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Discussion Starter · #1 ·
I know that film source DVDs are encoded with 3:2 pulldown to get the correct frame rate out at 480i. A thread about the new Samsung RPTVs got me thinking and I have several quesitons:


1) Does anyone know if 3:2 pulldown is also employed for HDTV broadcast movies?


2) Is the answer different depending on if it is broadcast in 720P or 1080i?


3) Is there some other frame rate compensation used in HDTV?


4) Is a HDTV monitor with 3:2 pulldown useful when viewing HD video material?


-phil
 

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Hi Phil,


I'm not following you around - honest :)


But why would an external line doubler or scaler ever have to worry about 3:2 (or 2:2) pulldown if the DVD player already accounted for it in its 480i output?


Pulldown just means that the progressive field generator uses the correct number of input interlaced fields when building the correct number of output progressive fields.



Ken
 

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With proper encoding the HDTV or DVD's decoder shouldn't need to know about the 3:2 sequence because the redundant frames (fields) are (should) be eliminated by the encoder as a part of the compression process. What shoud happen is that the encoder detects the duplicate frames (fields as the case may be) and replaces them with "repeat" flags, thus avoiding sending the duplicate data - and gaining you an extra 20% or so compression/bandwidth effeciency.


The compression effeciency should be even higher when the HDTV signal is encoded in 720p because if the 720p signal is running at 60 frames per second, then three out of every five frames will be duplicates, for which the ATSC/MPEG standards also allow the duplicate frames to be replaced with repeat flags. Else the obsure "table 3" which enumerates the 18 ATSC standards actually defines 480p, 720p, AND 1080p (!!!) among the available formats - all at 24 FRAMES/second.


Of course things arent always done as they should be. So you really only need 3:2 pull-down recognition in the receiver when film source is encoded at 60 fields/second video, such as with a tradional analog broadcast.
 

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OK, I thought I understood this, now I'm a little confused. So tell me if the following is correct.


DVD out at 480i -> needs 3:2 pulldown to accurately get 480p

DVD out at 480p -> no need


HDTV 480i and 480p same as for DVD

HDTV at 720p -> no need, already progressive

HDTV at 1080i -> If this is 30 frames or 60 fields/s it needs 3:2 pulldown if you desire to convert it to either 720p or 1080p - Is this correct?


Thanks

Matt
 

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Quote:
Originally posted by MattJ
OK, I thought I understood this, now I'm a little confused. So tell me if the following is correct.


DVD out at 480i -> needs 3:2 pulldown to accurately get 480p

DVD out at 480p -> no need


HDTV 480i and 480p same as for DVD

HDTV at 720p -> no need, already progressive

HDTV at 1080i -> If this is 30 frames or 60 fields/s it needs 3:2 pulldown if you desire to convert it to either 720p or 1080p - Is this correct?


Thanks

Matt
DVD out at 480i -> needs 3:2 pulldown conversion in the receiver/display to convert to to 480p or 720p or whatever.


DVD out at 480p -> either its a properly encoded DVD and its already progressive, or its 480i on the disc in the 3:2 conversion is being done in the player.


HDTV at 720p -> already progressive, if the broadcasters arent a bunch of idiots then they are sending this out at 24fps with repeat flags that tell the tuner/decoder which frames to repeat to make it display with proper pulldown at 60 frames/second. Hopefully they are doing this because then you save a bunch of bandwidth which means you dont have to compress as much detail out of the picture. Or you can use the bandwidth savings to run multiple program streams, but that's another story.


HDTV at 1080i -> yep this can get tricky, because 1080 (contrary to popular myth) be broadcast as 24 progressive frames/second, 30 progressive frames/second or 60 interlaced fields per second. Film source hopefully being sent at 24, or with the proper repeat flags - so that you likewise can use the saved bandwidth to squeeze in more detail, all done at the encoder.


The tricky part being if you have a ATSC decoder (such as a MITS) that puts out only 1080i - which will take a progressive 1080 signal and (hopefully) properly generate the 1080i signal with the 3:2 pulldown for display on a 1080i monitor. Likewise if your receiver downconverts to 720p, there is still no problem with a properly encoded signal based on film or 24 fps video. The strangeness occurs if you feed the output of a 1080i receiver into a display that downconverts it to 720p. This might occur with some plasma or LCD or whatever units, but it doesnt have to be that way if the display is properly designed. Somewhere I read that plasmas and DLPs can refresh at something like 50 kilohertz, and LCDs can be made that are "flicker free", cause they'll refresh at whatever rate you feed them.


