I don't understand the hype for 1080p/24 output

[Bohdy]

I'm not sure if this is the right forum to address this, but I wanted to try and understand this madness.

Correct me if I am wrong, but with a standard interlaced 2:3 pulldown applied to Blu-Ray films, most TV's with built-in IVTC will restore the movie to the progressive pulled-down 23.976 framerate. The 24 mode that (avsforum) people are going crazy about is just the console doing the IVTC and "pull-up" instead of the the TV right? I hear people claiming "wow, the motion is so much better, blah blah blah", but who can really see the difference between 23.976 and 24 fps?

Can someone explain this for me or point out where I am wrong in my thinking?

 

[RobertR1]

The 24 native is preserved and then speed up to multiples of 24 (24 itself is too slow) thus no pulldown is done and you don't get "judder." Judder is like lack of aliasing. Once you notice/understand it, you can't ignore it

 

[Bohdy]

Can you explain why there would be judder at 23.976 fps? Or why the TV can't just speed it up to 24fps internally?

 

[RobertR1]

Can you explain why there would be jidder at 23.976 fps? Or why the TV can't just speed it up to 24fps internally?

http://www.avsforum.com/avs-vb/showthread.php?t=731479

A perfect thread about 24 with lots of techincal (too much for me) information from insiders. Enjoy

 

[Bohdy]

http://www.avsforum.com/avs-vb/showthread.php?t=731479

A perfect thread about 24 with lots of techincal (too much for me) information from insiders. Enjoy

It sounds like either they don't understand the IVTC process, or I don't understand how HDTV's do it. Ordinarily, any redundant frames (that cause judder) are removed during IVTC. If it is a matter of syncing each video frame to the display device's refresh rate (do LCD screens even have a refresh rate?), then it would be trivial for the device to simply display one frame every third refresh @ 72hz after IVTC. 1080p/24 sounds like more avsforum voodoo to me, because the fact is that Telecined 1080i + IVTC is trivially interchangeable with 1080p/24.

 

[Colourless]

Problem is TVs display at 50Hz, 60Hz or 100Hz depending where you are and what sort of TV you have and results in speedup (PAL sets) or judder (NTSC and HiDef). The idea with sending native 24 FPS signals is the TV can instead run at 72HZ or 120 HZ or some other multiple of 24 and display the film source with no judder.

 

[Bohdy]

The idea with sending native 24 FPS signals is the TV can instead run at 72HZ or 120 HZ or some other multiple of 24 and display the film source with no judder.

But my point is that if the TV can do that with 24fps input, it can just as easily do the same with telecined 60i input, since it is perfectly interconvertible with the original frames.

 

[Ventresca]

The 24 native is preserved and then speed up to multiples of 24 (24 itself is too slow) thus no pulldown is done and you don't get "judder." Judder is like lack of aliasing. Once you notice/understand it, you can't ignore it

Well you dont speed up the movie, if a display is 24p capable, it will diplay 24 flame/sec. without altering the speed of the movie in any way.

The 24p is not too slow, since it is the standard speed the most part of the professional movie are made of, and you must shoot at 24p if you want to have a "film look motion cadence" to make your film looking professional and not like unprofessional video

Bye,
Ventresca.

 

[Ventresca]

But my point is that if the TV can do that with 24fps input, it can just as easily do the same with telecined 60i input, since it is perfectly interconvertible with the original frames.

Yes, but doing a reverse telecine does not generally came without loss of quality.

Then you display a 24p source as 24p all the frames dont get any kind of processing, so no loss of quality.

As i understand it, when you do a reverse telecine, to extract the original 24p from a 60i source, the originals A, B, and D frames can be recovered by using two fields from the 60i source, but the C fame can't because it is splitted across the second field of the third 60i frame and the first field of the fourth 60i frame, you must need to decompress and recompress it into a new 24p frame, and this introduce a loss of quality.

Thats why this conversion should be avoided if possible.

Bye,
Ventresca.

 

[Bohdy]

As i understand it, when you do a reverse telecine, to extract the original 24p from a 60i source, the originals A, B, and D frames can be recovered by using two fields from the 60i source, but the C fame can't because it is splitted across the second field of the third 60i frame and the first field of the fourth 60i frame, you must need to decompress and recompress it into a new 24p frame, and this introduce a loss of quality.

Thats why this conversion should be avoided if possible.

No, sorry but that is not correct. Given a standard telecine pattern, all frames can be recovered perfectly. With some non-standard patterns TV's can IVTC incorrectly, inducing some quality loss. There are adaptive inverse teleciners that can determine the right pattern most of the time, however.

 

[Ventresca]

No, sorry but that is not correct. Given a standard telecine pattern, all frames can be recovered perfectly. With some non-standard patterns TV's can IVTC incorrectly, inducing some quality loss. There are adaptive inverse teleciners that can determine the right pattern most of the time, however.

I did not said you can't recover all the frames perfectly, but you must need to do a decompression and recompression process to recover the C frame because it is splitted across two diferent fields , and , unless it is an uncompresseed video, this process introduce a loss of quality.

Well, i am 100% sure this is what happens for digital video with standard 2:3 pulldown, because i work with it.

Bye,
Ventresca.

