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Author Topic: Why 24 fps  (Read 4352 times)

benn600

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Why 24 fps
« on: January 22, 2009, 09:41:12 am »

When I see 24 fps content, I dislike smearing in fast motion.  Why aren't we pushing frame rates up to 30 or 60?  That is FLUID motion!  Why do people like 24 fps with stuttering--the film-like feel?
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mark_h

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Re: Why 24 fps
« Reply #1 on: January 22, 2009, 01:21:29 pm »

Movies are filmed at 24fps, so for playback you actually want 24 fps or a multiple there of to avoid stuttering.

Video is filmed at 60fps in the US and 50fps in Europe.

Mark
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glynor

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Re: Why 24 fps
« Reply #2 on: January 22, 2009, 03:32:17 pm »

High frame rates (above 30) look way too "video".  Useful for sports and other things with fast motion.  Terrible for everything else.

One note...

Video is filmed at 60fps in the US and 50fps in Europe.

Not true.  Most older NTSC "video" (US) is shot at 60 fields per second (59.94, actually, but that's getting very nerdy).  The equivalent European standard is 50 fields per second.  A field is NOT a frame.  A field is half of an interlaced frame (the even or odd lines).

The NTSC standard calls for 29.97 frames per second (fps), broadcast interlaced at 59.94 fields per second.  How the video is recorded usually depends on it's source.  If it is sourced on film, it is usually 23.976 fps (called 24p) and then telecined using a 3:2 pulldown technique to match the 29.97 broadcast frame rate.  If it is sourced from video (usually now with nice High-Def digital cameras) it is typically recorded at the higher 60i frame rate (59.94 fields per second) only if it is content that needs the higher frame rate (sports, car racing, and other similar subjects).  Other types of content (dramas, sitcoms, etc) are almost always shot at either 24p or 30p.  Even though the new HD standards allow for higher progressive frame rates (like 720p60) it is very rarely used.  Generally HD footage is shot and broadcast at either (again, NTSC, Europe is the same but the numbers are 25 and 50):

720p30 - most HD content is broadcast at 720p30 OTA.
1080i60 - sports content
1080p30 - some content but generally not broadcast OTA (usually only on digital cable or satellite broadcasts).

The main issue comes up when content with fast motion is shot with crappy video cameras that don't handle motion blur properly.  When you are shooting on film the way it works is simple.  The "shutter" on a film camera is actually a rotating pie wedge.  It blocks off (and thereby "closes" the shutter) for only a tiny a fraction of a second, but the film is exposed for the majority of the time that it is the "current frame".  Therefore if a subject is moving in the frame, it will "blur" in the shot (because it moves during the relatively long exposure).  Just like if you set your personal still camera to 1/30th of a second and take a picture of a person running across the field of view.  They will "blur" across the frame in the photo.  This makes movement on 24p movies shot on film look nice, because it creates a soothing (and realistic) motion blur effect for moving subjects.

However, most cheap consumer video camcorders do not do this.  They effectively manage exposure and shutter speed more like an automatic point-and-shoot still camera, and use very high shutter speeds.  Therefore, even though the frame is on screen for the full 1/30th of a second, when it was captured the shutter was only open for 1/200th (or less) of a second.  This makes managing proper exposure easier (the lens aperture doesn't need to compensate for low shutter speeds as much, and you can hand-hold the camera), but it makes fast motion look jerky and stuttering.

Pro camcorders don't have this limitation and can properly mimic a true film camera even though they don't use a rotary disc shutter.  Proper motion blur can also be added back in during Post in the Non-Linear Editor suite (and a good editor will do this).
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glynor

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Re: Why 24 fps
« Reply #3 on: January 22, 2009, 03:47:23 pm »

Interestingly, the only reason that the 60Hz field rate for NTSC broadcast was chosen was that it matched the 60Hz alternating current cycle of the power grid used in the United States.  This helped early kinescope cameras record early live television broadcasts, because it was very easy to sync the film camera to the proper frame rate (the AC cycle frequency was just used as a shutter trigger).  It also helped to avoid wave interference (which produces rolling bars on the screen), which was initially difficult and costly to shield against in early broadcast equipment.
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mark_h

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Re: Why 24 fps
« Reply #4 on: January 22, 2009, 04:01:12 pm »

One note...

