Time Code and the
High Definition Camera
By Charles Parra, Denecke Inc.
Why use 30 fps Time Code when shooting at 24 fps film?
30 fps Time Code is used when transferring film to 29.97 fps
video for video/workstation editing or when transferring to mag film.
Every fourth frame at 24 fps matches every fifth frame at 30
fps. The film transfers at a rate of 23.976 fps when converted to video at 29.97
fps during the 3-2 pulldown. At the same time the audio is also slowed down from
30 fps to 29.970 fps. This results in an exact match in the video transfer.
Recording 30 fps time code at 48 KHz means that the transfer house has to sample
your tape at 47.952 KHz (using special decks) to achieve the .1% pulldown
necessary for the 3-2 pulldown. If the recorder supports the 48.048 KHz sampling
rate then the tape can be played back at 48KHz, which produces the same .1%
pulldown using the standard available decks. The audio can then stay in the
digital domain. NTSC video frames are composed of two interlaced fields, each
containing half the picture information. One field contains all the odd scan
lines, the other all the even scan lines. When 24 frame film is transferred to
NTSC, the first film image is placed in three successive video fields and the
second frame image is placed in the next two successive video fields. This
process is repeated until the end of the film. Artifacts of this process can be
seen if you play the videotape in very slow motion. There are many good
resources available for more information on the 3-2 pulldown process.
How does this relate to 23.976 fps and HD?
High Definition Cameras are in fact capable of running 24 fps progressive
scan. This is different than NTSC, which is 29.970 interlaced. If you use 24P (P
= Progressive), it must be down converted to 29.97 fps NTSC for video assist or
editing on an Avid. Also, when running 24P the audio TC must run at 30 fps.
Currently the down conversion from 24P is not practical. A solution is to run
the HD camera at 23.976. The High Def conversion to NTSC video can then use the
available standards converters.
Again, every fourth frame at 23.976 fps matches every fifth
frame at 29.970 fps. What does this mean to the audio Time Code? Just run it at
29.97 fps! The same sync relation from the 3-2 pull down chart holds true. The
difference being that there is not a speed change on the audio or video and the
sampling rate can stay at 48KHz.
What do I do on the set?
Until modern audio recorders support the 23.976 frame rate
all we can do is "Cross Jam." What is really important here is that
the jam sync occurs on the "00" frame. The zero frame is the start of
the second (real time or not). All Denecke products have always done this so
there is no problem there. If your recorder jams on the zero frame, i.e. Nagra
IV-STC, then you can cross jam directly without having to use one of our boxes.
HHB and PD-4 owners can cross jam using the Denecke Syncbox or GR-1. Simply feed
the 23.976 TC out of the HD Camera and into the Syncbox or GR-1 set to 29.97 Non
Drop and jam sync them as usual. Then simply take the 29.97 code over to your
recorder (set to 29.97 Non Drop) and jam sync or external feed the recorder with
the Syncbox or GR-1. The Time Code Slate is handled in the same fashion. Set it
to 29.97 and jam it either from the camera or the cross-jammed Syncbox or GR-1.
Treat this as a 29.97 video shoot. Keep in mind that this procedure only works
for Free Run / Time of Day shoots. Be aware that if the camera is powered down
for a battery change, etc. then the camera code will not be concurrent when
powered back up. In this case you will need to re-jam everything. As always,
communication is very important between the camera and sound departments!
An alternate method and the simplest way for Free Run shoots
is to feed external code to the HD camera. This is useful in multi-camera
shoots. Jam sync a Syncbox set at 23.976 to your recorder set at 29.97 ND.
Attach the Syncbox to the camera and feed it the Syncbox code. A software
upgrade is available for all the Denecke Products (TCXO preferred for high
stability) to enable the generation of 23.976 fps. The catch is that HD cameras
want to see Tri-Level Sync + Time Code for absolute frame accuracy. When feeding
external time code only, without Tri-Level sync reference, the camera can be in
sync or one frame off but not more than that. This can easily be corrected in
post, especially when using a Time Code Slate. When external sync is fed to HD
cameras, this sync becomes the master reference for the camera and must be
extremely stable.
Another option is to ignore the 23.976 on the camera
completely. Run your audio recorder with Time of Day (29.97fps) and jam sync
your slate as usual. You can take the output of a Syncbox that has been jammed
to your recorder (29.97fps) and feed it into track 2 of the HD camera that can
be used in editorial. The audio will stay at 29.970 fps when it is transferred.
Ed Novick used this method on the Spider Man movie with no problems. Also, you
don’t have to worry about the camera being powered down. Just make sure to
communicate with post so they don’t get mixed up with the time code on the HD
camera audio track!
Record/Run time code is a bit more complicated. The 23.976 TC
can be sent via RF or hard wired to the Syncbox or GR-1 (set to 29.97ND). It is
necessary to re-jam at the start of each take since the time code clock is
stopped when the camera isn’t running film. The re-jam process is automatic
with a GR-1 in Jam Continuous mode (JC). The GR-1 locks on to the next valid
(00) frame anytime there is a break in the time code clock (camera stopped).
What is the Evertz After Burner?
The Evertz After Burner converts 23.976 fps video to 29.970
fps NTSC video. The time code clock is also converted from 23.976 fps to 29.970
fps. This is a rackmount unit that requires AC power. There is a 5 frame offset
during the conversion to 29.97, but this can be handled in post. The drawback is
that you have to be hard wired (at least for the video feed). There should be a
corresponding audio delay to compensate for the video delay. Check out the
Evertz website at www.evertz.com for more information.
Many thanks to Ray Cymoszinski for his invaluable help and
guidance.
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