for the types of camera moves you can make and still have your film look good when projected at 24fps. If you pan too quickly, for example, the motion kind of falls apart and looks jerky. There's too much change from frame to frame at 24fps for it to look good to the viewer. The same issue applies to the onscreen display. Some of the animations in the OSD produce changes over large areas of the screen, and at 24fps, the result is not pleasing.

We could potentially do a 24fps onscreen display, but we would have to adjust the animations to work within that constraint, and that's quite a bit of work that is not near the top of our priority list given some of the other things we're working on.

Jerry: That's interesting. I've actually seen that on an older DLP projector -- it looked better at 60fps than it did at 24. It's all down to the projector's internal processing. One thing I do on my Sony is to enable dark frame insertion. I lose a fair amount of light output, but the motion appears much smoother on 24fps sources because the projector is more closely emulating what an actual film projectors shutter mechanism does.
 
@MikeKobb - does this stand true for any device that has a panel refresh rate of 120Hz, or does it depend entirely on the processing of the panel/projector.

On a totally different subject matter and apologies to the OP - but Mike, was that your Home Cinema build I followed over on AVS recently - the one with the basement excavation to provide a lower floor level. Mighty impressive I must say - if it was.
 
Depends on the projector. Glad you enjoyed the thread. :) There seems to be a problem with the images right now, though. I need to check on that.
 
Thanks Mike!

I suspect you are right and the projector is doing its own frame interpolation- the projector is a Digital Projection Insight Laser. They are about to release a new input board that supports the newer HDCP and that will allow me to try to integrate a Lumagen. I am curious to see where the best settings end up.

I don't mind some judder in panning, as that is what makes film look the way it does. But sometimes it can look too much.

I also don't mind the resync from 60 to 24 (or multiple of 24) to watch a movie.

I do like the menus in UHD/60. It looks really nice!
 
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