An Explanation of Film-to-Video Frame Rate Conversion for NTSC

To better understand the upcoming concepts, one must be armed with some basic knowledge of how film gets transferred to video, as well as the nature of interlaced versus progressive display. As such, the following information is not intended to be a definitive paper on the subject, but should serve as a good introduction for all.

The visuals and animations presented here, though large in file size, are key and will reward repeat viewing.

Motion pictures are comprised not of motion at all, but numerous stills shown in rapid succession. For the films we all watch at the theater, 24 frames are shown in one second (24 frames per second, or 24fps). The NTSC television system differs from film in this regard, making it complicated to show film on video.

Televisions create their image by drawing (scanning) lines of light on the CRT face, left to right, top to bottom, to produce a picture over the entire screen. The resultant images that make up the motion picture are comprised of two interlaced fields: that is, the first field consists of all the odd lines (1 through 525), and the second field consists of all the even lines (2 through 524). The result is that only half of the video's display is drawn every 60th of a second. A simulation of this is shown on the left. Field 1 is scanned, and then Field 2 is scanned. Traditional talk quotes NTSC television as having 30 frames per second (as opposed to film's 24), each being comprised of two interlaced fields. This is actually misleading: The NTSC interlaced system shows 60 unique images per second, but each one uses only half of the vertical resolution available on the display. Only if the source material contained 30 unique frames per second could you could say that two fields form a single frame but in reality, video material such as the evening news is true 60 fields per second. So we don't want to think of interlaced televisions in terms of frames but rather in terms of fields, interlaced fields, and 60 of them per second.

The principal drawbacks of an interlaced display are (A) visible line structure, (B) flicker caused by the rapid alternating of the fields, and most important, (C) artifacts such as 'feathering' (also referred to as 'combing') and 'line twitter'. Visual artifacts like these last two occur anytime the subject or the camera is in a different position from field to field. The subject will be in one position for one field, and in another position for the next, resulting in jagged edges (feathering) or shimmering horizontal lines (twitter).

The animation on the right shows an example of an interlaced display trying to show a tomato moving from left to right. Each field shows the tomato a little farther to the right than the previous. Because the fields are interlaced, jagged vertical edges can't help but exist, except during for the last two fields (5 and 6) where the tomato is stationary. The further back you are from an interlaced display (or the smaller the display is), the less this and other artifacts are noticed. If you want to see the effect in real life, just stick your nose up to an interlaced TV. Focus in on an objects edge that is stationary and wait for it to move. You will notice this right away.

At left is an interlaced image of a skier. Not only is the flicker annoying, but have a good look at the ski-pole: It comes and goes because its so fine it can only be found in one of the two interlaced fields. This is line twitter. This artifact manifests it self when fine detail is less than 2 scan lines high. It is exasperated during vertical movement as the fields alternate. Often fine detail is filtered before being encoded to minimize these artifacts when played back at home on your interlaced display device. Because of this, we have yet to experience the full potential of DVD.

The preceding basic knowledge of interlacing is necessary to understand the transfer of film to video, because it is an important factor in what we end up seeing.

Motion picture photography is based on 24 frames per second. Time to call to mind all that math you learned in school and realize that 24 doesn't go into 60 very easily. To boil it down a little, our challenge is to make 4 frames from the film fit as evenly as possible across 10 video fields. We can't just double up the fields on every fourth film frame or we'd get a real 'stuttered' look. Instead, a process is used known as 3-2 pulldown to create 10 video fields from 4 film frames. This form of telecine alternates between creating 3 fields from a film frame and 2 fields from a film frame. Hence the name 3-2.

Consider now our flow chart of the 3-2 pulldown performed on four frames of this movie scene:

/images/dvd-benchmark-part-5-main.jpg (76125 bytes)

Pretty cool right? It is and it isn't. 3-2 pulldown inherits much of the artifacts we described when talking about interlaced video. A anytime a field follows one made from a different film frame (noted above by the "!" icon), there exist the possibility for anomalies in what we see, feathering and twittering being great examples. Absolutely any differences between the two film frames that make up the video frame (the last field of one frame and the first field of the next frame), be it brightness, color, or especially motion, are going to result in some artifact as the two fields merge on screen. Even our little animated synthesis of the final interlaced product, which actually contains 10 interlaced pieces, shows evidence of such anomalies as the flying police cars move ahead. Such is life.

