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Challenges of 24fps for domestic projectors


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Background

 

Hi,

I've been following discussion in this thread about performance of late model projectors when showing content shot at 24fps: 

 

A number of the recent posts about different 24fps display modes and the effect on visible jitter or judder have been suppressed from view, being considered off topic. (I had been tempted to make some quick observations there myself,  but am glad I didn't as I'm sure these would have been hidden as well!)

 

I haven't had much personal experience with home projectors but I will make a few remarks, which may set the ball rolling for any others inclined to comment.

 

Traditional projectors

 

Mechanical 24fps projectors in public cinemas used at the turn of the century would typically employ a 2 or 3 blade shutter to interrupt the light beam. Each frame was not illuminated once, but strobed two or three times.  As noted  at https://en.wikipedia.org/wiki/Movie_projector#Shutter :-

 

It is the gate and shutter that gives the illusion of one full frame being replaced exactly on top of another full frame. The gate holds the film still while the shutter is open. A rotating petal or gated cylindrical shutter interrupts the emitted light during the time the film is advanced to the next frame. The viewer does not see the transition, thus tricking the brain into believing a moving image is on screen. Modern shutters are designed with a flicker-rate of two times (48 Hz) or even sometimes three times (72 Hz) the frame rate of the film, so as to reduce the perception of screen flickering. (See Frame rate and Flicker fusion threshold.) Higher rate shutters are less light efficient, requiring more powerful light sources for the same light on screen. 

 

This technique helped even out the flickering. There was the inevitable shutter closure needed to mask the pull down of the next frame, and then there was an additional shutter closure or closures mid-frame. 

 

With stronger illumination levels, human eyes can perceive flicker at higher rates. (For example CRT computer monitors operating at 50Hz would appear very flickery, 60Hz was a distinct improvement, and 75Hz was much smoother.  In Australia, late model CRT sets were made available with a 100Hz scanning rate to avoid the noticable flicker of 50Hz. )

 

Public cinemas in the mid-20th century used relatively low levels of brightness for a number of reasons. High lamp brightness:

  • risked burning of the print through excessive temperature rise (while a particular frame was being illuminated by an early projector lamp which emitted a lot of heat)
  • would have made flicker more noticeable
  • would have made motion seem more jerky in certain scenes.

 

Even today, public cinemas generally use rather low levels of illumination. This causes human irises to dilate, which tends to result in a less clear image on the retina for many people, unless their focus (natural or assisted with corrective lens) is very good. It also forces the rods and cones to operate at a reduced light level where the response time is slower. The persistence of vision is increased, allowing adjacent frames to be blurred together to some extent by the human eye.

 

Blurring is also introduced when the 24fps movie is shot. Traditionally a shutter angle of 180 degrees is used in the movie camera, meaning that each frame of the film is exposed to light over a period of 1/48th of a second. If you examine individual frames of 24fps movies that were shot during periods of fast  motion of the camera (e.g.fast panning) or of the subject (e.g. the legs of a person running) you will see a great amount of blur.  This amount of blur is unavoidable if using 24fps and a traditional exposure time.  [Sport is typically captured at television rates (50i, 50p, 50i, 60p) and not at the cinema rate of 24p. Exposure times for modern TV cameras capturing sport tend to be very short, much less than a 180 degree shutter angle. This shortness of the exposure helps if motion interpolation is added later, as the source frames/fields are relatively crisp.]

 

 

Domestic projectors

 

Domestic projectors can typically perform inverse telecine so that a 60i signal from an old NTSC DVD of a film can be reconstituted to 24fps [or 23.976fps]. The advantage of converting a 60i [or 59.94i] NTSC DVD of a film to 23.976p is that the judder is removed of the slight unevenness in delivery of unique frames. The judder is noticeable with scrolling credits at the end of a movie. Instead of a uniform slight jerkiness (jitter) there is an uneven jerkiness (sometimes called judder).  However for many years most DVD players have been able to perform the inverse telecine function themselves, so it has become a less important feature for a 24fps capable projector. And Blu-ray players will generally be able to deliver  24fps [23.97fps] directly from 24fps Blu-ray discs.  I guess there may still be some AVRs in use that can do 60i but not 24p.


Movie directors work around the slow frame rate of 24fps by avoiding scenes that would look too jittery/juddery. Pans can be done very slowly, leading to an amount of jitter between frames that is largely hidden by the high exposure time of 1/48th second; or they can be done rapidly with extreme blur that is tolerable for a fraction of a second. Zooms are best done as dolly shots with fairly slow movement of the camera along the dolly track.  

 

However even when care is taken to perform a pan or zoom slowly, the raw 24fps frames will betray noticeable jitter if displayed on a large screen and/or at high brightness. That is why modern home projectors typically include a frame interpolation mode to hide jitter in such scenes.

