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Videos are made out of frames. In simple terms, each frame is a picture

Framerate is a property of video that indicates how many frames the video displays per second. It is denoted in FPS (frames per second). Common video framerates are 23.976/24 (NTSC/Film), 25 (PAL) and 29.97 (NTSC).

Types of Frames:

There are several different types of video frames when video compression is used.

  • I picture (key frames) - frames that do not reference other frames, and so they contain all the information needed to render the current frame. Usually occur at scene changes. Compression rate of about 7:1
  • P picture (predictive) - references earlier I and P frames, storing differences data. Compression rate of about 20:1
  • B picture (bi-predictive) - references past and future frames. Compression rate of about 50:1

3:2 Pulldown

NTSC television sets refresh at 60 fields per second (30 frames per second), while film is at 23.976 (24) FPS. 3:2 pulldown allows 24 FPS film to be displayed at 30 FPS by first splitting each frame into 2 fields. The first field contains the odd numbered lines in the frame (top field), the second frame the even numbered lines (bottom field) - when two fields are combined, they reproduce the original frame. Then, certain fields are duplicated to create an additional frame for every 4 frames.

For example, these are 4 frames in a 24 FPS film:

1 | 2 | 3 | 4

Using the interlaced nature of NTSC, these frames are split into odd (T - top) and even (B - bottom) fields:

1T 1B | 2T 2B | 3T 3B | 4T 4B

We now apply duplication of certain fields to create the extra frame (* indicate repeated frames):

1T 1B | 1T* 2B | 2T 3B | 3T 3B* | 4T 4B

The image below demonstrates the same process.


As you can probably notice, because certain frames contain an extra field, the repeated fields will cause the smoothness of the picture to be affected, causing what is known as Telecine Judder.

3:3 Pulldown

Some modern displays can now refresh at 72 Hz (72 fields per second). Because 72 is a multiple of 24, the telecine