Frame Rates and Interlacing
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Frame Rates and Interlacing

October 26, 2019

There’s no experience quite like sitting in
a movie theater and watching a film. Since it’s inception, film has had a special look
and feel to it that many video producers have tried to emulate.
In this segment, you’ll learn about interlaced footage, progressive footage, frame rates,
and footage conversion. A solid understanding of these key concepts
will help make your video, look like film. Knowing the difference between interlaced
footage, progressive footage, and frame rates might not be the most glamorous subject, but
it’s a crucial component to understanding why film has traditionally looked so different
from video. A working knowledge of these subjects will also help when you need to convert your
footage from one frame rate to another. Traditionally, television has been recorded
and played back at 29.97 frames per second. When television standards were being developed,
engineers were attempting to get the most amount of motion information using the least
amount of bandwidth to control the cost of production and transmission. The solution
they came up with is interlacing. Rather than sampling a full image 30 times
per second and displaying all the horizontal lines of the television picture to form each
frame, Interlaced footage is made up of fields sampled at the faster rate of 60 times per
second. In order to compensate for the faster sample rate, each field only contains half
of the image. One field contains the odd lines of an image, known as the upper field, and
the other contains the even lines, known as the lower field. The odd and even fields alternate
60 times every second, which is fast enough to form what is perceived by viewer as 30
complete frames. Let’s use a moving pendulum as an example.
As the pendulum swings from one side to another, over the period of 1 second, 60 samples of
motion are taken, but each sample only contains half of the lines that make up the image.
By using the process of interlacing, engineers created a method that allows for smooth recording
and playback of fast paced action such as sporting events. This also gives video the
“reality” feel that separates it from film. Video shot in this manner is sometimes referred
to as 60i. It’s important to note that the effective frame rate of 60i is still 29.97
frames per second. Film, on the other hand, has traditionally
been shot and played back at 24 progressive frames per second. Progressive images, unlike
interlaced images, sample the entire image. Every second, film samples the image 24 times,
which is considerably less than the 60 samples interlaced video takes, but each sample contains
the entire image. This difference in frame rates is one element that accounts for the
unique, surreal feel that film has, and is a key element in achieving the film look.
Another thing you’ll find when watching film is glass-smooth slow motion.
In film, this look was originally achieved by a process known as over-cranking. Over-cranking
refers to the process of recording the film at a faster rate, say 48 frames per second,
then playing back that footage at 24 frames per second. The result is stunning slow motion.
In the past, this has been very difficult to achieve with interlaced video. However,
consumer demand and advances in technology have brought some options to the table for
video producers. Many newer camcorders and DSLR’s have the ability to shoot 60 progressive
frames per second. Like the 29.97 interlaced frame rate, footage
shot in this manner takes 60 samples of motion per second. The difference is that footage
shot in 60p samples the entire image. When you play out 60 frames at the rate of 24 frames
per second, the footage that originally is one second in real time, will now last 2.5
seconds, slowing the footage to a 40 percent playback rate.
Because each frame is a complete image, this can be done without any loss of quality. However,
due to the intensive processing power it takes to record at this rate, many cameras can only
record 60p footage at a resolution of 1280×720. So if you’re planning on shooting the rest
of your footage at 1920×1080, you’ll have to up-convert your slow motion footage.
In order to achieve the film look, you’ll want to shoot your footage in 24p for standard
motion, and 60p for slow motion. But if your footage has already been shot in 60i or your
camera doesn’t have the option to select those rates, you’ll have to convert your 29.97 interlaced
footage. Many edit programs allow footage shot with
mixed frame rates to be dropped right onto the same timeline. These programs will use
various methods in order to conform your footage automatically when you export your final output,
however these methods will often times produce mixed results.
Converting footage from 29.97 to 24p is a complex process, and many producers prefer
to use effects such as timewarp in After Effects in order to have more precise control of their
conversions. Another option is purchasing plug-ins for
your editing software designed specifically to convert footage from one frame-rate to
another such as magic bullet frames or twixtor. While these programs will take a chunk out
of your wallet, their results are often worth it.
But what about converting that 60p footage you shot to play back in slow motion? There’s
a simple way to achieve this within premiere Pro CS5. Many other non-linear edit programs
should have similar features. In this example, we’ll select the footage
in the project panel. Notice that premiere will show you the frame rate of the file at
the top of the window. Right click the file, select modify, then interpret footage. Now
select assume this frame rate and Change the frame rate to 23.976. Notice that this will
change the duration of your shot. Now place the footage on your 24p timeline, and you’ll
see that your footage plays back in perfect slow motion, without a drop in picture quality..
When the goal is making video look like film, understanding frame rates, interlacing, and
progressive footage can really help you on your way. In our next segment, we’ll discuss
the use of lighting in order to further your efforts to make your video, look like film.

