24p: What You Should Know

Nov 23 2010

Background

Long ago, filmmakers decided to standardize shooting film at 24 frames per second (FPS) primarily due to the high cost of film and state of the technology at the time.  Film became popular worldwide and every piece of production and distribution equipment was made to support this frame rate.

When television came into existence, the cathode ray tube (CRT) display technology was used.  CRT design at the time used a multiple of the power line frequency which is 60Hz in North America and 50Hz in Europe. The National Television System Committee (NTSC) standard developed in the United States in the 40’s and 50’s actually ended up slowing down the CRT refresh rate by 0.1% to 59.94Hz to compensate for some distortion that occurred due to the nature of the way NTSC transmits information.  Later, the Phase Alternate Line (PAL) standard was developed in Europe and had no such anomaly.

It should be noted that the refresh rates of PAL and NTSC are field rates because PAL and NTSC transmit frames using interlacing which is essentially a means of compression.  In other words, it allows more lines of resolution in a given amount of bandwidth, albeit at a slower frame rate.  When a frame is interlaced, it is divided into two fields with each field representing half the frame.  The fields are then transmitted in alternating fashion such that the frame rate is equal to half the field rate.  Thus, the frame rates of PAL and NTSC are 25fps and 29.97fps, respectively.

Naturally, people wanted a way to take film content shown in cinemas and display it on their televisions.  However, this presents a problem because film and video frame rates are different.  To compensate, “telecine” was created which is the general term used for “pulling down” film frames into video.

For PAL, the solution was rather simple: the film frame rate was simply sped up 4% from 24fps to 25fps.  This technique, also known as 2:2 pulldown, does have the affect of slightly altering the sound of the film and displaying video slightly faster than intended.

2:2 Interlace Pulldown

For NTSC, the solution was a bit more complex. First, the film rate was slowed by 0.1% to 23.976fps.  Next, frames of film are displayed in an alternating sequence of two or three fields in order to match the NTSC field rate.  This technique is also known as 2:3 (or 3:2) pulldown.

2:3 Interlace Pulldown

You can see that NTSC presents a couple of problems. First, due to the interlacing, there are dirty frames with interlacing artifacts. Dirty frames are those which have fields of two frames of film.  The second problem is that the original film cadence has been altered because frames are being divided into non-equal numbers of fields and thus displayed in non-equal amounts of time.

Now that we have an understanding of the history, let’s take a look at the present.  High-definition (HD) television displays today will typically be operating at a 50Hz or 59.94Hz refresh rate.  However, unlike the NTSC/PAL displays in the past, HDTV displays of today are also capable of displaying full progressive frames at their refresh rates (1080p, 720p, 480p) in addition to two interlaced fields composing a single frame (1080i, NTSC, PAL).

 

Present Day

Film is in a much better state with today’s HDTVs than the PAL/NTSC days.  Most every film is transferred to Blu-ray at a frame rate of 23.976 (24p) (note: today, most players can also process film-based DVDs in such a way as to also create a 24p output).  If the display is operating at 59.94Hz, 2:3 pulldown will occur, however, there will not be any interlacing artifacts since full frames are now being presented instead of fields.

2:3 Progressive Pulldown

So, while we’ve achieved one victory here over NTSC by removing the interlacing artifacts, we still have the unnatural frame rate cadence due to each frame of film occupying different amounts of time.  This manifests itself in what is called judder.  Pulldown judder is especially noticeable in scenes with long panning shots or credit rolls, but it is always a problem as long as there is motion being captured on film.  Judder is probably best described as a hitching motion.  A lot of times, it may not even be noticeable if you’ve become accustomed to watching content with judder.

Fortunately, there is a solution to both the 2:3 pulldown judder problem and the slight 2:2 pulldown speedup problem and that is to obtain a display that is capable of maintaining the proper film cadence by accepting the native 24p frame rate offered by Blu-ray and displaying at a refresh rate that is a natural multiple of 24p.  Actually, this is exactly what cinemas today do. A typical cinema will display each frame of film twice to obtain a 48Hz refresh rate or even 3 times as often to achieve a 72Hz refresh rate.

 

Recomendations

Due to our perception of light and the higher light output of most HDTV displays compared to cinema projectors, it is best to obtain a display that can provide proper film cadence at least 3 times the 24p frame rate (~72Hz); otherwise, the lower multiples can result in a flickering effect, especially in brighter scenes.  It is also important to recognize that many displays on the market will accept a 24p input, but will internally apply 2:2 or 2:3 pulldown to achieve 50Hz/59.94Hz output.  Even displays supporting proper film cadence may require you to engage a special mode or change the default configuration to avoid sub-optimal pulldown.