As long as you are watching your movies on an ordinary interlaced display, there is not much more to tell you. What you see at home is pretty much what we've shown as the interlaced content in the above illustration. But should you have the fortune to be using a progressive display TV, the following comes into play.

Progressive displays, such as high-performance CRT/LCD/DLP/D-iLA projectors and the new HDTV-ready TVs, can show progressive scanned images as opposed to interlaced. In order to do this, the display must scan at a higher rate, 2x the speed of NTSC. Because we are scanning at twice the speed, we can draw an entire frame in the same amount of time it takes an interlaced system to draw a single field. We learned above that an interlaced display shows 60 fields per second. But with progressive, each "field" is now a complete picture including all scan lines, top to bottom, so we will now call it a frame, and we are showing 60 of those per second. (Of course, only 24 of those are unique if the source is film based) The benefits of a progressive display are no flicker, scan lines are much less visible (permitting closer seating to the display), and they have none of the artifacts we described for the interlaced display, as long as the source material is progressive in nature (film or a progressive video camera).

But sources which are truly progressive in nature are hard to come by right now. Movies on DVD are almost always decoded as interlaced fields yet all of the film's original frames are there, just broken up. What we're going to talk about next is how we take the interlaced content of DVD and recreate the full film frames so we can display them progressively. The term commonly used to restore the progressive image is deinterlacing, though we think it is more correct to call it re-interleaving, which is a subset of deinterlacing.

Deinterlacing (or re-interleaving) involves assembling pairs of interlaced fields into one progressive frame (1/60 of a second long), and showing it at least twice to use up the same amount of time as two fields. The need for 60 flashes on the screen each second stems from a biological property called the Flicker Fusion Frequency, meaning how many flashes that we need to see each second so that we (our brains) fuse the image into one where we don't see a flicker.

For every film frame that had three fields made from it, the third field is a duplicate of the first, and (if the MPEG-2 encoder is behaving properly) won't even be stored on the DVD. Instead of encoding the duplicate fields, the DVD flags repeat_first_field and top_field_first are used to instruct the MPEG decoder where to place these duplicate fields during playback.

The progressive output of a DVD player should assemble 2 fields from each film frame and create a complete progressive one that looks just like the original film frame. You should now be thinking that the DVD will once again have 24 frames to show in one second. But the progressive display is still expecting 60 complete frames per second. In order to space them out, the DVD player shows the complete frames in this order: 1, 1, 1, 2, 2, 3, 3, 3, 4, 4 and so on.

This form of display gives us a moving image very close to the original film. It has a tendency to "judder" a bit though, as every other film frame lasts 1/60 of a second longer than the previous one. Even our little synthesis of the final product, which actually contains 10 pieces, shows this judder. In the future, both the player and the display could increase their display rate above 60 fields per second, to 72 per second. At that point, the fields would only last 1/72 of a second, permitting the player to show every film frame three times (24 x 3 = 72), eliminating the motion judder, and also helping us with the Flicker Fusion Frequency problem (60 flashes per second are just barely enough in a well lit viewing environment). This would look like:  1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4 and so on. 72 fps will only work with film based sources though, as it is a multiple of 24. It will not work well with video sources which are 60 field per second.

The re-interleaving process we've just covered is specific to 24fps film material which is MPEG-2 decoded (as interlaced fields). It's really a matter of putting the right fields together so it's fairly simple. Deinterlacing native NTSC interlaced video material is much more complicated. In such video material, each field is a unique image in time, and in order to be deinterlaced at an acceptable level, it requires getting into motion-adaptive and motion-compensation algorithms to overcome the inherent problems of the interlaced material. There is no best method, and the two mentioned are expensive to implement.

(Note: NTSC does not really run at 60 Hz; it is technically 59.94 Hz. The industry rounds it up to make it easier to read. If you did play back video at 60 Hz instead of 59.94 Hz, you would end up with a dropped frame approximately once every 20 seconds.)

- Brian Florian - Secrets of Home Theater and Hi Fidelity

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