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History of 29.9



Phil, 
I don't know where you got your figure.

The NTSC subcarrier frequency is precisely 3579545 Hz, and there are
precisely 227.5 subcarrier cycles per line, and 525 lines per frame.  This
means 3579545 / (227.5 x 525) gives you the frame frequency.  The lowest (I
believe) integer ratio of that is 1431818/47775, which is 29.97002616431
Hz.

Taking 24/30  (or 4/5)  of that leads to the 23.976020903  film frame
speed.

BTW, very few people seem to realize that this also means the 4:2:2 clock
speed is not precicely 27 MHz in the US.  It is actually 26.99999657 MHz.

If you were to number every video frame from 0 through 29 and increment
seconds when rolling over from 29 to 0, you'd have a nearly accurate
version of the real running time.  Notice, however, that you will need 3000
ticks to reach 100 seconds, but in 100 real seconds you have passed only
just over 2997 frames.  Drop-frame timecode (maybe better named skip-frame)
compensates this by starting every minute except the tenth minute with
frame number 02.  The numbers 00 and 01 are dropped from the numbering
scheme.  Thus, 18 frames are gained every 10 minutes, for a total of 108
frames per hour.  Added to the 107892.0942 real frames which have played
during the hour gives 108,000.0942 apparent frames, which is very close to
the 108,000 we would expect in an hour running at 30 fps.

Combine all that with the 3:2 pulldown required to use "24" fps film in a
"30" fps video environment and you start to appreciate the complexity of
editing in NTSC countries.

Disclaimer: I receive no compensation from the National Television Systems
Committee.

Respectfully submitted

Craig Fearing

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