Frame Rates and Interlacing

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 1280x720. So if you're planning on shooting the rest of your footage at 1920x1080, 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.