Telecine  -  2:3 or 3:2 Pulldown

Telecine converts film to television - 24 fps to 30 fps.

Which is it -  2:3 or 3:2 ??

Telecine normally goes from film to TV, so the number of frames are increased - not decreased, so most call it 2:3 pulldown, but many call it 3:2.  To keep it straight, use 2:3 for telecine and 3:2 for inverse telecine.

This is where the confusion comes from.  The word "pulldown" would seem to indicate "less", but we are converting to more.  Although the origin of this term is difficult to trace, the word pulldown probably refers to the diagram used to explain it - where a series of 4 frames are shown above, and then "pulled down" to a larger series of frames below.

As to the origin of the numbers, two explanations are found depending on the source :

• for every other frame, the process takes 2 fields, and "pulls them down" to make 3 fields (2:3) - this is the one that seems most likely to be the valid explanation
• the final series of fields, there is a pair, then a triplet, a pair, then a triplet (i.e. 2:3:2:3:etc  -  stated simply as 2:3).  This is less likely the true meaning, since the colon represents a "ratio" - not a series

Telecine vs Inverse Telecine

Telecine uses a 2:3 pulldown to convert from 24 fps film to 30 fps TV.  

Inverse Telecine uses a 3:2 pulldown to take a television video 30 fps source and converts it to 24 fps.  This is usually used to reverse the effects of Telecine, which definitely cause artifacts.

Movies vs TV

A movie is usually made on a 35mm Film Negative, at 24 frames per second. The first thing that is done is the film is slowed down by 0.1% to make it 23.976 Frames per Second. This is done because the actual television frame rate is 29.97 which is FPS is 0.1% slower than 30 FPS. However, I will refer to the two rates as 24 and 30 for simplicity.

Television uses 30 frames per second.  It actually uses 29.97 fps, but for ease of discussion we use 30 fps.

Telecine, also known as the 2:3 pulldown, is the process of converting 24 fps to 30 fps.  We need 6 additional frames each second.  This means six frames need to be repeated - but which frames ?? Obviously, you would not want to repeat six consecutive frames.  Instead you want to spread out the effect, so that it is not obvious.

Why is the Ratio Wrong ??

If you start with 24 fps and end up with 30 fps, the actual ratio is 4:5, not 2:3.  The creators of this standard did not use the total frames to name it.  Instead, they used the fact that some frames (where each frame has 2 fields) are converted to 3 fields - hence the 2:3 ratio  -  it describes converting 2 fields to 3 !!

The 2:3 Pulldown Process

The pulldown process uses a 4-frame (8 field) model.  This model describes what is done with every 4 consecutive frames - where they are converted to 5 frames (10 fields).  Then you take the next 4 frames and do the same thing with them.  This is why IMO it should be called 4:5  or  8:10 pulldown.

• Every 8 fields of the movie, are converted to 10 fields for Television.
• This is the same say saying every 4 frames of the movie are converted to 5 frames of television.
• 24 fps is converted to 30 fps

• Frame1 is is copied 2 fields to 2 fields
• Frame 2 is copied 2 fields to 3 fields
• Frame1 is is copied 2 fields to 2 fields
• Frame 2 is copied 2 fields to 3 fields
• and the sequence of field conversion continues:  2-->2,   2-->3,  2-->2,  2-->3, etc.

 In the end, what was running at 24 fps is converted to 30 fps (the actual conversion takes 23.976 fps and converts them to 29.97 fps).

The Added Fields

For every 8 fields we add 2 fields.  The additional fields must come from somewhere.  If you simply copy the previous field every time, it will give an odd, brief pausing of the action every time you copy a field (2 identical fields in a row means no motion).  So what they do is to stagger the 3-field segments, copying Field1 for the first 3-field segment, then copying Field2 for the next 3-field segment, etc.  Assuming each frame is Field1, Field2 (F1, F2)  -  then the subsequent 3-fields are composed as:

F1, F2, F1  then  F2, F1, F2, etc  -  this pattern repeats for every two 3-field segments, as shown below, shaded in Yellow.  The 2-field segments remain the same of course, as F1, F2.

24p vs 60i (24 progressive frames vs 60 interlaced fields)

Film is shot using 24 frames, where each frame is a photograph.  Some purists believe that 24 frames per second has a richer look to it than 60 fields per second interlaced video.  Digital camcorders are now available that approximate 24 fps film, by shooting with 24 fps non-interlaced video (progressive scan) - and then converting that to 60i on the tape.  During playback it will have the look and feel of 24 frames per sec, even though it is 30 fps (60 fields per sec).  This format is called 24p.

Better Telecine - the Panasonic 2:3:3:2 Method

Panasonic took 24p one step further, and refined the pulldown technique.  Notice that with 2:3 - out of every 5 frames, two of them have fields from two different original frames.  In the diagram below, using 2:3, field C is comprised of B-C, and field D is comprised of C-D.  Using the Panasonic method, all frames except for one (frame C is comprised of B-C) contain the original fields. This is identical to 2:3 except that it clears up the 4th frame !!