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QUESTION: When I play the two outputs side by side the 24fps is noticeably less smooth than the 30. 

This is probably related to your computer's monitor refresh rate. 

For most computers/LCD monitors the *refresh* rate is 60 - and this is a problem for 24 frame video since the computer has to interpolate frames at odd increments to make up for the fact that 60 is not evenly divisible by 24.

Your monitor needs a refresh rate that is evenly divisible by 24, for 24 frame video to play smoothly - such as a rate of 72. OR, the refresh rate needs to be high enough that the interpolated frames are of short enough duration that they are not perceived as jutter.

This is partly why better home theater displays are capably of a true 24P refresh rate, or can refresh at the much higher 120 hz rate. 120 Hz is divisible by 24 by exactly 5.

If you monitor is refreshing at 60 (as most are) then 30 fps will look smoother than 24, especially for certain levels of contrast and motion.


The integration point of human sensitivity in sight, sound, and touch for "individual impulse stimuli" vs "continuous stimuli" begins at approximately 12 to 16 Hz) though it can be lower, as low as 4 hz for touch). For stimuli with lower frequencies, we perceive individual impulses, vs a continuous stream - but related to this is the differentiation between "on" and "off" of the stimuli - i.e. contrast.

I.e. in sound, where we transition rom hearing a series of distinct "clicks" vs a "continuous tone", or in touch, where we feel distinct "taps" vs a vibration. In vision, we begin to relate to motion in 8 to 12 Hs range, with smooth motion appearing above 16 to 20 fps.


The region between 16hz and 75 Hz is very dependent on contrast, that is, the difference between the absolute value of ON vs OFF. But also critical is the length of the ON vs the OFF signal.

But flicker can be perceived as high as 100 Hz and higher.

The phosphors in old CRT type PAL monitors were formulated with a longer persistence than those for NTSC monitors because PAL refreshed at a 50Hz rate, and NTSC refreshed at 60Hz.

In your LCD monitor, the backlight is typically going to be operating at a very high rate (NOT related to the refresh rate), so it won't flicker - but the REFRESH rate will have an impact on the perception of *motion*.


Motion is processed by a different area of the brain than still images. Things that help trick the brain into perceiving smooth motion include motion blur, number of images per second, and importantly the *equal distancing* of the stream of images.

While the region between 16 fps and 24 fps may have a varied perception of smoothness of motion, at 24 fps, smooth motion is "normally accepted" - but interestingly studies have shown that human's have a different emotional response to higher frame rates - between 24 fps and 72 fps, higher frame rates result in the perception of being "more real". There is no improvement in the "reality" sensation above 72 fps. A hypothesis is that as frame rates increase, the area of the brain that processes that experience shifts with higher frame rates.

In the case of creating a sequence of images intended to trick the brain into perceiving motion, the length of time eace image is exposed will affect the amount of motion blur.

Note that in the film "Saving Private Ryan", in the battle sequences, they set the camera shutter to a very short 45 degrees, to get a "staccato" sharp effect. While you still perceived motion, it had a "snappiness" to it.

Some film cameras allow the shutter to be opened as much as to 220 degrees, allow for increased motion blur - and some digital HD cameras allow for an "equivalent" 360 degree shutter - but in that case, the motion blur is SO long that it "feels like video".

Cinema 4D: If at 24 fps, the equivalent shutter angle for a 1/48th exposure is 180 degrees. At 220 degrees, the equivalent exposure is close to 1/40th 

24 FPS films in theaters: Most modern projectors use two or three bladed shutter to project the image on screen two or three time between pull downs - this eliminates flicker that would occur from a single bladed shutter cutting light off at 24 fps.

ShowScan: Showscan is not about making motion seem "more smooth" it's about making the total experience "more real". Running the film at 60fps allows for a small single bladed shutter, resulting in much more light hitting the screen. Also, Showscan is shot on 65mm (70mm projected) so the image area is nearly 4 times that of 35mm. The large image area and high frame rate together reduce the appearance of film grain (and dust), and greatly increase the perception of "being real". Showscan has a nearly 3D feel to it (without the need for glasses, LOL). 

However, there are reasons that this is not necessarily desireable in a narrative film - Stu has this to say on the subject:

Prior to the introduction of sound, films were shot and projected at a widely varying number of rates - from 16 fps to over 26 fps.

In the silent era, each studio had their own standard rate system, and in theaters not controlled by the studios, some theater owners increased frame rates of projection from what was shot, in order to fit in more films per day.

In the sound era, 24 fps was chosen because a single standard was needed for sound, as variations in filming and projection would result in objectionable sound issues - among other things EARLY sound was NOT optical, it was on a separate disk (i.e. a record), that had to run in sync with the film.

Hysteresis synchronous motors allow for accurate motor speed based on AC line power frequency. 24 fps is easy to "sync" using a system built with hysteresis synchronous motors, as is 30 fps. But other "off" frequencies are less straight forward. (a HS motor running at 240 Hz needs only a direct 10:1 gearing ratio for one revolution of a shutter in the camera, etc etc).

24 fps is more economical to shoot than 30 fps, and gives acceptable motion smoothness.

Nevertheless, many Todd-AO "roadshow" pictures were shot at 30 fps, such as the musical "Oklahoma".

Optical sound tracks can work at various different frame rates, including rates lower than 24 fps - the 24fps was chosen more due to the need for a single standard, and a stable speed, which was easiest to achieve using line powered synchronous motors. In 35mm, consider that the film travels at 90 feet per minute. But in 16mm it's 36 feet per minute. Sound tracks don't care about fps, but feet per minute, and 16mm typically used optical tracks as well.

24 fps was considered the best trade off for economy (less film shot), stability, smooth motion, ease of engineering projection systems that ran at the rate, etc etc.


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