Another concept to be aware of is frame interpolation.  Some display or source devices may attempt to multiply the frame rate, but instead of repeating the frame several times, the device will interpolate frames to be displayed between source frames which can create a smoothing motion resulting in a more video-like picture (also known as “The Soap Opera Effect”).  This is not something that is desirable because this was simply not intended by the filmmaker.  The filmmaker understands the strengths and weaknesses of the film frame rate and makes very specific decisions based on that by adjusting things such as shutter speed, aperture and frame rate when filming.  By employing interpolation, you can’t possibly enjoy the film as it was intended to be seen.

When using a home theater PC (HTPC), we often must manually change our display refresh rate to “23Hz” to achieve proper 24p output.  As Blu-ray disc (BD) playback software matures, I expect to someday see this happen automatically without user intervention as some hardware playback devices are capable of.  MPC-HC, a free open-source software (FOSS) player, is capable of BD-Lite playback and automatic refresh-rate adjustment based on the detected media.

Another item of interest for HTPC users with 50Hz displays and PAL film-based DVDs is SlySoft’s ReClock software which can compensate for the 2:2 pulldown speedup by resampling the audio and correcting for the pitch problem.

Thankfully, we can reproduce film experience in the home theater more accurately than ever and hopefully, your display and source devices can properly support 24p.

For more information on the subject of frame rates and refresh rates, see our guide.

Note: As we discussed in the comments, some of the current trade names used to indicate proper 24p support by manufacturers include:

  • LG: Real Cinema
  • Panasonic: 24p Cinematic Playback
  • Samsung: Cinema Smooth
  • Sony: 24p True Cinema

Comments

Very interesting article, I had no idea really what 24p really meant.   So what is the take-away for HTPC users?   It sounds like from your comments that TV's aren't smart enough in most cases to auto-select the proper resolution, if say I inserted a Bluray.   Did I get that right?    If I did, would it make sense then to write a small app that sets my TV resolution through it's serial interface based on the media being played back? 

Native 24p is typically what is encoded to a BD for film-based material. The BD player should always give the option of outputting the native 24p or performing the 2:3 pulldown to achieve 59.94fps output. If the native 24p output is used, the TV may also internally perform a 2:3 pulldown, but if you have the right type of TV, it will maintain the film cadence by displaying and reapeating each frame an equal amount of time.

For HTPC use, a lot of times, it is a manual affair to output 24p if you switch between broadcast content (1080i, 720p, 480i) and BD. For broadcast content, you'll need to be in 59.94Hz refresh rate, but if you stay at this refresh rate, a software BD player will perform 2:3 pulldown so you need to change the refresh rate to 23Hz to get 23.976Hz.

This switching scenario really should be handled by the software player because it knows exactly what the frame rate of the content is. BDs are not always a pure 24p either as extras might be NTSC or PAL and sometimes the disc might also be exactly 24fps instead of 23.976fps (although, I believe this is fairly rare for commercial BD). If it is 24fps, then 24Hz would be the proper refresh rate setting.

To make things just a bit more confusing both 23.976fps and 24fps can both be considered 24p. Smile

Another thing to note about HTPCs is that Intel hasn't gotten 24p right yet. Even though their driver might allow it in their Clarkdale processor IGP settings, they output 24Hz instead of 23.976Hz. What ends up happening is that there are repeated frames displayed every so often which sort of defeats the purpose of avoiding 2:3 pulldown.

I'm really starting to dislike you and Andy.  I had no plans to buy a new HDTV in the near future even though none of my HDTVs support 24p.  After reading Andy's VT25 review and now this I feel like it’s a necessity.

I'm going to need your billing address...

Money mouth

That's the burden we must bear for perfection. It's not like 2:3 pulldown is unwatchable, but once you notice judder in 2:3 pulldown, it can drive you a little crazy.

From what I understand, wouldn't every 120Hz TVs correct this problem? 60Hz divided by 24Hz gives 2.5, so the need of 3:2 pulldown. However, 120Hz divided by 24 gives exactly 5. So, at 120Hz, each frame needs to be displayed 5 times. Each frame is displayed the same amount of time.

Am I correct in my understanding?

