M-AUDIO Pro Tools Recording Studio - Manual (Page 1051)

Chapter 47: Working with Synchronization 1033
The same problem occurs when audio is re-
corded into Pro Tools without a resolved SMPTE
source (for example, if it was recorded before the
current session). The audio cannot be accurately
synchronized with an analog tape recorder or
video tape deck, since the Pro Tools audio was
not recorded referenced to the SMPTE time code
from the analog tape deck or video tape deck.
Resolve All Components of Your System
When striping time code, make sure that the
time code generator and the video record deck
are resolved to the same crystal reference. For
example, when striping 29.97 Drop Frame time
code onto a VTR, both the SMPTE generator and
the VTR should be resolved to the same “black
burst” or house sync generator. During play-
back, the master deck should be resolved to
“black burst” or house sync.
This convention provides compatibility for your
tape between the record and playback passes,
and when it’s played back in other facilities on
different equipment. This also means that when
playing back a tape striped with time code, the
playback deck should be resolved to the same
synchronization rate as the record deck was re-
solved to at the time of the striping. When you
stripe time code using an audio transport, it
should be “free-running” and unresolved, but
should be resolved with a house sync-referenced
synchronizer during playback.
Know the True Frame Rate on Your Media
If you get your media from a production com-
pany instead of recording them yourself, be ab-
solutely sure that they indicate the SMPTE frame
rate used on the tape correctly. Several time code
formats use the same frame count (such as
23.976 fps and 24 fps) yet they run at slightly
different frame rates per second.
A Digidesign SYNC peripheral (and some third-
party products) can be used to help determine
frame rate.
29.97 fps Non-Drop Rate Not Recognized
29.97 fps Non-Drop is a slightly slower version
of 30 fps Non-Drop time code. When 29.97 fps
Non-Drop is used with color video, each video
frame matches up with each SMPTE frame with-
out having to use a drop-frame coding. This
makes any frame number mathematics much
simpler, since no frame numbers are dropped.
Some hardware and software devices do not rec-
ognize 29.97 Non-Drop as a separate frame rate.
For example, any standard SMPTE-to MTC-con-
verter does not explicitly recognize it. You must
set the convertor to expect 30 fps Non-Drop in-
stead. In fact, many devices that read SMPTE
work acceptably by reading 29.97 Non-Drop if
they are set to expect 30 fps Non-Drop.
Any SMPTE reader that uses the time code num-
bers to make real-time calculations (as Pro Tools
does when it tries to trigger and synchronize to
SMPTE) also needs to know that the frame for-
mat is 29.97 and not 30 fps. Since Pro Tools al-
lows this choice of frame rate this does not really
pose a problem. The problem exists because
many users cannot readily distinguish 29.97
from 30 fps.
Even though NTSC tapes only use 29.97, some
production companies will distribute video
work prints striped with 29.97 fps but mark
them as “30 fps NTSC,” by which they actually
mean 29.97 fps Non-Drop.
By the time you get the tape, you may have no
idea what’s actually on it. Feeding 29.97 Non-
Drop to Pro Tools when it’s set for 30 fps Non-
Drop will result in timing errors of about 1.8
frames per minute, causing audio playback to
trigger out of sync.