Alesis ADAT Manuel du propriétaire Page 296

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296
Synchronization, Video and Midi Files
Emagic
Adat Edit
code is usually referred to as SMPTE time code, or just
SMPTE (pronounced: ›simptee‹).
Drop Frame (df)
One technical problem was the frame rate of 29.97 fps. Only
whole frames can be counted, but counting up to the 30th
frame in every second would cause a deviation between the
time code time, and the actual time (a difference of about 5.4
seconds for a typical feature film). So, the following trick was
employed: in every minute 2 frames are left out (›dropped‹),
except in minutes that can be divided by 10.
This may sound complicated, but is actually simpler than ›one leap-year every 4 years
except in years divisible by 100, apart from those also divisible by 400‹!
To describe this method, ›drop‹ or ›df‹ is added after the frame
rate. Because it is used so often with 30 fps, it is more common
to indicate non-usage by adding ›non drop‹, ›nd‹ or ›ndf‹ to avoid
confusion.
The 29.97 nd format is seldom used (just like the pointless 30
df), because the timecode time ›drifts‹ from ›real‹ time.
Frame Rates
The SMPTE frame rate must be set the same for all connected
devices; you can’t have different frame rates in one system.
The diagram below shows the various frame rates with the
respective duration of a frame, or subframe:
Frame Rate [fps] Frame [ms] Bit [+ s] Source
24
41,67 520,8
Film
25
40 500
PAL
29,97 drop
33,37 417,1
-
29,97
33,37 417,1
NTSC color
30 drop
33,33 416,7
-
30
33,33 416,7
NTSC S/W
Note
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