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Old 09-15-2012, 09:16 PM - Thread Starter
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Hi,

Just had a question about DVD playback or suitable material for playback in 1080/24p.

I have a Samsung Plasma which runs Cinema Smooth and an Onkyo blu-ray player outputting at the above rate.

Are DVD videos both movies and TV series better when played at 24hz?

Also I heard that special features on blu-ray discs should not be played at the 1080/24p rate, is that true?

Thanks in advance!!
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Old 09-16-2012, 06:42 AM
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Originally Posted by billyboy35 View Post

Hi,

Just had a question about DVD playback or suitable material for playback in 1080/24p.

I have a Samsung Plasma which runs Cinema Smooth and an Onkyo blu-ray player outputting at the above rate.

Are DVD videos both movies and TV series better when played at 24hz?

Not all DVDs are suitable for 24hz conversion. They have to have film-based cadence. If they were originally video instead of film, they won't have 24hz content to recover.

As to "better", there are several factors:

How sensitive are you to to pulldown judder? Some people are and some aren't. It doesn't matter to me at all.

How well does your gear do the conversion?

Even film-based DVD can sometimes cause conversion trouble. Depending on how they were edited you may see playback glitches.

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Also I heard that special features on blu-ray discs should not be played at the 1080/24p rate, is that true?

They must be presuming the extras are video-based. That's often true, but not always.

-Bill
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Old 09-16-2012, 01:47 PM - Thread Starter
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Thx for the reply Bill!

Can you please tell me how you determine if a movie has film-based cadence?

Also when you say video compared to film, whats the difference?

Thanks
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Old 09-16-2012, 02:11 PM
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Originally Posted by billyboy35 View Post

Thx for the reply Bill!

Can you please tell me how you determine if a movie has film-based cadence?

You just have to know, or guess. There may be software that will analyze a DVD and tell you, but I don't know what it is.
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Also when you say video compared to film, whats the difference?

Film is from a film camera, like most movies from the beginning of cinema.

Video is from a video camera, initially recorded on magnetic tape, then on digital storage. Often used for concert videos and some TV.

Film is traditionally 23.976hz, often called 24hz, although 24.0hz itself also exists.

Video in NTSC regions like North America has been about 30hz. In PAL countries is has been 25hz.

DVD decoding produces 480i60 in NTSC, 576i50 in PAL. If the NTSC DVD was originally film-based, you can try to recover the 23.976hz content, which may or may not be feasible in any given case.

-Bill
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Old 09-16-2012, 02:19 PM - Thread Starter
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Thx for the information mate!
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Old 09-17-2012, 01:32 PM
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BTW, the 25 for PAL and 30 for NTSC was based on the AC frequencies which were 50 hz in PAL countries and 60 hz in NTSC. Since analog video was interlaced then each field was timed to the AC frequency. This is not needed anymore at all with modern electronics who can do this all with onboard timers.

The fractional for 23.97 is for drop frame. Sometimes you time the decoding to the audio which would definitely be noticed if a packet is left out and drop a video frame (or two.. ). IOW, there are 24 frame per second in the file if encoded that way. Video cameras including many of the consumer ones will do 24 fps if for nothing more than to make for smaller files or record for a longer time. 24 frame was used on DVD and BD because it could store longer films at higher encode rates. And you could probably create one file then remux for an NTSC version and a PAL version. Back in the late 90s and early 00s encoding for a DVD was time consuming and not so anymore. And modern decoders can display any frame rate. It's only a matter of how many microseconds you hold the frame before swapping the buffer. The device just displays whatever frame is current. The camera app on my Galaxy Nexus phone encodes 1080p at 17 fps and 720p at 19. My BD player will play those files but if I run it through the Movie Studio app on the phone it won't but I think it doesn't like the audio encode which is 32K whereas the original is 64K.

