Why 29.97 and not 30?

mhbstevens wrote on 8/11/2004, 9:29 AM

This is probably a silly question but I have always wondered and never asked why is NTSC the fractional 29.whatever fps and not a nice round 30?

Mike Stevens

Comments

Former user wrote on 8/11/2004, 9:55 AM

When TV was B&W, it was truly 30fps. When color was created, they had to slow the signal down in order to insert the color information. That is how it got to be 29.97.

Dave T2

rs170a wrote on 8/11/2004, 9:56 AM

Simple answer:
Because of the change to the color system many years ago.

Complicated answer ((from the Leitch SMPTE Time Code tutorial):
When the NTSC color video system was introduced, a small change was required in order to add the additional color information to the transmission signal. A color subcarrier was added, at 3.579545MHz, and the signal rates of all other portions of the video signal were then derived from this new subcarrier’s frequency. This newly defined relationship changed the horizontal and vertical scanning frequencies slightly. The new
horizontal scanning frequency was defined as 2/455 times the subcarrier
or 15,734.26Hz, while the vertical scanning frequency (field rate) was
defined as 2/525 times the horizontal frequency, or 59.94Hz. For
historical accuracy, the previous horizontal scanning frequency was
15,750 Hz, and the vertical scanning frequency was 60 Hz.

(another) Mike