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Old June 22nd, 2008, 02:18 AM   #1
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What is and isn't judder?

While some, Sony and JVC, AVCHD camcorders cannot record progressive -- others, Canon and Pana, can. Moreover, some progressive camcorders can record only 24p while others can record 25p/30p. It's clear in reading some posts that there is confusion BOTH between how these different modes actually LOOK and WHY the modes look different. The term often used is "judder" but this term is way too often used wrongly.

Low Frame-rate Judder
The word judder is applied to the lack of smooth motion that results from the low “sampling rate” of 24p, 25p, and 30p capture. This type of judder is inherent to low sampling rate video and is part of what’s called “film look.” It makes no difference if you like it or not--this judder is inherent in film. If you go to the movies, you see judder when objects move through the frame--although film shooters know how to keep it from becoming too extreme. Video shooters tend not know how, or don't use the fluid-head tripods which limit panning speed, and so tend let it get totally out of control.

Pulldown Judder
We know 24p material has 2-3 pulldown added so it can be broadcast at 480i60 and 1080i60. (Programming that uses 720p60 also uses 2-3 pulldown. This type of pulldown, however, does not manipulate fields.) The result: Frame 3 and Frame 4 contain two fields that come from two different film frames. Despite these mixed frames often being called JUDDER FRAMES, they really only create judder when they appear in a series with “non-judder” frames. It is this pattern (cadence) that is Pulldown Judder.

Several Blu-ray and HD DVD players offer the option of a 1080p24 output. This connection directly passes the 24fps material on an HD disc. Doing so eliminates the need to apply either type of 2-3 pulldown. Pulldown judder, therefore, is prevented. Low Frame-rate Judder, of course, cannot and should not be removed.

Eye Tracking Strobing
The situation where each frame is shown twice, is present when p25 and p30 video is shown on displays at 50Hz and 60Hz, respectively. Repeating the same frame twice causes moving objects to “strobe.” This illusion created by our eyes when they automatically follow (track) a moving object—and the moving object, in the same position, is flashed twice. (The effect can be lessened by flashing the image 3 or 4 times.) Often called judder, it is not. It is an “eye tracking artifact.” Film shooters know how to control it. For example, by tracking a moving subject. This keeps the moving object in the same place within the frame. (Of course, background details still strobe, but folks are looking at the foreground subject.) Those who have long shot 25p video are far better at this than new to 30p shooters.

High Shutter-speed Strobing
When one uses a shutter-speed much above what's equal to 180-degree shutter, anything moving RAPIDLY though the frame will "strobe." Unless one is looking to create a Saving Private Ryan FX -- seeing strobing is a clear sign of a newbie shooter or a camcorder that provides no control over shutter-speed, or both. High shutter-speed strobing is the result of a too high shutter-speed while Film Judder is the result of a too low frame-rate.

Which is why those shooting 24p must be sure they set their shutter-speed to 1/48th > 1/60th just as those shooting 60i must limit shutter-speed to 1/60th > 1/100th.
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Old June 22nd, 2008, 09:01 AM   #2
 
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Steve...

I've observed a very objectionable double image with my EX1 and my HD110 when I shoot in progressive mode at either 24p or 30p. I shoot a lot of panoramas in which the camera pans the entire scene, creating a constantly changing frame, not just motion within the frame. I make a concerted effort to keep my pans rate very slow, however, the "motion blur" looks more like judder than blur to my eye. If I do a frame grab, there is visible and real motion blur, no double image.

I always assumed this was a weakness of the long form GOP compression these cameras use. Could it be, as you say, an eye tracking artifact of a 60 hz display?
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Old June 22nd, 2008, 07:15 PM   #3
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Steve...

I've observed a very objectionable double image with my EX1 and my HD110 when I shoot in progressive mode at either 24p or 30p. I shoot a lot of panoramas in which the camera pans the entire scene, creating a constantly changing frame, not just motion within the frame. I make a concerted effort to keep my pans rate very slow, however, the "motion blur" looks more like judder than blur to my eye. If I do a frame grab, there is visible and real motion blur, no double image.

