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

How are NTSC Videos Telecined?

NTSC based video runs at approximately 30 fps (actually 29.97 fps), but film runs at 24 fps.  To convert a film that runs at 24 fps to run at 29.97 fps, it is first necessary to slow down the video by 0.1% to 23.976 fps.  Then approximately 6 frames are added to the video each second, bringing the frame rate to 29.97 fps.  This is done by adding one extra frame to each group of 4 film frames.  Although they could simply duplicate 1 out of every 4 frames to produce the extra frame, this method is not used.  This is because the duplication of one frame would cause that frame to be displayed for twice as long as the other 3 frames, which leads to jerkier motion.  Fortunately, film producers can make use of the field-based nature of video to more gradually introduce the extra frame.  Instead of adding a whole new frame at once, 2 fields are introduced separately to each group of 4 film frames.  Since 2 fields make up a frame, this method is equivalent to adding 1 new frame.  However, since the 2 duplicated fields are not added at the same time, this reduces the jerkiness of the video.

The process of adding 6 frames each second to the video takes place in two steps.  First, the film frames are split into fields (half frames).  While splitting them into fields, every odd film frame has one of its fields duplicated.  This means that every odd frame  is converted to 2 + 1, or 3 fields.  The even frames are simply converted to 2 fields without any field duplication.  This leads to an alternating field pattern of 3, 2, 3, 2, ...  For this reason, NTSC telecine is known as 3:2 pulldown.
Now that the frames have been converted to fields, the fields are put together in pairs to form the frames that will be seen on a television.  However, since all the odd frames were converted into 3 fields, each pair of fields does not correspond to the original film frames.  Rather, 2 out of 5 of the new frames will have fields from different film frames.  The fields from different film frames are actually just the duplicated fields that were produced when the odd film frames were converted to 3 fields instead of 2.

Since I'm sure that the above description was rather confusing, here are some diagrams that demonstrate what I am trying to say:

Source film frames

Source film frames split into fields
(T = top field, B = bottom field;  The underlined fields are duplicates.)

Fields combined in pairs to produce television-ready output
(The underlined fields are duplicates.)

As you should be able to see, the result is that the 4 original film frames have been converted into 5 frames for a television.  There are no entire frames that have been duplicated, but two of the fields (1T and 3B) are repeated in the final output.  Because the fields are duplicated from different frames at different times, the video plays smoothly.
Here are some sample frame grabs of NTSC telecined video.

Telecining Patterns in NTSC Video

If you look at the diagrams above, you'll see that some of the television frames are different from the original film frames.  Specifically, TV Frame #2 and TV Frame #3 each contain fields that do not belong to the same source film frame.  This difference in the fields isn't visible when watching a video on television.  However, when watching the same video in full resolution on a computer monitor, interlacing artifacts are visible.
TV frames that contain interlacing artifacts appear twice within each group of 5 TV frames.  The 3 remaining frames are progressive (non-interlaced) and thus do not contain interlacing artifacts.  By searching for this pattern of 3 progressive frames and 2 interlaced frames, it is possible to identify video that has been telecined.  Here is another small diagram that demonstrates the pattern:

You might have noticed that I rearranged the pattern to show 3 progressive (non-interlaced) frames followed by 2 interlaced frames.  Because the pattern of telecined video continually repeats itself, it is acceptable to start looking at the frames at any point in the group of five frames.  I have chosen this starting point because I find it easier to look for a 3p-2i pattern rather than looking for a 1p-2i-2p pattern. (p = progressive, i = interlaced)
You can recognize this pattern in my sample frame grabs of NTSC telecined video.

If you capture your video in low resolution (XXX by 240), then your capture card drops one field of the video.  In this case, you captured video looks like this:

Once again, I have changed the starting point of the pattern for simplicity.  Since there is only one field in low resolution video, there are no interlaced frames.  However, there is one frame that is duplicated in each group of 5 frames.  Therefore, the pattern is 4 unique frames followed by a fifth, duplicated frame.

How to Identify If Your NTSC Video Is Telecined

Here are the steps you should take to determine if your video has been telecined:

1)  First, you should try to capture your video at full resolution (XXX by 480).  When you capture a video with a vertical resolution of 480 lines, you are capturing both fields of the video.  You need to be able to see both fields of a video in order to identify the interlacing artifacts that are present in telecined video.
It is also possible to recognize telecining in a low resolution captured video (XXX by 240).  However, this method is more difficult and does not rely on the presence of interlacing artifacts.

2)  Open your video with VirtualDub or another frame-accurate video editing program.

3)  Locate a part in your video with lots of continuous motion.  Portions where the camera quickly pans across a scene are probably the best choice.

4)  For 480 line video:  scroll frame-by-frame through the high motion part of your video and try to identify frames with and without interlacing artifacts.
For 240 line video:  scroll frame-by-frame through the high motion part of your video and try to identify frames that appear to be duplicated.

5)  For 480 line video:  if you find that the pattern in your video is 3 progressive (non-interlaced) frames followed by 2 interlaced frames, then your video has been telecined.  (Here are some sample frame grabs of NTSC telecined video.)  If your video consists entirely of progressive or interlaced frames, then your video has not been telecined.
For 240 line video:  if you find a pattern of 4 unique frames followed by 1 duplicate frame, then your video is probably telecined.  If all the frames during the high motion scene appear to be unique, then your video probably wasn't telecined.