 

Used in moderation, frame interpolation can allow the dreamy, other wordly, impression created by a traditional 24fps raw frame rate to be retained for the bulk of a movie, kicking in only in specific circumstances such as during a steady zoom or pan.

Used at a high setting, frame interpolation can make a 24fps film look like a television production shot at 50i or 60i (frame rates often used for shooting daytime television soap operas). Some people perceive this more fluid look as a cheap look, very different to the cinema. As a separate issue, high interpolation settings generally result in distracting artefacts. The motion can become quite artificial looking.

Another technique to assist apparent motion fluidity is the insertion of extra frames that are blank. This technique has been used with LCD screens. The eyes are not exposed to a single frozen frame for so long that the eyes become conscious that the single frame is actually a still frame rather than a glimpse of a scene in motion. The brain in effect continues to process the scene as a moving scene and is not distracted by individual clearly frozen images.

 

Challenges that are new

 

There are two new challenges:
 

  • 24fps 4K Blu-rays are being released with HDR, resulting in some parts of some scenes being very bright, increasing the speed of response of the light receptors in human eyes and making jitter very noticeable during moderate speed pans and with anything bright in the scene that is moving at a moderate or fast pace. This effect increases the usefulness of (or need for) frame interpolation, at least to some extent. (Streaming services may also provide some 24fps titles with HDR, e.g. some of the 4K Netflix titles.)
     
  • The high visual resolution attainable with 4K sources and 4K projectors (particularly in combination with large screens, and generally bright projector performance), allows the eyes to fix on a very detailed part of the scene. As soon as motion occurs in that part of the scene, the detail becomes very noticeably blurred if the director uses the traditional long exposure time of 1/48th second per frame.  The greater the clarity of the detail when the scene is static, the more obvious the blurring of detail when the scene moves.

 

Arguably, HDR and 4K are incompatible with traditional 24fps cinematography!

Edited by MLXXX
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Thanks for you post mate, you have pretty much covered it but I will add a few important points.

 

The way movies are shot is typically incomparable with high resolution. Most of the time the cameras aperture is wide open, or close, which gives a very narrow depth of field where the subject is in focus and everything else is not to draw the viewers eye to what the director wants you to see. Focus accuracy is absolutely critical and since focus is performed manually by measuring the distance from lens to subject it is rarely accurate. Even a very small focus error has a devastating effects of resolution, no use having 4K plus cameras if they are not perfectly focused.

 

As has been mentioned slow shutter speeds are also used, partly because there is often not enough scene lighting to use a faster shutter, but also to make sure there is LOTS of motion blur to cover up the side effects of a low frame rate, visible jitter in medium speed panning shots. Low speed and high speed pans are fine its only the medium speed panning shots that have real issues and often motion blur is added in post production where its is deemed necessary to hid low frame rate jitter.

To put motion resolution into perspective, if anything in the scene or the camera its self moves the distance of just one pixel during the cameras shutter open period the on screen visible resolution is emediatly halved. Now when you consider how small a pixel is at 4K you begin to understand that its going to be pretty much impossible to get anything like 4K unless the camera and scene are completely still, not very useful for movies.

To get high resolution we NEED high shutter speeds and high frame rates, more pixels can't help. The movie industry has shown no interest in going down that path.

 

Projectors and TV's have long been able to display 24fps natively, by natively I mean a new and different frame is displayed every 1/24th of a second. Its important not to confuse frame display rate and "refresh rate" as they are quite different.

Unlike CRT and Plasma which where "refreshing" displays,  the TV's and projectors available today all use the sample and hold display method and do not require refreshing so that term is redundant. Sample and hold displays "hold"  the state of their pixels until new data is presented that is different. The term "update rate" is appropriate for these displays as they only update their pixel output when input data is different to what was already being displayed. Sending the same pixel data multiple times is pointless and results in no on screen change.

 

To get smooth motion, or as smooth as the video allows, the display MUST use an update rate that either exactly matches the video frame rate or is an exact multiple of it. TV spec sheets quote display rates like 50, 60, 100, 120, 200 and 240Hz. 50 and 100Hz is used for 25/50Hz video source, 60, 120 and 240Hz is used for 30/60Hz video. Note that 24fps also divides perfectly into 120 and 240Hz so TV's can use 120 or 240Hz for the display of 24fps content. Now remember that sending the same frame data to a sample and hold display multiple times is no different to sending it once, nothing changes on screen so 240Hz looks exactly the same as 24Hz when frame interpolation is not in use, we get one motion update every 1/24th of a second. 48, 72 and 96Hz are also compatible with 24fps display and work the same as 24 native, current TV's don't run those update rates as far as I know but projectors may and probably do, manufactured don't say. Remember a higher rate is not better for native 24fps display and even with interpolation 48 would be fine, that would allow one real frame followed by one interpolated frame, plenty.

