Pulldown (a.k.a. 3:2 Pulldown)

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Pulldown

The Short Answer

The term "pulldown" or "3:2 pulldown" originates from the method of converting film to video, a process called telecine. This is how 24 fps film is transferred to 30 fps NTSC video without there being an apparent change in the speed of the film.

A more accurate term for the method of conversion is "2:3 pulldown" as this is the most common pulldown used. Film in the U.S. is shot at 24 fps for theatrical release. NTSC video in the U.S. is played back at 30 fps (or more precisely, 29.97 fps). In order to convert the film to video without changing the apparent speed of the film, a pulldown is used which "extends" four frames of film across 5 frames of video, converting an original frame rate of 24 to 29.97. This method is also used to convert 24p video (SD or HD) to 29.97 fps (either SD or HD).

More Info

Its important to understand the basic idea behind pulldown since today it has broader implications than just converting film to NTSC video. In the current state of HD video production and post production, converting frame rates is now a big part of this process. 24p has become a commonplace term today, yet it does not expressly imply 23.98 or 24 fps- it could be either of the two rates. Since its possible to shoot either of these two frame rates, it also means that many of us may be faced with the task of trying to convert 24p video to 29.97 fps for SD or HD and we could easily encounter problems if the "wrong" frame rate is used during production. We'll look at the standard pulldown patterns in use today, and in the process hopefully clear up any confusion on how we convert between 24 fps and 30 fps (and the other way around).

In addition, many cameras (SD and HD) that shoot at 24 fps use a pulldown to record those frames to tape, converting 24 fps to 29.97 fps. Although some cameras photograph and record at 24 fps, the majority do not. HDV, DVCPRO HD and XDCAM HD are among formats that photograph at one rate, yet use a pulldown to record to tape. See HD And Pulldown Recording later in this article to see how HD devices add a pulldown as they records to tape or file based media.

2:3 Pulldown

Although usually referred to as a 3:2 pulldown, the standard pulldown uses a different cadence: 2:3:2:3, and would be more accurately labeled as a 2:3 pulldown. As mentioned earlier, 4 frames of 24 fps material are spread across 5 frames of 29.97 video.

This is accomplished due to the nature of NTSC video, which is interlaced (2 fields per frame). Frames of 24 fps film are recorded to video by duplicating certain frames to more than 2 fields of video (more than one frame).

The cadence starts as follows: the first frame of film is recorded to the first 2 fields of video, creating a whole video frame. The next film frame is recorded to the next 3 fields of video, creating 1 1/2 video frames. The cadence then repeats, one frame goes to 2 fields, then the next frame goes to 3 fields, and the process continues over and over.

The Key To Success Is 23.98 fps

The above scenario sounds simple enough, except that the math doesn't quite work: an exact frame rate of 24 doesn't evenly convert to 29.97. In order to make the conversion with a traditional pulldown, The 24 frame source needs to be precisely 23.976 fps (typically referred to simply as 23.98) to convert to 29.97. This does work:

23.98/29.97 = 4/5

How a 2:3 pulldown works with 24 fps film and 24p video:

• With 24 fps film: During the telecine transfer, the film is run through the film scanner slightly slower than 24 (by 0.1%), running at 23.98. The speed difference is not noticeable to the viewer.

• With 24p video (SD or HD). There are two flavors of 24p: 23.98 and exactly 24. Most cameras that shoot 24p are set up to record at 23.98 by default. For the U.S., 23.98 is a must in most cases (for just this reason). For conversion to 29.97 via a pulldown, the 24p video should be 23.98.

Compatibility Between HD And NTSC

Since a lot of HD video shot today for TV must still be down-converted to standard definition at the NTSC frame rate for the majority of transmission to U.S. households, then HD video for TV should ideally be shot at 23.98 or 29.97 (or 59.94p). Although the NTSC standard, which is analog, has been discontinued for transmission, a digital flavor of this format has replaced it. And a large amount of consumers are still using analog NTSC equipment, whether an analog NTSC television, a DVD player or VHS deck.