Back in the 50's the US TV stations used projectors with 5 blades in them illuminating the 24 fps frame 5 times meaning that when the camera on the multiplexer refreshed it took whatever frame was current and that worked (5 * 24 = 120 frames which coordinates the 60 hz field refresh). In Europe they just sped the film up to 25 fps.
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Old 09-17-2012, 02:44 PM
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^ The 23.97 is for compatibility with broadcast telecine.

In historical NTSC standard definition analog TV broadcasts, the actual frame rate was 29.97 frames per second or 59.94 interlaced fields per second (rather than the "nominal" 30 and 60) for technical reason related to keeping enough bandwidth separation between the video and audio components of the modulated broadcast signal while staying within the limited frequency bandwidth assigned to each broadcast channel. That video rate has been carried over into modern day. (NOTE: There is no similar issue with 50Hz PAL broadcasts.)

This is why 60 Hz power line interference lines crawl slowly up your TV screen; the 60Hz power line rate is slightly faster than the 59.94 refresh rate of the TV.

The TV fields making up the broadcast signal don't arrive butted end to end; there's a gap in between them where nothing is displayed -- the sync "blanking" interval -- which gives the TV a chance to get its act together and get prepared to start displaying the next field. There's enough slack in that sync interval between fields (i.e., that waiting period before the next field begins) that the TV can easily stay synced with the real video signal even though the video rate is slightly different from the power line rate.

For movies recorded at 24 frames per second, converting them for TV broadcast requires adding in the extra frames so the TV gets the frame rate it is expecting. This process (telecine) is done by breaking each frame of film up into 2 interlaced fields and then repeating the fields in a regular "cadence". Basically you need 4 frames (8 fields) of film to happen in the same amount of time as 5 frames (10 fields) of video. A typical "field repeat cadence" might be 2-3-3-2. The result is that some of the fields (interlaced half-frames of the film content) are shown on screen slightly longer than the rest -- which is where "cadence judder" comes from.

But since video is not really 60 fields per second but rather 59.94, the FIRST thing you have to do is slow the film down just a hair to maintain the same timing relationship between film frames and video frames. Thus 23.97 frames per second for film.

That slow down -- 0.1% less than the "nominal" rate for both the video and film rates -- is too tiny for the eyes or ears to spot -- amounting to roughly one less frame every 33 seconds. For example, it is so tiny that no "pitch adjustment" is needed for the audio.


For home theater, modern electronics can handle the difference between 24.000 and 23.97 frame rates without problem. I.e., the player can generate the correct output timing regardless of which is on disc, and so the disc formats give studios the option of putting the content on disc either way.

In what follows I'll ignore the difference between 24.000 and 23.97 frame rates -- as that all gets handled automagically.


Now I mentioned "cadence judder" above. That's a VERY slight ratcheting of what is supposed to be smooth motion when film is displayed at video rates due to some fields of the film being held on screen slightly longer than others as part of that telecine process of raising the film frame rate to the video frame rate for display on the TV. You've been seeing cadence judder your whole life -- any time you watched a film on broadcast TV. It turns out the brain is VERY good at ignoring cadence judder, so unless you are unusually sensitive to it, odds are you've never noticed it, and won't even notice it now unless you have a chance to compare normal and "judder free" display side by side. One good place to look for it is in the credit scroll at the end of a movie, which will advance in a slightly ratcheting fashion due to cadence judder.

But these days, modern electronics allow for reasonably priced TVs that can display images at EITHER a 30 or 24 based refresh rate. Modern TVs will typically display 60 frames per second of TV (the normal 30 frame rate with each frame doubled). For film rates, the TV refresh rate will be a multiple of 24 -- 48, 72, and 96 being the most common -- differing by how many times each film frame is repeated on screen before the next one gets displayed. (These repeats reduce the chance you'll see "flicker" due to the slowness of 24 frames per second -- theater movie projectors do a similar thing, breaking up each frame into 2 repeats with a mechanical shutter mechanism.) This allows for the possibility of displaying movie content WITHOUT cadence judder.

Blu-ray movies are recorded on disc as 1080p/24. If you have a TV that can accept /24 input and "do the right thing" with it, then you get cadence judder free display of that movie.