 

60Hz is not divisible by 24 so if 60Hz display is used for movies the result is what I call "judder" which is distinctly different to low frame rate "jitter" and far more objectionable IMHO, it gives a disjointed run-stop-run-stop sort of motion that looks like dropped frames, which in effect it is. One frame of video is displayed for 3 time periods and the next for 2 time periods, its a mess and if you ever see it you know that you are not getting 24fps native. The same thing happens if 24fps movie are encoded into 60Hz video, streaming services are renowned for that.

 

Sample and hold displays do not flicker because there is no blank or black period between frames, pixels go from one on state to another on state with no black in between so there is no flicker what so even no mater what the update rate. While this is good there is a down side and that is motion blur to the eye. Sample and hold displays are limited to about 300 lines of resolution in motion be they LCD, OLED or any of the projector technologies.

Black frame insertion can be used to emulate an old film projector and significantly improve motion resolution as seen by the eye - brain BUT there are serious problems with that approach. One is flicker, lots of flicker, and the brighter the picture the more noticeable it becomes. The second is a big issue for projectors and HDR on TV's and that is brightness. If a black frame in inserted between every real frame we get a 50% duty cycle and image brightnes drops 50% which is obviously not useable. Updating the display at say 120Hz can allow the insertion of a much shorter black period, one display update out of five, which improves both flicker and light loss but there is still flicker and we will still loose about 20% light.

 

There is another serious down side to BFI, a reduction in motion blur sounds like a good thing BUT it is exactly what we DON'T want for smooth motion. Remember 24fps content has lots of motion blur for a reason, the reduction of visible jitter. The mastering displays are sample and hold and its expected that the final viewing display will be as well these days. So if the projector reduces motion blur without using any motion interpolation to smooth things out jitter will be visibly increased relative to what was intended.

 

So, we are left with only two ways to reduce visible jitter in 24fps movies, increase motion blur and/or insert extra frames between the real frames AND use motion interpolation to create data to put in those extra frames that is different to the real frames.

Since sample and hold displays all have the same motion resolution they can all display 24fps content with the same visible jitter when no motion interpolation of BFI is used, each frame of video is presented to the viewer unaltered, warts and all. Pixel response time isn't a factor either because sample and hold totally dominates what we see. All else being equal faster pixel response makes jitter more visible not less. A case in point is OLED V's LCD TV's, OLED has much faster pixel response but motion is not smoother than LCD. Smoothness is is dominated by the sample and hold display method and motion interpolation performance, if used.

 

When if comes to projectors, JVC's DILLA and Sony's SXRD technology are peas in a pod, they are both proprietary version of LCoS and have the same native motion performance, which in turn is effectively identical to LCD and OLED TV's because they are all sample and hold displays.

 

24fps content is either displayed unaltered at 24 motion updates per second or its not, so if display "A" is running 24 motion updated per second and display "B" is visibly smoother its because display "B" is using motion processing to smooth motion, either adding blur, interpolated frames or both. Now the manufacturer may not tell you whats actually going in the projector and my even be down right deceptive in their marketing to suck in the less knowledgable, but if 24fps content on their display looks smoother than the same 24fps content on other displays when all motion processing is allegedly off you know for a facts something is amiss.

 

If motion is super smooth you have the "soap opera effect" which is what a 60fps camera provides. I note that many people say that dislike the "soap opera effect" yet go on to say they like motion that is as smooth as 60fps video and hold this up as a great attribute, super smooth = soap opera guys.

Its not a crime to like super smoothed motion but movies are not shot that way, just man up and admit you like the "soap opera" look. 😉

 

Having said all this, it find it vary rare to find any scenes in modern movies that are not smooth with only a low level of interpolation applied because film makers know what they are doing. I can't remember the last time I observed jitter on my JVC projectors, even with scrolling titles which show up jitter easily, they are perfectly smooth and never a dropped frame to be seen. Just works.

 

If I want to see jitter I can view on my VT60 Plasma TV with interpolation off. With 3 times the motion resolution of sample and hold displays jitter is bloody obvious and distracting on the rare occasion when the camera operator and or director do the wrong thing, reminds me of real film projection in a cinema and thats not a good look IMHO. Sample and hold displays take the edge of jitter by their very nature and are incapable of showing film like jitter even with all motion processing off. How much jitter reduction is desirable beyond that with the use of interplation is very much up to the individual viewer, whats better to one viewer can be worse to another.  So there is no "better" as such its only whats "better" for you.

 

I find the whole motion smoothness thing a beat up, I haven't seen any motion related issues I thought needed fixing in many years when playing Bluray content on a projector. JVC's have displayed 24fps natively since the very first X series if not before.

Streaming content on the other hand is a mess and thats just one reason I avoid it.

 

Edited by Owen
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