Unfortunately, not every 120Hz LCD can display proper film cadence. It is important to know exactly how the display gets to 120Hz. One common way is simply multiplying 60Hz by 2 and that means 2:3 pulldown is being applied. Further, the display may accept 24p input, perform 2:3 pulldown to 60 Hz and then multiply to 120Hz.

The other thing that most often happens with 120Hz LCDs is frame interpolation is used to display 120Hz.

When a manufacturer supports proper 24p cadence, they might apply a name to it like Samsung's "Cinema Smooth"

From experience...my 120Hz Sharp LCD does not handle it properly Cry

When your HTPC is connected, what is the refresh rate of the video card? 120Hz? 60Hz?

Most 120Hz LCDs will not accept 120Hz as an input. There are some PC monitors that do and are used for 3D gaming.

So, if I want to use a software bluray player like TMT, I need a monitor or TV that accept 120Hz input?

Not at all. Displays that support 24p with correct film cadence accept the native 24p input (23Hz from your HTPC). The display then displays it at an even multiple. As an example, one of my displays will accept the 24p input and display each frame of film 4 times in equal amounts of time for a display rate of ~96Hz.

Right. 120Hz is the end result output, not the input. No consumer electronic video format is 120Hz, they're all a multiple of 24, 30, or 60, the TV is doing the processing up to 120Hz or 240Hz, or 480Hz for LCDs.

Ok. I understand now the need to switch resolution to 24 when watching a bluray and switch back to 60hz for tv.

If TVs with 120fps input would be available, there would be no need to switch between resolutions.

The problem with that though is expense and it isn't really necessary because broadcast and film standards do not have framerates that high. The display simply needs to accept the native content frame rates and display them properly. The source device should be capable of automatically detecting and outputting the native frame rates.

Remember that 120Hz, 240Hz, 480Hz, etc. are typically interpolation features born out of trying to account for LCD tech deficiency in the area of motion resolution. They aren't really necessary, perse -- and undesirable with film. This is one reason why it is important to understand exactly what the TV is doing with 24p.

My small brain is starting to understand this. Whenever I turn on my HTPC my Samsung plasma says the input is 60Hz, regardless of it I'm using TMT5 or watching normal TV from 7MC. That means, when watching a Bluray in TMT, I am not getting an optimal framerate and the display is probably doing this 2:3 pulldown. I know my TV supports "Cinema Smooth", so where should I start looking for options to output to 24p? My Nvidia driver? TMT5?

Awesome news for you woodchuck. You can enjoy proper film cadence with Cinema Smooth on your display. What you are currently doing with TMT5 and the refresh rate set to 59Hz is forcing TMT5 to perform the 2:3 pulldown so that 59.94Hz is output to the display.

What you need to do next time is change your display's refresh rate to 23Hz when watching a 24p BD. To do this, you just go into the NVIDIA Control Panel or Advanced monitor settings under the Windows display settings and apply it.

You might also need to go into your Samsung's menu system and change the mode to Cinema Smooth. On my Samsung plasma, it remembers that I have chosen Cinema Smooth and automatically engages when it receives 24p and disengages when 59.94Hz is input.

The sad thing is to manually switch between 24hz and 60hz when watching a bluray of live tv.

Definitely a bummer to have to switch refresh rates to get this to work.

I wonder if there are any TVs which detect 3:2 pull down at 59.94Hz to get the original 24p content, then do the conversion to 120Hz by repeating each 24p frame 5x?

Myself, I won't upgrade to a new TV until it either takes 120Hz input directly or will do what I describe above, as I'm just not willing to fight the battle of switching refresh rates from 24Hz to 60Hz depending on what the content is.

RehabMan wrote:

I wonder if there are any TVs which detect 3:2 pull down at 59.94Hz to get the original 24p content, then do the conversion to 120Hz by repeating each 24p frame 5x?

That's not possible, they are just frames at a given rate to the TV.  The limiting device in this scenario is the PC, almost every other device (streamer, BD deck, etc) will switch automatically.

For a htpc, only one of these would be acceptable:

1) The player should have an option to switch resolution to 24p according to video it's reading...

or

2) TVs with 120Hz input: the software player could play 24p in 5:5 without any pull down.

Anything else requiring manual operation would not be accepted by wife Wink

... deleted duplicate post... reposted below...

(which still didn't end up where I expected...)

Too bad comments weren't handled just as threads in the forum...

babgvant wrote:

That's not possible, they are just frames at a given rate to the TV.  The limiting device in this scenario is the PC, almost every other device (streamer, BD deck, etc) will switch automatically.