Of course if you have a reciever that takes a 1080i output from whatever source and sends it into display that actually downconverts and deinterlaces it to force it down the throat of a display that runs at an actual refresh rate of 720p -- then you get what you deserve.





HDTV
 

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Discussion Starter · #6 ·
Ok Ken,


It sounds like we're going to have a showdown in Sugar Land. :)


After further research, I've decided that I was confused about what 3:2 pulldown was doing. I thought it was doing more than just helping with deinterlacing. I thought it was somehow helping cure the 24 fps -> 60 fps cadence mismatch. It looks like we won't be solving that until we get 72 fps sources and displays.


-phil
 

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Phil,


Too Funny!


I didn't notice until you just mentioned it!


I live in the Lakes right behind CompUSA and CircuitCity.


Now that I know where you're from I see more merit in your words. It's the end of the day and I'm tired but I'm certain that it does relate to the 24fps and 60fps. Otherwise why is it for film sources?


Someone will help us out here.


Nice to meet you!

Ken
 

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It seems the discussion on this topic has ended, but this question has been bothering me as well.


Based on the explanations here, the you are stuck with the cadence issue on any current HD technology. Even if you have a projector capable of 720p or 1080p at 48 or 72 Hz, any receiver that meets the ATSC guidelines will have to output at 30 or 60Hz. Does this make sence to anyone? I thought I saw where Sony's HD cameras are 24fps or some multiple. Does that mean even HD "Video" media goes through some temporal jumping jack before being displayed on an HD "Video" device?


Seems strange. Can anyone clarify this? Where are my perceptions incorrect?


Brian
 

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Here's a link to an excellent, thorough explication of 3:2 (or, more correctly 2:3) pulldown: 3:2 Pulldown Explained


Briefly: The function of 3:2 pulldown is to enable conversion of 24-frame-per-second film to 30-frame-per-second video. Since our longstanding TV system is interlaced, that means converting to 60 fields (2 interlaced fields per video frame) per second. Since 60 is not an even multiple of 24, the solution is to use the first film frame for the first two video fields, the second film frame for the next three video fields, the third film frame for the next two video fields, and so on. This works well, although it introduces an artifact called "judder" to horizontally moving objects, since they are, in effect, speeding up and slowing down from frame to frame.


Now, this is all ducky so long as the display is interlaced, but what if you want to make it progressive? Current progressive-scan sets put up 60 frames per second, so 60-field-per-second interlaced video must be converted to 60-frame-per-second progressive. The easiest way to do this is simply to combine the first and second fields from the interlaced source into a single progressive frame, scan that frame twice, then combine the next two fields, scan them twice, and so on. But if you do this with material that has been converted from film, you wind up with some video frames that contain fields originating from different film frames, which can look pretty awful.


So to do the de-interlacing correctly, it is necessary to detect 3:2 pulldown when it is present and undo it during the de-interlacing process to ensure that film frames match up to video frames. Then it has to be reapplied to the assembled frames so that you wind up with 60 frames per second (first two video frames from the first film frame, next three video frames from the second film frame, next two video frames from the third film frame, and so on) instead of 48.


So that's what it's all about. Whether 3:2 pulldown compensation should be applied is strictly a function of whether the material being displayed progressively originated on film or on video.


Incidentally, this means you don't need to worry about this if the display is interlaced.
 

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So is that a bonus point for 1080i displays?


Also, it doesn't answer the question why the new HD standardsare not applied on the production end?
 

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Quote:
Originally posted by Briands
So is that a bonus point for 1080i displays?


Also, it doesn't answer the question why the new HD standardsare not applied on the production end?
Yes, it's a bonus point for 1080i displays. On the other hand, I'd much rather watch 720p material (what there is of it) at 720p rather than 1080i.


Not sure what you mean in the second question. Do you mean why isn't all film-based material transmitted at 24 fps rather than 30 or 60? Probably because SOP for video transfers has been to make them conform to video frame rates, so that's what's available. The encoder on the broadcast end could undo that, but that might require somebody paying attention!


It's too bad that manufacturers seem uninterested in making displays that can run at 72 fps as well as 60. That would solve some problems.
 
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