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Re: The history of 29.994



Phil Budden wrote:
> 
> Not wanting to appear too ignorant but could one of you on the other side
> of the Pond please tell me the history behind running NTSC at 29.994?
> Also, when do you use simply 30 f.p.s. instead of 29.994 and when do you
> drop frames and so on ... and on ...
> 


Well Phil, I'll give it a go . . . but it may be lacking in the minutiae that
you'll get from some of our less technically challenged members (Chris Bacon &
Bill Topazio come to mind).  These are vague recollections from long ago
reading, so if I am off, Group, please add your corrections.

Your question actually embraces two different rate questions; the video rate
of 29.97, and time code rates that evolved around it.

The video rate of 29.97 was arrived at in the early development of
broadcasting as a compromise that allowed the simultaneous broadcasting of
both the color and the black & white television signals.  The 29.97 rate is a
base rate that satisfied two concerns; it dovetailed into the broadcasting of
the black & white signal without interfering, and secondly it is a rate that
could be mathmatically raised to a higher frequency (similar to how you raise
through octaves in music) for broadcast.  Oh . . . and I think that the
original quiding factor was that it was tied to our "line" rate (national
electrical rate) of 60 Hz - into which 29.97 was the closest available fit.

Thus when time code emerged we had 29.97 fps time code.  This 29.97 time code
rate when observed over the course of an hour actually produces an extra 3
seconds 8 frames and so evolved the neccessity for Drop Frame time code which
corrects for this error by dropping 2 frames every minute except every 10th
minute resulting in one hour being correctly reflected after one hour.  For
all but long format work, this minute error has little effect on our work, so
most people use Non-Drop Frame time code.

30.00 frame time code is used in field audio recording when production is
known to be for syncing in an NTSC environment.  The idea is this; that which
is being shot at either 24.00 fps or 30.00 fps will be transferred at,
respectively, either 23.98 fps or 29.97 fps (an approximate .01% slow down). 
Thus the field audio must be slowed down by the same degree to accomplish
syncronization.  So the 30.00 fps field rate time code when resolved to a
29.97 reference (typically 29.97 video, or time code depending on the method
of syncing) in the post production environment results in the correct .01%
slow down, and thus syncronization.

Hope this helps,

Tim Bond

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