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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 picture will