I always assumed this was a weakness of the long form GOP compression these cameras use. Could it be, as you say, an eye tracking artifact of a 60 hz display?
I'm doing a story for Broadcast Engineering on this topic. I have EX1 24p samples sent to me by someone who was convinced his new EX1 was broken. And, it did look horrible. He sent the same video to Getty Images and they said it looked "normal" GIVEN his zooms and pans were way too fast.

I had a wonderful discussion at NAB with Larry Thorpe -- of Sony CineAlta fame -- about a BBC paper that explains WHY these problems can arise. Both judder and strobing become more visible the "harder" the edges of moving objects. This is not simply the Sharpness one can adjust -- it is how the camera is designed, ie, COST.

Bottom-line -- CineAlta and Varicam really do shoot 24p like film. With lower cost cameras it appears that one must work harder: lower sharpness; use a wider than 180-degree shutter; never pan unless one is following a subject; and use an adaptor that lets you mount a prime lens dialed for minimum DOF. (That keeps BACKGROUND judder and strobing WAY down.) And, don't zoom. (For 30p, you may want to try 1/48th not 1/60th.)

His 24p JERKED past in a pan.

30p will strobe -- double images.

Do you have a different look for each?
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Old June 22nd, 2008, 07:41 PM   #4
 
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I've experienced both. And I like the term "jerky". Pans jerk past the frame. Sharp edges appear doubled. Both are equally distastful. I upped the shutter speed, which is opposite what you described, and the problem seemed to go away. I'll try the opposite, turning shutter off.
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Old June 22nd, 2008, 10:33 PM   #5
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I've experienced both. And I like the term "jerky". Pans jerk past the frame. Sharp edges appear doubled. Both are equally distastful.
Then why do you shoot 24p? Why not shoot 60i or 60p but with the good film camera techniques .

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Old June 23rd, 2008, 12:39 AM   #6
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I'll try the opposite, turning shutter off.
The standard shutter for 24p is 180-degrees. Do not turn shutter off -- WAY too much motion blur! Try 230-degrees.

There is no simple solution for 25p and 30p. The normal shutter-speed for them are 1/50th and 1/60th. With the old JVC HD1, we used to combat strobing by setting the shutter-speed to a mid-point between 1/30th and 1/60th. This added extra motion blur that obscured the double-images. Some folks simply went for 1/30th, which I felt was way too much blur.

Please report on 24p and 30p separately. Try shooting a busy intersection. Also to stay on topic, I guess we should only talk about the Canon.

Remember, the typical goal of shooting 24p is to mix with CineAlta/Varicam or with film. Or, to create a 24p BD disc or to go to film. To do this, pulldown is removed before editing. That means to REALLY see what you've shot you need to see it at 48Hz or 72Hz. It also means you need experience with what CineAlta/Varicam looks like.

If you just want a 24p look or 24p for Vimeo, then you only need to get a look YOU like. I don't know your goals.
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Old June 23rd, 2008, 07:18 AM   #7
 
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Thanx for your time and input, Steve. No, I don't shoot web video. Interestingly, zooms look fine. Most of my work is for independent feature "film" production. I will send dailies off to the director or DP on DVD, sometimes it is video on hard drive, I guess. The distribution media is, typically, DVD. One of these days(not holding my breath), I'll distribute in BD.

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Old June 23rd, 2008, 07:43 AM   #8
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Steve, you say "Bottom-line -- CineAlta and Varicam really do shoot 24p like film. With lower cost cameras it appears that one must work harder: lower sharpness; use a wider than 180-degree shutter; never pan unless one is following a subject; and use an adaptor that lets you mount a prime lens dialed for minimum DOF. (That keeps BACKGROUND judder and strobing WAY down.) And, don't zoom. (For 30p, you may want to try 1/48th not 1/60th.)"

I don't understand why this would be the case though. What is different about a Varicam and a much cheaper HPX500 for example? Would you expect judder on the 500 and not the Varicam? How about a JVC HD100 and a Sony F900 - if the JVC shows judder that the F900 doesn't, why would that be? Long GOP compression?