Not true.  Most older NTSC "video" (US) is shot at 60 fields per second (59.94, actually, but that's getting very nerdy).  The equivalent European standard is 50 fields per second.  A field is NOT a frame.  A field is half of an interlaced frame (the even or odd lines).

Yes indeed.  Was trying to avoid complicating things ;)

Mark
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jmone

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Re: Why 24 fps
« Reply #5 on: January 22, 2009, 05:08:34 pm »


The NTSC standard calls for 29.97 frames per second (fps), broadcast interlaced at 59.94 fields per second.  How the video is recorded usually depends on it's source.  If it is sourced on film, it is usually 23.976 fps (called 24p) and then telecined using a 3:2 pulldown technique to match the 29.97 broadcast frame rate. 

How true - ever since I've been using "detect/remove 3:2 pulldown" in the video drivers and Reclock to monitor the frame rate of the material being played most of my NTSC material is actually 24p.
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Daydream

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Re: Why 24 fps
« Reply #6 on: January 22, 2009, 09:22:24 pm »

Interestingly, the only reason that the 60Hz field rate for NTSC broadcast was chosen was that it matched the 60Hz alternating current cycle of the power grid used in the United States.
One round of applause to you glynor! :) That is one small killer detail that usually on many (maybe most) guides is left out, like those FPS numbers were chosen out of the blue. Great that you mention it.
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jmone

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Re: Why 24 fps
« Reply #7 on: January 22, 2009, 09:48:37 pm »

One round of applause to you glynor! :) That is one small killer detail that usually on many (maybe most) guides is left out, like those FPS numbers were chosen out of the blue. Great that you mention it.

I'd also hazard a guess that there is a high correlation between PAL and 240v/50hz countries as well
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aldonc

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Re: Why 24 fps
« Reply #8 on: January 22, 2009, 10:08:26 pm »

Since we're on a nerdy trend... Here's a pretty good explanation why the frame rate is 29.97 fps and not 30 for color broadcasts - http://groups.google.fi/group/sci.engr.advanced-tv/msg/108815e3089c4d53. Note that it was 30 Hz in the "good old" B&W days (ditto for PAL/25hz countries). Because PAL/25Hz countries chose a different color carrying technique when they adopted color broadcasting (PAL vs NTSC) they did not have the audio interference problems that the 29.97 Hz change took care of.

Since a lot of NTSC material was shot on film, the 3:2 removal does restore the signal to it's original film-based frame rate. Some of the HD content is also shot at 24fps (actually 23.976 as Glynor points out), whether video or film, and the 3:2 pulldown is added since it needs to be broadcast at a "30 Hz" frame rate.

This 29.97Hz anomaly has also been carried into the DTV/HD broadcast world (primarily in the US) where the standard is now ATSC.

aldon

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mark_h

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Re: Why 24 fps
« Reply #9 on: January 23, 2009, 03:03:27 am »

How true - ever since I've been using "detect/remove 3:2 pulldown" in the video drivers and Reclock to monitor the frame rate of the material being played most of my NTSC material is actually 24p.

We do the same thing in our home cinema - the video processor (Lumagen in my case) does the 2:2 pull-up to return the material to correct 24fps cadence.  Once you live with 2:2 it's very easy to spot the 3:2 cadence stutter on 60Hz movie playback.

Of course, most blu-ray players will output native 24fps source from blu-ray discs meaning you no longer necessarily need video processors (though I still use one for other reasons).

Mark
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glynor

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Re: Why 24 fps
« Reply #10 on: January 23, 2009, 09:18:25 am »

Since we're on a nerdy trend... Here's a pretty good explanation why the frame rate is 29.97 fps and not 30 for color broadcasts - http://groups.google.fi/group/sci.engr.advanced-tv/msg/108815e3089c4d53. Note that it was 30 Hz in the "good old" B&W days (ditto for PAL/25hz countries).

Yeah.  I didn't go into the "why" it was changed from 60Hz even to the weird 59.94Hz rate of current broadcasts (due to color).  I figured I was geeking out quite enough, but thanks.  You took it one step further!   ;D
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xen-uno

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Re: Why 24 fps
« Reply #11 on: January 23, 2009, 09:56:37 pm »

Fascinating ...
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