HD And Pulldown Recording

High-end decks and cameras photograph a specific frame rate, like 23.98, and record to tape at that frame rate. These include decks like D5 and HDCAM SR. To photograph and record a different frame rate, the recording speed (frequency) of the deck can be changed and record to another frame rate like 29.97 interlaced (59.94 Hz). This is a 1 to 1 relationship between photographed frames and recorded frames.

On the other hand, there are different manufacturers creating cameras and decks that use a pulldown to record HD video to tape (or to file-based media as well). DVCPRO HD on tape (and in some cases to P2 cards), HDV and XDCAM HD and EX are examples of a completely different method of recording. These decks or cameras have a fixed recording speed, 59.94 Hz (which translates to 59.94 fps progressive or 29.97 fps interlaced). In order to accomodate different frame rates, manufacturers came up with an ingenious idea: photograph at one frame rate, but record to tape at another.

Therefore, during a 24p recording, the actual amount of frames photographed are 23.98 in one second. Then, in order to record those frames to tape, without changing the speed at which the camera records, a pulldown is added to the 24p material to end up recording at 29.97 fps. Similar to film transferred to tape, extra fields are added to the original frames to end up at the new frame rate.

Because these new video fields are "extra", certain video editing applications can remove these extra fields that were recorded, and the result is video that has returned to its original 23.98 frame rate. This process is often referred to as reverse telecine, which is covered next.

2:3:3:2 Pulldown

This modified pulldown cadence is used by Panasonic and other manufacturers to shoot 23.98 progressive video and lay it into a 29.97 interlaced video recording (or a 59.94 progressive recording). This pulldown pattern is referred to as "advanced" pulldown". This pulldown is employed as its easier for digital applications to remove or add this pulldown than the traditional 2:3 pulldown.

The pulldown can also be easily removed during capture into an editing system like Final Cut Pro, which will return the footage to 24 fps (23.98) by stripping out the duplicated fields or frames.

For more information on how Panasonic recording works (which is also a great overview on the common practice of using a pulldown for recording with other manufacturers as well), go to http://documentation.apple.com/ and click on the link for Final Cut Pro Help. In the page that opens, choose the document titled "Final Cut Pro 7: Professional Formats and Workflows". From there, choose Working with DVCPRO HD > DVCPRO HD Frame Rates.

A Confusing Array Of New Terminology

One of the issues that camera departments and post production staff face is they are now being given choices for video formats that seem contradictory in nature.

Here's an example of a choice on a Panasonic HD camera:

        1080i/24pA

At first this may sound confusing- how can something be interlaced and progressive all at the same time? The simple answer: the 24p stands for the photographed rate. The 1080i stands for the recording format. In other words, 24p video is recorded with a pulldown to 29.97i.

Notice that the above format also has an "A" in its description. This stands for an advanced pulldown. Whether or not the recording is done on a Panasonic device or another manufacturer, the "A" will often be included to denote that an advanced pulldown was used. Although some devices may record with an advanced pulldown without adding the "A" in the name of the format. Check the user manual of a particular device to see how it handles this issue. As a reminder, the advanced pulldown is what makes it potentially simple for an editing system to remove this pulldown on capture into the system.

Another format example is from Sony:

       24p/60i

Again, this refers to photographing 23.98 fps progressive, but adding a pulldown to record to tape, disc or memory card at 29.97 fps interlaced.

Reverse Telecine

This term is used to describe converting 29.97 video back to an original frame rate of 23.98 (or 24). Called inverse telecine and pulldown removal as well. Useful for many purposes:

• When film is transferred to NTSC compliant video, it takes on a 29.97 frame rate. When editing film digitally, where the film negative will eventually need to be cut for traditional film printing, it is advisable to do a reverse telecine (converting back to 23.98 or 24) so that when working, you have a 1 to 1 relationship between video and film frames, making sure that the negative cutting will be frame accurate.