Last post did not end up where I wanted...  Retry.

I don't know about not possible.  The TV is obviously receiving 3 frames, exactly the same, followed by 2 frames exactly the same (assuming HDMI digital input)... in a repeating pattern.  Seems fairly simple to watch for changes in the frames and note the 3:2 cadence (yes... you would have to buffer the last frame in-memory and keep a couple of counters).  Seems that because I can see the algorithm in my head, it should be possible.

Practical?  Not sure... depends on what kind of hardware capabilities these TVs are built with these days.

I guess that's the other way I'd upgrade my TV... If Win8 (or whatever) decided to start switching the refresh rate depending on content (full screen only).

// Dean

RehabMan wrote:

Last post did not end up where I wanted...  Retry.

I don't know about not possible.  The TV is obviously receiving 3 frames, exactly the same, followed by 2 frames exactly the same (assuming HDMI digital input)... in a repeating pattern.  Seems fairly simple to watch for changes in the frames and note the 3:2 cadence (yes... you would have to buffer the last frame in-memory and keep a couple of counters).  Seems that because I can see the algorithm in my head, it should be possible.

Practical?  Not sure... depends on what kind of hardware capabilities these TVs are built with these days.

I guess that's the other way I'd upgrade my TV... If Win8 (or whatever) decided to start switching the refresh rate depending on content (full screen only).

// Dean

It probably would be possible to maintain a frame buffer to detect 2:3 pulldown and undo it, but there's not a whole lot of reason to do it when TVs can accept the native rate and display it properly.

The better approach is really for the playback device to output the detected media type at the proper rate. If CE devices can do it, there's absolutely no reason HTPC software can't do it other than lack of initiative thus far.

swoon wrote:

It probably would be possible to maintain a frame buffer to detect 2:3 pulldown and undo it, but there's not a whole lot of reason to do it when TVs can accept the native rate and display it properly.

The better approach is really for the playback device to output the detected media type at the proper rate. If CE devices can do it, there's absolutely no reason HTPC software can't do it other than lack of initiative thus far.

I agree with you except for the nasty glitchyness that happens when you switch refresh rates and the TV tries to resync.  Now if TV/video card makers can solve that problem as well, then I'm all for it.

Otherwise, I stand by my desire to have one refresh rate that does it all.

In the sense that anything is possible given enough time and effort, sure I'll give you that.  I should have said "not possible with a lot of extra VPP, and even then probably difficult to get right"... 

Either way, it's never going to happen; this problem has already been solved in pretty much every modern CE device.  This is a player issue, not the OS.  MS should have built it into MC, ArcSoft into TMT, Cyberlink into PDVD, etc. - it's inexcusable that a $100 BD deck/streamer gets it right and they can't.

Awesome indeed swoon! Thanks! Now does anyone make a manual resolution changer that can be accessed under mediacenter's "extras"? Mikinho? Bueller?? Anyone???

There is a manual one already, see dgaust's Refresh Changer.

 

I know someone will release their automatic one soon.

I wrote a hotkey application that you can use if your remote supports key press macros, or inside of 7MC you can use the DTBAddin to automatically switch resolutions.

It should be noted that all managed UI applications (which includes 7MC) leak memory when the resolution changes, so over time if you leave them open it can cause issues.

This doesn't have direct hotkey support, but hotkey support via something like AutoHotKey or ATI Hotkeys, correct?

I've been using the 12noon refresh changer + AutoHotKey + Harmony, but I think I will give yours a try as the 12noon doesn't always seem to change reliably for me and sometimes requires a couple tries to switch.

babgvant wrote:

I wrote a hotkey application that you can use if your remote supports key press macros, or inside of 7MC you can use the DTBAddin to automatically switch resolutions.

It should be noted that all managed UI applications (which includes 7MC) leak memory when the resolution changes, so over time if you leave them open it can cause issues.

Your tool is awesome! It is simple, reliable and powerful.

swoon wrote:

 

When a manufacturer supports proper 24p cadence, they might apply a name to it like Samsung's "Cinema Smooth"

Is there a master list or can we get one made of HDTVs that support proper 24p?  I'm in the market for one this holiday season and I HATE JUDDER!  Many months ago it bothered me so much that i went in search for its cause. This article explained it better than any I read before. THANK YOU.  So I know Samsung's CS... what other makers and models, please?