Steve
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Old June 23rd, 2008, 11:29 AM   #9
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I'll have to test again to be sure, but I seem to recall that shooting with the A1 in 25F (I have a PAL model), judder was not more noticeable than what I see in a movie theater. It helps a LOT to:
-select a shutter speed around 1/25s
-have a relatively narrow depth of field and a relatively "soft" picture (turn sharpening down)
-move the camera slowly.


Pans are especially difficult, which is probably the reason why you never see them used in movies. They use travelling sometimes, however.
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Old June 23rd, 2008, 01:01 PM   #10
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Several Blu-ray and HD DVD players offer the option of a 1080p24 output. This connection directly passes the 24fps material on an HD disc. Doing so eliminates the need to apply either type of 2-3 pulldown. Pulldown judder, therefore, is prevented. Low Frame-rate Judder, of course, cannot and should not be removed.
I think that native 24p output from a BD or HD DVD to a compatible display will still repeat the frames 3:3 at 72 hz. At least it is symetrical.

24fps viewed at the motion picture cinema I believe uses a double bladed shutter, flashing each frame twice at a 48hz rate, equivalent to a 2:2.

It is possible to shoot smooth looking motion at 24p, I've done it with the EX1, and not quite sure why it was so hard with the XH-A1, except perhaps my motivation wasn't as strong. But Steven Dempsey has shot lots of smooth 24F from the Canon XH-A1.

Going to the EX1 changed my software workflow somewhat, including making use of Sony Vegas 8.0b. By default, it adds motion blur so unwittingly I may have improved my technique for no other reason than that. But I never shoot 24p without a fluid head tripod, and some shot planning.

I have noticed in one particular long panning scene across a landscape, a "threshold" speed below which is fluid and smooth, above which strobes. It's a very subtle transition, at a very slow pan speed.

Bill Ravens, you will be the master of 24p.
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Old June 23rd, 2008, 01:43 PM   #11
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I am not sure about movie theatre projectors mine at home has 3 blades so flicker rate is 72hz but the frame rate is still of course 24fps for my sound films. This will be the same as the newer 72 hz plasma displays. Some projectors used for film to video transfer have 5 blades producing 120hz flicker. This of course has no effect on frame rate judder.

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Old June 23rd, 2008, 01:57 PM   #12
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I am not sure about movie theatre projectors mine at home has 3 blades so flicker rate is 72hz but the frame rate is still of course 24fps for my sound films. This will be the same as the newer 72 hz plasma displays. Some projectors used for film to video transfer have 5 blades producing 120hz flicker. This of course has no effect on frame rate judder.

Ron Evans
Agree it's not judder but it's one of the subjects of Steve's opening post, EYE TRACKING STROBING, a real phenomenon.
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Old June 23rd, 2008, 02:53 PM   #13
 
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Bill Ravens, you will be the master of 24p.
24P R US

seriously, I think the correct approach is to just eliminate pans.
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Old June 23rd, 2008, 09:08 PM   #14
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I am not sure about movie theatre projectors mine at home has 3 blades so flicker rate is 72hz but the frame rate is still of course 24fps for my sound films. This will be the same as the newer 72 hz plasma displays.Ron Evans
1) The reason to use 3 blades is it raises the Flicker Fusion Frequency which is necessary with projectors with very bright lamps. So with 24fps film, each frame is flashed 3 times which also helps reduce strobing. This is one of the reasons why a 72Hz flat-panel is of value when playing a 24p BD through a 24p connection to a flat-panel. In Europe, refresh of 25p is typically 100Hz. Which IMHO is too high -- 3X is better. This is why, as folks buy 120Hz displays find, and I find in Region 50, 100Hz or 120Hz makes film look like video. The optimum refresh rate is 72Hz (24p), 75Hz (25p), and 90Hz (30p). Amazingly, the 3X value used by modern film projectors. :)

2) Why are cameras different? Simply put, cheaper cameras extend frequency response -- to get some fine detail -- by boosting mid-frequencies (edges). The Sharpness control makes minor adjustments to the built in hump. The hump, plus the greater contrast of cheaper camcorders, make edges HARD. (Cheaper camcorder not only can't avoid contrast, adding contrast is a very cheap way of making a lens with low MTF appear sharper.) The harder the edge -- the more you see its motion, thus the more Film Judder and Strobing.

