Framerate

The framerate for film or video is how many full screens are displayed per second. It's measured in fps (Frames per second). Even though analog video isn't progressive, for purposes of comparison it's useful to use the same units. Interlaced video does also have a fieldrate, which will always be twice the framerate. Film has a framerate of 24fps, PAL uses 25fps, and NTSC 30/1.001fps. The strange number for NTSC is because of changes that were made from the original 30fps black and white signal so color could be added. It's commonly noted as 29.97fps, or occasionally 30fps. Sometimes NTSC and PAL signals are identified by a combination of their framerate and type (interlaced or progressive) or fieldrate and type. 25i or 50i refers to PAL while 30i or 60i is generally used for NTSC. This is generally in reference to a digital signal with either NTSC or PAL characteristics. Film can be referred to as 24p.

Film to Video Conversions

In order to display material from film, like movies and many televisions shows, it's necessary to change the framerate to match either PAL or NTSC, depending on the television. For PAL this is generally accomplished by speeding it up approximately 4%. Each field is half of an original film frame. The audio pitch will be raised slightly in the process. Converting film to NTSC is a little more complicated since it's 25% faster than film. The solution here is to repeat fields. Since 24fps is 6 frames (12 fields) less than 30fps, repeating one out of every 4 fields will give you a 30i picture that plays at the same speed as the original. Reducing the speed to 24/1.001 and then duplicate 1 out of every 4 fields, will result in an exact NTSC framerate.

The process of duplicating fields to convert film to analog NTSC is called telecine, referring to the name of a machine that performs the conversion. Digital video can use field duplication for this conversion as well, or it can use a technique called pulldown. Pulldown flags tell the decoder that's reading frames to duplicate fields in a telecine pattern. There are two major advantages to this. File size is reduced because no fields have to be stored twice. Also, due to differences in compressing interlaced and progressive video, what is stored will be closer to the original film frames. This is called 2:3 pulldown, referring to the sequence of 2 fields displayed for one frame followed by 3 (2 original and a duplicate) from the next. Since only the playback framerate is important, when pulldown flags are added the framerate of the file is now 29.97fps.

Other Pulldown Patterns

Pulldown can be used for other things besides film to NTSC conversions. PAL's framerate of 25fps also works well as both a source and destination format for pulldown. Film only needs one more frame per second to reach it, so duplicating a single field out of every 24 (12 frames) would give you a 2:2:2:2:2:2:2:2:2:2:2:3 pulldown pattern and audio identical to the original. Progressive PAL can be converted to NTSC using 2:3:2:2:3 pulldown so that 5 frames become 12 fields. Since pulldown only works with progressive sources, if you want to use it on interlaced PAL you'll have to deinterlace first, but that's a subject for another guide.



Version History

v1.0 2007.08.24 Original by Rich Fiscus
v1.1 2007.11.28 Interlacing information added by Rich Fiscus