Where 29.97 fps Came From
Understanding the origin of common frame rates is a real asset in recon video analysis. Speed determination requires knowledge of position and time, with time being the more finicky. Where the heck does 29.97 fps come from? Here’s the brief rundown.
Silent films. Operators hand-cranked projectors, varying speed for effect, but generally maintaining 16 to 22 fps. Multi-blade shutters were used to reduce flicker by flashing each frame two or three times, thereby doubling or tripling perceived frame rate.
Here comes sound. Audio changed the ability to run a wishy-washy frame rate, because audio was printed on the filmstrip. So pitch varied when frame rate varied. Not cool. For audio quality, film economy, and editing math, 24 fps became the international standard.
Television. Now comes TV, where footage isn’t the limiting factor, bandwidth is. This is where interlaced video originated. They needed a signal that wouldn’t flicker but also wouldn’t eat too much bandwidth. So, two fields were transmitted sequentially: field 1 (odd lines) then field 2 (even lines) 1/60th of a second later, forming one frame. Why 1/60th? The frequency of North America’s power supply was 60 Hz and matching it prevented interference patterns like hum bars. One field every 1/60th of a second = 30 full frames per second. That’s where 30 fps comes from. In countries using 50 Hz power (most of the world), the frame rate was 25 fps.
Color TV. Color came along and complicated things further. Black-and-white TVs were everywhere, and in an effort to ensure they didn’t become obsolete, engineers devised a signal that separated luminance (brightness) from chrominance (color). Older TVs ignored the color portion. However, the color subcarrier interfered with audio, so they reduced the frame rate by 0.1%. 30 fps became 29.97 fps (and 60 fields became 59.97 fields per second). This wasn’t necessary for the 50 Hz folks and they’ve stuck with 25/50. Much simpler.
So, that’s how we got 24, 25, 29.97, 30, and multiples thereof. That history really blew my hair back, and I hope you found it interesting as well.
Thanks for reading, keep exploring!
Lou Peck
Lightpoint | JS Forensics