Frame Rate (fps)
Definition
Frame rate is the number of images recorded or displayed per second, expressed in frames per second (fps) or in hertz. Cinema uses 24 fps. Television in 50 Hz regions (Europe, Australia and most former PAL and SECAM countries) uses 25 and 50 fps, while North America and Japan use rates divided by 1.001, inherited from NTSC colour: 23.976, 29.97 and 59.94 fps. ITU-R BT.709 lists 24, 25, 30, 50 and 60 Hz for HD, with the variants of 24, 30 and 60 Hz divided by 1.001, and ITU-R BT.2020 extends the list up to 120 Hz for UHD. The fractional rates complicate timecode, which uses drop-frame counting at 29.97 fps. Sport and live events are usually produced at 50 or 59.94 fps, and 120 fps is used for high-frame-rate capture and slow motion. Mixing frame rates in one chain causes judder, duplicated or dropped frames, or loss of sync.
In the field
A frame-rate mismatch often goes unnoticed until the signal reaches a switcher, a recorder or an LED processor, which then repeats or drops frames or fails to lock. The usual practice is to set cameras, capture cards, media servers and outputs to the same rate before rehearsal, and to check each menu, because 29.97 and 30 fps, or 59.94 and 60 fps, are easily confused.
Related lessons
Sources
- ITU-R BT.709-6 : cadences HD de 60, 50, 30, 25 et 24 Hz, et 60, 30 et 24 Hz divisées par 1,001 ; pas de cadence fractionnaire en 50 Hz · itu.int
- ITU-R BT.2020 : paramètres UHD, fréquences d'images jusqu'à 120 Hz · itu.int
- SMPTE, Understanding Standards: Time Code : cadences fractionnaires et comptage drop frame (ST 12-1) · smpte.org
- Wikipédia, Frame rate : usages 24, 25, 29,97, 50, 59,94 fps et haute fréquence d'images (pointeur secondaire) · en.wikipedia.org