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Re: 24/29.97/30 frame rate conversions





> Could someone please point me to descriptions of frame rate conversion
> practices for moving from video to film and vice versa.  In particular,
I'm
> interested in when/where the .01% slow down/speed ups are introduced
> and what is normally done with audio (e.g., sampling at 48048 and
> playing at 48000). And, if there are any "standards" for which frames to
> drop when turning 30fps material into 29.97 fps material.  (I know of the
> dropframe timecode standard... but I didn't think that would necessarily
> apply here).

Hi Eric,

The 0.1% speed change you refer to comes into play whenever material that
has an integral frame rate (e.g. 24, 25, or 30 FPS) is converted to or from
NTSC television, which has a frame rate of 29.97 FPS.

The audio must be slowed down or sped up, depending on which way you're
going, to stay in sync with the image.  Practically speaking, it means the
audio must be recorded at precisely the same speed as the image, then dubbed
at the new speed.  The resulting 0.1% pitch change is unnoticeable for all
intents and purposes, and is usually left "as-is" in the analog domain.

If the audio is digitally recorded, the sample rate is affected by the speed
change.   To obtain a reliable digital output, sample rate conversion is
generally required, either in the recorder itself or in a separate device.
Most digital audio recorders capable of off-speed operation also provide
analog outputs in that mode.  Using the analog outs may be preferrable,
since digital sample rate conversion is a complex process subject to
artifacts in some cases.

There is a misperception about 29.97 and 30 FPS time code.  In all time code
standards except 29.97 non-drop, the hours, minutes, and seconds correspond
to clock time.  Only the frame count changes to reflect the frame rate in
use.  29.97 non-drop and 30 FPS time code are exactly the same (i.e. both
count from 0 to 29 frames before rolling over to the next second), but 29.97
NDF is slowed down by 0.1% to match the NTSC video frame rate.  Its hours,
minutes, and seconds run slow compared to clock time.  For situations where
that is undesirable, the 29.97 drop frame standard was developed.  In drop
frame time code, it is only the count that is dropped--the actual video
frames are still there!

Best regards,
Christopher Bacon




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