With $100K camcorders the lens MTF is way higher, the CCDs capture far more real (non-aliased) fine detail, and very sophisticated (COSTLY) frequency shaping circuits extend frequency response without adding the hump.

Film inherently has soft edges. It took a huge amount of work to get CineAlta to mimic film.

3) As Tom mentions, it is not safe to simply pan slow. Your slow pans need to be at the correct speed. And, fast pans need to be at the correct speed. The reason I don't shot 24p is not that 24p looks bad, it's that I don't have the experience to make it look good.
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Old June 23rd, 2008, 10:13 PM   #15
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A more accurate explanation of Judder

Seeing a whip pan create double images in a Roy Rogers movie at 6AM, reminded me that I left out "split frames" and thus incorrectly described Pulldown Judder. Here, is more accurate information.

Low Frame-rate Motion Judder
The word judder is applied to the lack of smooth motion that results from the low “sampling rate” of 24p, 25p, and 30p capture. This type of judder is inherent to low sampling rate video and is part of what’s called “film look.”

Split Frames
We know 24p material has 2-3 pulldown added so it can be broadcast at 480i60 and 1080i60. The result: Frame 3 and Frame 4 contain two fields that come from two different film frames. Despite these mixed frames often being called JUDDER FRAMES, they really are SPLIT FRAMES. With motion, a frame can contain two very different fields of the same object.

Pulldown Judder
We know 24p material has 2-3 pulldown added so it can be broadcast at 480i60 and 1080i60. only create judder when they appear in a series with “non-judder” frames. The 2-3 pattern (cadence) is Pulldown Judder. Therefore, 24p sent via 60i to a 60p display has three types of artifacts: Motion Judder, Split Frames, and Pulldown Judder.

Several Blu-ray and HD DVD players offer the option of a 1080p24 output. This connection directly passes the 24fps material on an HD disc. However, to display at 60Hz, another type of 2-3 pulldown is used. (This type of pulldown, however, does not manipulate fields.) Therefore, 24p sent via 24p to a 60p display has two types of artifacts: Motion Judder and Pulldown Judder.

If the display runs at an even multiple of 24, then only Motion Judder is left. Motion Judder, of course, cannot and should not be removed.

Eye Tracking Strobing
The situation where each frame is shown twice, is present when p25 and p30 video is shown on displays at 50Hz and 60Hz, respectively. Repeating the same frame twice causes moving objects to “strobe.” (It also occurs if 24fps film is flashed twice.)

Strobing is an illusion created by our eyes when they automatically follow (track) a moving object—and the moving object, in the same position, is flashed twice. (The effect can be lessened by flashing the image 3 or 4 times.)

Often called judder, it is not. It is an “eye tracking artifact.” Film shooters know how to control it.

In fact, the very same things done to CONTROL Motion Judder work to reduce strobing when shooting 25p or 30p. For example, by tracking a moving subjects which keeps the moving object in the same place within the frame. Of course, background details still strobe, but folks are looking at the foreground subject. Minimizing DOF reduces background shutter.

High Shutter-speed Strobing
When one uses a shutter-speed much above what's equal to 180-degree shutter, anything moving RAPIDLY though the frame will "strobe." Unless one is looking to create a Saving Private Ryan FX -- seeing strobing is a clear sign of a newbie shooter or a camcorder that provides no control over shutter-speed, or both. High shutter-speed strobing is the result of a too high shutter-speed while Film Judder is the result of a too low frame-rate.

Which is why those shooting 24p must be sure they set their shutter-speed to 1/48th > 1/60th just as those shooting 60i must limit shutter-speed to 1/60th > 1/100th.
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