I believe this is a sampling of the trade names used to convey proper 24p support:

LG: 24p Real Cinema
Samsung: Cinema Smooth
Panasonic: 24p Cinematic Playback
Sony: True Cinema

Note how they all use "Cinema", so this is probably used by others as well. If you're unsure, I would take some BD where you know you can see pulldown judder and make sure the TV can properly handle it.

Ya know .... right about now .... I can image a Looney Toons character turning to look at the audience with a bit of a smirk and say

"Confusing, isn't it." 

Maybe Bugs munching a carrot, or perhaps Wyle E. Coyote just as the TV set blows up in his face.  Sealed

I don't mean to distract from this thread so please forgive my attempt at levity.  I do find this informative and interesting.

Just so you know, the real reason for the 59.94 was due to the color subcarrier choosen. The frame rate resulted in the subcarrier repeating at 15 fps and thus color editing of videotape required cuts at 15 and not 30. A miscut would show as a slight horizontal jump in the picture (as was often seen in late night used car ads).

Why was the subcarrier choosen at 3.58mhz which required this change? Well the committee had originally wanted to move the sound subcarrier highter and use a different subcarrier for color but one (and only ONE) engineer complained. He found that the move would introduce buzz in the sound of some of the tv sets his company made and thus pushed for the difference in subcarrier that changed the frame rate and forever stuck us with drop frame time code to keep clock time consistent with picture!

The set maker in question? Philco.

This full story was related in a SMPTE Journal some 20 or 30 years ago by egineers working on NTSC standards.

 

John

That's an interesting bit of history I did not know. It seems almost unbelievable that we have relied on NTSC for over 50 years as a primary broadcast mechanism and it will likely forever haunt us.

Interesting.  On a TV that has 24p input but doesn't do true 24p output, is it generally considered better to output 23hz and let the TV do the pulldown or 59hz and let the computer do the pulldown?

oliverredfox wrote:

Interesting.  On a TV that has 24p input but doesn't do true 24p output, is it generally considered better to output 23hz and let the TV do the pulldown or 59hz and let the computer do the pulldown?

It shouldn't really make a difference as the pulldown method should be identical whether at the source or the display.

swoon wrote:

It shouldn't really make a difference as the pulldown method should be identical whether at the source or the display.

That makes sense.  Seems odd to even bother having a 24p input that doesn't do true 24p and doesn't make any difference if pulldown is done on the player or TV side.  I guess it's just marketing then. LOL

oliverredfox wrote:

I guess it's just marketing then. LOL

I'd guess that it's more of a compatibility feature than marketing.

FWIW - I've had better results doing pulldown in the decoding device (PC, HD300, BD player, etc.) when connected to my TH-C42FD18.

Andy-

I tried your reschange app and for some reason 1080p@23 will NOT stick.

It goes to 24Hz instead of 23.976Hz. Is there something I'm missing? Do I need to do this:

http://www.avsforum.com/avs-vb/showthread.php?p=18039274#post18039274

and edit my ATI .xml? In running an ATI5850 w CCC 11.4. The ATI hotkey doesn't work - so I thought your app may change the refresh rate similar.

If I change it manually from the desktop it works fine. (But that's not too WAF) I use madVR's built in refresh changer when playing .mkv, but since the auto refresh in TMT5 is inconsistent (I disable it) I would really like your to work.

Thanks

st3lvio wrote:

 

It goes to 24Hz instead of 23.976Hz. Is there something I'm missing? Thanks

If you select 23Hz in my app it will choose 23Hz; you can't trust what CCC displays.  You shouldn't have to mess with any XML files or anything.

That is what I was expecting (I set it for 23Hz) - but it would not work for me unfortunately.

I'll try again tonight - do I need to run the .exe "as administator" or have UAC turned on?

My HTPC is on an Admin account and UAC is off to avoid any conflicts.

Switching back to 60Hz is not problem - so I'm bummed thisis not working for me.

Thanks

It is probably working.  Check it through the Windows interface.

This is what it shows when selecting 23Hz. This app is perfect (if I can just get it to work as you do)

23Hz ResChange gives me 24Hz in Windows

I see your using an Intel iGPU - could that be the cause?

st3lvio wrote:

I see your using an Intel iGPU - could that be the cause?

Check on the "Adapter" tab.

I'll have to do it tonight. I don't want to RDP in from work.

Thanks - I'll report back.

Ok, changing the quick key from ALT+Number 01 seemed to have solved my issue.Embarassed

Thanks for your help!

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