• When working with a 24p camera that records at 29.97i or 59.94p like Panasonic SD and HD camcorders are able to do. During capture into an editing system like Final Cut or Avid, if the advanced pulldown is removed, you will end up with 24p (23.98) video in the computer. This is helpful if you need to output directly to film or output to 24p DVD's. Manufacturers of HDV video also use a similar technique of recording 24p video to 29.97i. Removing the pulldown not only returns the footage to 24 frames, but it de-interlaces the video, resulting in a progressive frame.

It should be noted that software and hardware can be used to convert telecine film assets from 29.97 to 23.98 or 24 (in Avid and Final Cut). When converting footage back to 29.97, software and/or hardware can be used to convert 23.98 or 24 back to 29.97. The main advantage of 23.98 is that there is an extremely easy out to monitors and decks (at 29.97) through a standard pulldown.

Going From 30 To 24

If film or video is shot at 24 frames, we already discovered how this footage can be converted to 29.97, as well as how to return it to its original frame rate via a reverse telecine procedure. But what about converting footage that is shot at 29.97 fps to 24 fps (or 23.98)? If the video is originally shot as interlaced 29.97 fps video, then converting the footage to 23.98 fps will create problems. Its doable, but there will be a quality hit.

When interlaced video is shot, each field is captured one after the other. In other words, an interlaced video frame is not really one complete picture from a single moment in time. As we saw earlier in the section Interlaced and Progressive Scanning, each field is captured slightly later in time from the previous field. If the footage is reverse telecine'd, then vital fields of information are removed from the video. The video will have motion artifacts as a result, a strobe-like effect. Remember, when a reverse telecine is done on footage originally shot at 24, then converted to 30, extra or redundant fields are added to the video. During the reverse telecine, only these extra fields are removed, so the final motion is as clean in the final 24 frame media as it was in the original 24 frame media.

How To Work With Video Shot Video At 24 and 30

If both sets of video footage need to be used in the same project, then one of these formats needs to be converted so all footage is at the same frame rate. Since converting 29.97i to 24p is going to create problems as we just discussed, it is preferable to convert the 24p footage to 29.97 and work with all the video at 29.97.

DVD's And Pulldown

As there are many choices today in formats and frame rates, we also need a basic understanding of how to create a standard definition DVD that will play on NTSC TV's. If the film/video is 23.98, it does not need to be converted to 29.97 before creating the DVD. In fact, there may be an advantage to leaving the video at 23.98. For playback on NTSC TV's, the DVD player will add a pulldown to play back the video at 29.97 interlaced. When the video on the DVD is at 23.98 (progressive), this is sometimes referred to as a "soft telecine". There are benefits to storing the video on the DVD at 23.98:

• 24p sources do not need an extra step of being converted to 29.97 before creating the DVD media files.

• Because 24 frames requires less data than 30, there is a space savings when using 24p sources. This gives more room on the DVD for more video, higher bit rates for better quality, more space for DVD-ROM material, more motion menus, etc.

• For playback from a progressive scan DVD player, a progressive output is achieved by duplicating frames, not fields.

Working With 24 fps And PAL

Another consideration is converting 24 frame film and 24p video to PAL or HD that is PAL compatible. PAL has a frame rate of 25 fps, and HD in the PAL world is also at 25 fps. There are currently two main ways to convert 24 fps to 25 fps.

2:2 Pulldown: This traditional method is a 1 to 1 distribution of frames. Each film frame is sent to 2 fields of video (1 complete video frame). Because of the difference in frame rate, this method speeds up the film by 4%.

2:2:2:2:2:2:2:2:2:2:2:3 Pulldown: Another adoption is a different pulldown method, which avoids changing the speed of the footage. Its not the same as the 2:3 pulldown, instead the pulldown used is 2:2:2:2:2:2:2:2:2:2:2:3. With this method, each frame is sent to 2 video fields (one video frame), except every 12th frame is sent to 3 fields (1 1/2 video frames)