Only registered users can comment.

  1. Great video! What is the difference between shooting 24 and 30. I shoot on a t3i and I use 1920x1080p 30. But some people tell me I should be shooting in 24. What would you recommend? thanks 🙂

  2. While bandwidth was an issue, so was flicker and the fact that due to phosphor formulation at the time the images started to fade, so interlacing was used as a compromise.

  3. @JahnnaRandall that all depends on your distribution method. I believe, but I could be wrong, that displaying footage shot at 24 FPS will not look quite right on most TVs. If your shooting for web distribution neither frame rate should matter.

  4. You explained Interlacing very well ! Especially with the example of Pendulum. I have no more words to praise you. I'm interested to see more videos of yours.

  5. This is hands down the best video about video I've ever seen. I have been trying for years to properly explain to people why Film looks so damn good, and why 120hz LCD TV's make stuff look HORRIBLE when 120hz resampling is enabled.

  6. Rovi pro for Adobe Premier gives glassy smooth outputs no matter what size, frame rate or output format you use.

  7. Thanks, very helpful! I really understand now a couple of things which I'd never understand before. But, I've come out with some other doubts.
    -If I use the over-cranking technique to make a slow-motion scene, you're assuming that I really can't decide how much slow will be. In your example, at 6:02, you show when you interpret the footage at 23,97, the duration is 0:45. So if I extend or reduce the duration, it would be a little jerky, right?
    -59.94fps to 23,97fps…why? 59.94 isn't a multiple of 23.97 but 29.97. Correct?
    -Last, there are any visual differences from 23.97 to 24 or 59.94 to 60?

  8. Help! half of my film was shot in the wrong frame. It should have been shot in 1920X1280 it was shot 1280X720. Help me please!

  9. I submitted footage to the news yesterday and i treated the footage like i would with my youtube channel, apparently that wasn't the way to go.
    apparently they have a hard time playing footage back that has a frame rate other than 30fps

    i was extremely confused by this because in premiere or any video editing software that I can think of, there is never a problem when using clips with different bitrates, frame rates, aspect ratios, and resolutions.
    the footage will take whatever form it is rendered in.. no matter what what settings it had.

    so if someone understands tv can you enlighten me and I'm assuming the majority of people why that is?
    why is it so hard for them to playback any other footage that isn't in 30fps?

  10. These days US television are stuck with 23.976 versions airing on their 29.976 setup which is good for us UK guys having 25.000 smoothness.

  11. HA! the conspiracy of 24p! Too funny….
    (explanation of interlaced / non-interlaced video should help a LOT of people however…)

  12. At 1:34 and 2:30 this video gets it straight up completely wrong (despite most of the rest being correct). 60i is perceived as 60fps, the two half images are created and displayed at different points in time and never form a single whole image, 60i is 60 half images, not 30 full images. Dropping every other vertical line doesn't magically reduce the frame rate. The only time 60i ever becomes 30fps is when two fields are mushed into a single frame in a digital video format and when that video file is than played back incorrectly without unpacking the fields (aka deinterlacing). But that's a bug, not a feature. Properly deinterlaced 60i gives you a 60p video, if you get a 30p video you are doing it wrong.

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