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Photography

Choosing Progressive Versus Interlaced Video Recording

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You've likely seen a proliferation of numbers—1080p, 1080i, 720p and 24p, 25p 30p, 60p and 60i—on televisions and cameras, often on a shiny silver or gold decal. You might know these numbers are vaguely about video quality, but the exact meanings of these numbers are understandably murky. Even for pros they can be more than a bit confusing!

The first three numbers refer to the number of lines on a sensor or screen. The second five numbers refer to number of frames per second a video-image is recorded or played back. The letters i and p, as the tutorial title hints at, stand for interlaced and progressive, the main ways video images are created. These three technologies—number of lines, number of frames per second, and the way the images are recorded and displayed—are big factors in determining the resolution and quality of your video image.

This tutorial deals with the last of the set: interlaced and progressive methods of creating video images. These are the two choices for recording and displaying video footage, each with their strengths and weaknesses. This tutorial will help you choose and use each to the best of it's abilities.

Progressive Video Recording and Playback

Let's step back to the days of old fashioned cinema film for a minute. With a motion picture camera, motion is created by recording one still picture in rapid succession after another. This image is played back, projected, in the same way: one image after another in rapid succession. This creates the illusion of motion out of still images, just like a flip book. This is a progressive set of images or frames.

In the digital world, making progressive video works in pretty much the same way: digital still after digital still; 24 of them per second in traditional cinema filming, increasing to 30, then 60 and so on, each single image flashing to the screen in rapid succession. The higher the frame rate, the smoother the illusion of motion.

So far, so good. But why invent interlaced? Wasn't progressive good enough?

Interlaced Video Recording and Playback

Well yes, and no.

Interlaced has its roots in broadcasting. Television broadcasters needed a way to squeeze good-enough video images onto your screen with as little bandwidth as possible. Interlacing was the answer.

Imagine the film still as previously described. Now take an eraser and blank out alternating lines in each frame. Odd frames blank one half of the image, even frames blank the other. Even though the eye is only seeing half an image each time your brain puts together a whole.

Interlaced example
An example of how interlaced images work

With fixed bandwidth and a high refresh rate (60i, for example, which is 60 interlaced images per second), interlaced video provided a higher spatial resolution than progressive recording and transmission were able to provide. Interlaced made for a better quality look to television broadcasts.

As the half images of interlaced are processed quicker than a progressive capture, there is less time for the subject to move within the capture time and so the movement can be crisper and cleaner. Capturing an image progressively at the traditional cinematic 24 frames per second, the subject can move within the same capture time resulting in some blurring, the same as you would find in a longer stills camera exposure.

So Which is Better?

At the speed that images are recorded and played back, you’re obviously not aware of the process whether it’s progressive or interlaced; the human eye can’t keep up and the action should look smooth whichever system is being used. However, the finished results can produce rather different overall looks to a production.

Our ‘stills’ analogy from earlier doesn’t quite stand up because we’re now talking about digital sampling and when we say ‘still’, we actually mean image data. Data takes up space and requires time to be encoded and decoded; only milliseconds but that takes its toll on memory space and processing speed.

Computer monitors display video progressively, one still or frame after the other, so when you get an interlaced recording playing on a progressive monitor, things can begin to fall apart and weird lines can become apparent, particularly with fast movement as the progressive playback attempts to cope with the interlaced image.

Interlaced film playing on progressive monitor
An interlaced film shown on a progressive monitor will display the missing 'half' image lines 

Imagine two films, one recorded progressively and one recorded interlaced. Play the progressive one via a traditional broadcasting system that uses interlaced encoding and the system will play them reasonably well. No one will notice that the image is interlaced. But take the interlaced video and play it on a computer monitor or modern HD TV that uses progressive scanning and the problems begin. Half the data was never there in the first place, so the system attempts to create progressive images from half ‘stills’ resulting in the missing lines becoming apparent, particularly in fast movement.

 Which Method to Use in Filming

If forced to choose one method it would be progressive. Computer monitors display progressively, modern HD televisions display progressively and, despite the possible appeal of faster interlaced frame rates, you’re only ever getting half the data on screen at any one time, even when the recording is displayed on an interlaced system.

Record in Progressive, Most of the Time

Regardless of the HD size, the frame rate for most films was (and still is) 24 progressive frames per second. We’re used to it, we recognise it (on a subconscious level) and as filmmakers, we can tap into this subconscious expectation.

While the higher frame rates associated with interlaced video can be seductive, they give you a harder look, whereas the small movement allowed within each 24p capture can add warmth to the subject that overly crisp interlaced or higher frame rate progressive filming can take away.

The exception to this recommendation is if your project is predominantly about recording something that is moving fast: a sporting match or birds at the seashore, for example. Progressive recording will still probably be the right choice, but it is possible 60i will give you the smooth motion you're looking for. It's worth doing a test of your own to get a feel for the difference between the two.

Whichever method you choose, stick to it throughout your production. If you filmed interlaced, edit, render and playback interlaced. If you filmed progressive do likewise, keep it progressive.

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