 

[Bohdy]

I did not said you can't recover all the frames perfectly, but you must need to do a decompression and recompression process to recover the C frame because it is splitted across two diferent fields , and this introduce a loss of quality.

I am 100% sure this is what happens for digital video with standard 2:3 pulldown, because i work with it.

Bye,
Ventresca.

I don't know why you think there is recompression involved. Both the odd and even fields of the C frame are weaved with the B and D frames respectively, and as such can be extracted and re-weaved (usually leading to some duplicate frames, which are then decimated). Weaving fields is a lossless process.

This Wiki diagram explains it quite well

I also work with video...

 

[Ventresca]

I don't know why you think there is recompression involved.

I also work with video...

This is not a diagram of a reverse telecine

This is a diagram of a reverse telecine :

Like you can see, in orderer to recover the C frame you have need to decompress the green and the magenta frames and recompress them into a new 24p frame, this process introduce a loss of quality.

 

[Bohdy]

This is not a diagram of a reverse telecine

If you see how Telecine is done, you can see how to reverse it and that it is reversible. But your diagram is more suitable

Like you can see, in order to recovering the C frame you have need to decompress the green and the magenta frames and recompress them into a new 24p frame, this process, introduce a loss of quality.

That's where you are mistaken. There is no decompression or recompression being done (those words are not applicable in this context), but merely fields being separated and then weaved again. There is no loss of quality in this process as long as the final odd and even fields of each frame are the original odd and even fields of each frame (which they are in the case of proper Inverse Telecine). You could try to argue this, but you would be wrong. I know this 110%

 

[Ventresca]

There is no loss of quality in this process as long as the final odd and even fields of each frame are the original odd and even fields of each frame (which they are in the case of proper Inverse Telecine). You could try to argue this, but you would be wrong. I know this 110%

From what i know, there is loss of quality , in term of image degradation (you can find the technical explanation of why, here ) , othervise 24p Advanced would have not been introduced

The advanced pulldown avoids this as no 24p frame is segmented across two separate 60i frames, but it is intended only for editing purpouse as a way to capture and extract without loss of quality the 24p with the reverse telecine.



Bye,
Ventresca.

 

[Bohdy]

Nope , there is loss of quality, othervise 24p Advanced would have not been introduced

That's backwards logic. Presumably 24p was introduced to save the TV doing the IVTC (and as such making sure it's done properly), because I can't think of any other reason for it (considering that you are not correct). When dealing with a progressively encoded source, weaving is simply inserting odd and even fields together into a frame; it's totally reversible--even when the fields come from different frames.

Edit:
Actually, I see what you were getting at when you mentioned 24A ("advanced pulldown"). This basically depends on in what order the telecine and encoding were done. I'm not sure how the "next-gen" formats do it, but DVD can basically do it in a couple of ways. One is encoding the 24p content, with flags inserted into the stream to tell the player player to perform telecine at play-time. This is totally reversible. The other is telecine, then followed by interlaced encoding. This encodes the source differently, which could cause inconsistencies and some quality loss that you speak of due to the fields being mixed between frames. However, in the latter case I don't thing even the "24p mode" can perfectly reverse this unless it is using the 24A telecine pattern. In that case, though you once again wouldn't need to output in 24p if the TV understands this mode of telecine. Hopefully that explains the confusion correctly.

 

[Ventresca]

That's backwards logic. Presumably 24p was introduced to save the TV doing the IVTC (and as such making sure it's done properly), because I can't think of any other reason for it (considering that you are not correct). When dealing with a progressively encoded source, weaving is simply inserting odd and even fields together into a frame; it's totally reversible--even when the fields come from different frames.

The 24p advanced can't be displayed on a normal TV as i would look really jerky, it is intended just as a camera recording and editing format.

24p advanced is the only way you can avoid decompression or recompression (this always introduce loss of quality, unless you are dealing with uncompressed materail) , to have a recovered 24p that is first-generation quality.

When you deal with compressed video, like mpeg2, every time it's recompressed , there is a degradation and no one , i hope , would argue on this.

So with this in mind, as we alredy stated that you have need to decompress and recompress the video in order to recover the original 24p frame, it is pretty easy to understand why recovering the 24p from a standard 3:2 pulldown introduce a degradation, as explained here

It is all so well explained in the link i gave you, i dont have more to add


Bye,
Ventresca.

 

[Colourless]

When doing telecine traditionally some filtering is done to reduce flicker, which means it's impossible to recover the original 24p source.

 

[Bohdy]

When doing telecine traditionally some filtering is done to reduce flicker, which means it's impossible to recover the original 24p source.

That I haven't heard before. But it still doesn't explain how 24p output would be anymore useful than 2:3 interlaced in that case. The source is already ruined.

 

[Fruitfrenzy]

The most sensible reason I can see for doing the conversion in the DVD Player/Console or whatever is that they can use the flags in the bitstream to remove the guess work about which fields go together. MPEG2 and the more recent standards have (optional) flags specifically labelling fields that go together to make progressive frames. In the TV none of this information is avaialble so they use image analysis to guess. On occasion they get it wrong (particularly around edits/cut scenes) so the motion looks jerky.

Cost restrictions on consumer TVs obviously limits how much image analysis they can do in real time.