But what of SD-DVDs? They are recorded on disc as 480i/60. That means movies on disc have ALREADY had telecine applied to them. The cadence judder is already built it.

Theoretically you can detect the field repeat cadence in the SD-DVD video stream and REMOVE the repeated fields -- thus recovering the original movie rate of 24 frames per second. That can be fed to the same sort of TV as above for cadence judder free display.

But the reality is that this is actually pretty tricky.

First off, suppose what's actually on disc is a TV show? Well that was recorded at 30 frames per second -- video rate -- not 24. And if you force it to 24 for output you are *DEFINITELY* going to have problems. That's because there's no good way to decide which frames to discard to LOWER the frame rate. So you will get "frame drop stutter". This is a significant jerkiness of motion -- most easily seen in horizontal or vertical pans as the whole image is shifting at the same time. Frame drop stutter is much MUCH worse than cadence judder. Almost everyone will see frame drop stutter, and it is patently obvious that it was there if you turn off 24p conversion and view the video normally -- motion will look distinctly better.

So the first issue is that you don't want to apply 24p conversion to SD-DVDs that were recorded at video rate. Now this will usually mean TV shows and live concerts on SD-DVD. HOWEVER, some TV series are actually recorded at 24 frames per second -- i.e., they are produced like movies. And some movies are EDITED with video rate equipment (for a variety of reasons that need not concern us). The point is you can get surprised and discover a TV show works well with 24p conversion or a movie DOESN'T work well with it.

In addition to the luck of the draw on how the content was produced, there's also the problem that the process of creating the "transfer" -- the digital rendition of the movie on SD-DVD -- often introduces glitches. These are points where the flow of the cadence is broken, and thus the 24p conversion goofs up in deciding which fields to discard in its attempt to extract the original 24 frames per second film content.

Such glitches often come from "bad edits", where the process of editing the film broke the field repeat cadence.

Others come from the mechanical telecine process used years ago to produce video broadcast versions of movies -- the result of which has just been dumped onto SD-DVD without regard to the problems that might cause. Movies on such discs HAVE NO fixed relationship between film and video frames. There is no uniform repeat cadence to be found. And thus 24p conversion will get all confused.

Glitches like this also produce frame drop stutter, or even worse artifacts depending on how the 24p conversion is implemented. And depending on the nature of the glitch, how frequently they occur, and the type of 24p conversion algorithm in use, the ability to RECOVER after a glitch will vary. So the period of bad video may be brief, longer, or even indefinite if the system can't figure out how to recover the cadence lock.


So what to do?

First, consider applying 24p conversion only to newer SD-DVDs of newer movies. These are more likely to be on disc "correctly".

Second, learn what frame drop stutter looks like. The easiest way to do that is to play an SD-DVD of a TV show and compare it with and without 24p conversion.

Then go ahead and try 24p conversion -- assuming you have a TV that "does the right thing" with /24 input in the first place (since there's no point in using 24p conversion if your TV is simply going to raise it BACK up to a 60 refresh rate!). And just be prepared to turn off 24p conversion if you see frame drop stutter, as that's your clue that THIS particular SD-DVD disc is not a good candidate for it.


TVs will attempt to do cadence detection "on the fly" and switch between how they convert fields into frames to include the repeated fields or not. That is they will take a movie stream of 48 fields per second and produce 24 frames per second from that and THEN raise that to 60 frames per second by repeating WHOLE FRAMES instead of fields. This makes for better looking "deinterlacing". This is what most TVs mean by "reverse telecine". Some TVs will actually shift their refresh rate on the fly as well -- detecting incoming film rate content and switching to, say 72 frames per second refresh instead of 60.

Disc players that do 24p conversion have a different issue to deal with, which is that changing between /24 and /60 output on an HDMI cable requires a new HDMI handshake -- which takes at least 2 seconds and sometimes longer. And so the player has to take a different tack. It has to stick with a choice of film or video rate processing to avoid constant new HDMI handshakes interrupting the flow of the viewing. And thus glitches can last longer if the player gets confused by the content into making the wrong choice. On the other hand, letting the player do the conversion means you can get true /24 into a TV that DOESN'T try to switch refresh rates itself unless fed with /24 to begin with.
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Old 09-19-2012, 06:25 AM - Thread Starter
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Thanks for the detailed information. A lot to take in eek.gif but very helpful, I'll keep an eye on what movies I play at the /24 rate.

Thanks again for the feedback.
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Old 10-02-2012, 10:48 AM
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Please suggest some DVD (not Blu-ray) scenes that illustrate the improvement in playback using DVD 24p conversion.

I'm using an OPPO BDP-93 which, with the latest firmware, can output 1080p 24hz from DVDs..

jdg
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Old 10-02-2012, 11:51 AM
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Most recent movies DVD are capable of 24P extraction but not all players do so. I use an external video processor so I can set either video or film or auto cadence mode. Even many TV shows are shot in 24P as well. The only way I can tell the difference for sure is to look for the micro-judder when watching the credits at the end of the movie. The other day I accidently put a DVD movie into my BD player. I didn’t notice it until the credits and then I saw the micro judder. Then I realized it was my error. I have a DVD player with SDI output to take maximum advantage of DVD material. Many independent and foreign films are only distributed in DVD format. The quality can be quite good on recent releases.
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Old 10-02-2012, 01:44 PM
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Originally Posted by CRGINC View Post

Most recent movies DVD are capable of 24P extraction but not all players do so. I use an external video processor so I can set either video or film or auto cadence mode. Even many TV shows are shot in 24P as well. The only way I can tell the difference for sure is to look for the micro-judder when watching the credits at the end of the movie. The other day I accidently put a DVD movie into my BD player. I didn’t notice it until the credits and then I saw the micro judder. Then I realized it was my error. I have a DVD player with SDI output to take maximum advantage of DVD material. Many independent and foreign films are only distributed in DVD format. The quality can be quite good on recent releases.

Understood that the end credit scroll gives you indicator as to the source material and the judder. I am curious, however, where I might notice a difference between 1080p60hz and 1080p24hz during the film. I find that I am NOT sensitive to practically anything, but I do notice normal film judder during scans. I'd like to see an A/B comparison where outputting 24hz from a DVD makes a discernible difference in the amount of judder.

jdg
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Old 10-02-2012, 02:09 PM
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Originally Posted by Brian Conrad View Post

BTW, the 25 for PAL and 30 for NTSC was based on the AC frequencies which were 50 hz in PAL countries and 60 hz in NTSC. Since analog video was interlaced then each field was timed to the AC frequency. This is not needed anymore at all with modern electronics who can do this all with onboard timers.

While this is generally true, it is not absolutely the case. PAL has been used in countries with a 60hz electrical frequency as has NTSC been used with 50hz. It is my understanding that parts of Japan had 50hz for their electricity and the Japanese variant of NTSC supported both NTSC/30 and NTSC/25. PAL/30 has also been used in a few places (Brazil for one). Both encoding systems are not irrevocably linked to a frequency rate.

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Old 10-03-2012, 12:46 PM
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"Close enough for jazz" though. biggrin.gif

What I said was the history of these systems which I used to study up on even in my teens when I had access to the local TV station. It's probably been a while since electronics bothered with syncing with the line frequency given technology. I've written video players for games so that is my background but I'm not going write a white paper for a simple forum post. It would go over most people's heads anyway and I'm not sure what it would prove.

Oh, and BTW I've also had a career as a jazz musician. wink.gif
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Old 10-03-2012, 03:40 PM
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My post was not directed at you but merely to educate the forum readers that PAL and NTSC are not wedded to 50 and 60 hz respectively. This seems to be a widely held belief. I know that many people (you included) are aware of this but hardly the majority.

BTW, I've never been a pro musician, merely an amateur and lover of jazz. I admire those who have the talent to be a pro.

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