How Do 24 Film Frames
Equal
30 Video Frames?
Or, More Than You Ever Wanted
To
Know About Fields And Frames!
by Pam
Malouf-Cundy
The purpose of this article is to provide (as simply
as possible) a clear and comprehensive explanation of video fields and
frames and how 24 fps film frames (and digital systems) relate to them. If
that goal is accomplished, this will explain why a film project cannot
properly digitize as a video project, why an online will not precisely match
a cut negative or digital output, and more.
Let's Talk About
Fields
United States, 60 cycle (Hz) electricity pulses 60 times
every second. The pioneers of television and video technology had to create
a signal that could pulse 60 times each second. Video was created - 30
frames per second with two fields per frame - hence, 60 "pulses" per second.
NTSC color video standard calls for 525 lines of information scanned at a
rate of 30 frames per second, hence, each field scans 262.5 horizontal
lines. Some people refer to the two fields of a video frame as being
"interlaced." They are also referred to as "odd and even" fields, or "field
one and field two".
In Diagram 1, these fields are indicated by F1 and F2.It doesn't matter what
image is on a video field - the important point is that video must be respected as it's own medium requiring two fields
(one odd, one even) per frame. Not half a field from one frame and half a field from another -
each video frame must have its own two
fields!
Well, it'd be really perfect if film were 30 fps but
it's not!
3 - 2 Pull
Down
This created a problem - how to get 24 frames to equal
30! So, it works like this: 2 video fields for the first film frame, 3 video
fields for the second film frame, 2 for the third, 3 for the fourth and so
on - 2,3,2,3.. This is referred to as the "three-two pull down".
The first column shows four film frames. The next column
shows how that equates (gets translated) to 10 video fields. The next column
shows how that equates into 5 video frames.
We Can't See Those Repeating
Fields?
24 film frames per second are translated into 60 video
fields per second. The totality of what
material we see in one second remains the same. 1/60th of a second is invisible to the naked eye.
Everything
we've ever watched on tape or TV - that was
shot on film - does indeed have an extra field for every other
frame.
How Digital Systems Digitize
And Save Space
Now that we've established that there are two to three
video fields
per film
frame, let's analyze 24 frame capture (digitizing). Guess what? Another
techno wizard said, "If each of the two fields of a video frame is the same
image visually, let's digitize just one field and that will save hard-drive
storage space! We can then duplicate the fields during playback." Great
idea. This is also the root of all digital
skipping problems. But before we get into that,
let's get back to how systems digitize.
The digital systems all work with one field only for
digitizing. Avid takes the first video field and digitizes that. Lightworks
takes the second field and digitizes that.
Now, look at Diagram
2. Because only one field is digitized, you'll
see that the third (Avid) or fourth (Lightworks) frame becomes a duplicate.
The digital systems ignore and do not digitize this frame. In other words,
they throw this extra video frame out. This allows for true 24 frame capture
and film editing - which, of course, is wonderful.
When you play
back your cut or make an output (in a 24 fps
project), the digital system
recreates the extra fields based on the 3-2 pull
down formula (Diagram 2, column 5). The
digital system takes frame one of your cut and immediately starts the
2-3-2-3 field configuration. This is exactly
what happens when your cut negative is put on a telecine machine starting
with frame one. The great thing about this is that, if you are cutting
negative, your digital output will precisely match a telecine transfer of
your cut negative.
Making Sure Your Digital
Output
Precisely Matches Your Cut Negative!
As an editor, you must be aware that if you abort a final assembly (digital output) in the middle of
a reel or act, and then do a pick up on your digital system, you have
re-started the 2-3-2-3 configuration and therefore you may not match your
cut negative by one to two fields. Abort and
pick-up more than once, and you could be out of sync by several frames! For
example, you may have aborted at the end of a two field edit, where the next
cut would have
started with a three field edit. The 2-3-2-3 configuration will be restarted
at the pickup point and that next frame coming in on your output, will come
in with two fields only. But when your negative gets transferred, that frame
will be a three field edit - because the 2-3-2-3 configuration will not have
been interrupted.
Bottom line? When you are
doing your final digital output, you should not make any pickups in your
assembly. If you have to stop and make a
change, then you should start the output over starting with frame one of
your cut (reel or act) so that the 2-3-2-3 configuration remains constant
throughout the entire reel or act. This will assure that your cut negative
tape transfer will precisely match your digital output. You could try and do
a pick up on a two field edit, but the 3/4" decks (even the $10,000 Sony
9850's) are not guaranteed to be field accurate. By the way, because of
this, it's best to start your assembly based
on a timecode rather than manually setting
the 3/4" deck.
Why Do 24 fps Film Dailies
Stutter
When Digitized as a 30 fps Video Project?
Let's say you're cutting a show that isn't going to cut
negative because they are shooting it partially on film and partially on
videotape. So, you load it as a 30 fps video project and notice that your
digitized film dailies stutter upon playback. Sorry - but there is nothing you can
do about it!
|
|
|
Playback in a 30 fps video project is
different than in a 24 fps film project. Notice that each field is
played back twice, whereas playback in a film project uses the 3-2
pulldown field
configuration. |
When you digitize as a 30
frame video project, the digitizing and playback
process changes! One field of
each video frame is digitized and each field is
played twice during
playback. The 3-2 field configuration does not take place during playback in
a video project. Hence, the stutter effect . (We humans can see that extra
field when it's out of proper sequence!) Perhaps it goes without saying, but
original 30 fps video dailies digitize and playback accurately in a 30 fps project.
The only solution for us editors who need to cut 24
frame film in a 30 frame project, would be to digitize it at, for example,
level 75, where they digitize both fields. (Not really practical on a hard
drive storage level.)
Or, if you must mix tape and film dailies, you might
experiment with loading it all as a 24 frame film project - stuttering video dailies may look less offensive than
stuttering film dailies. Perhaps someday, Lightworks and Avid will have an
option to digitize both fields (besides AVR 75) - that would solve the
problem (but, of course, use double the storage space.)
Why an Online Tape to Tape
Master
Will Never Precisely Match Your 24 fps
Output or Cut Negative
Transfer
Here's where the fact that
each video frame must always have it's
own two fields comes into play most obviously. (Because there's not an even
ratio of film to video frames, the cut film and corresponding video edit
points do not line up evenly.) A video edit in the EDL might be slightly
longer or shorter than the corresponding film edit.
Look at Diagram
1 for a moment. Let's say you have a 2 frame cut
(frames A & B) that you want to online. It will never, ever online as 2
1/2 video frames. That's not possible - no more than having a 3 perf film
frame in the middle of a reel of 4 perf cut negative. Therefore, the online
will be either 3 video frames (6 fields) or 2 video frames (4 fields). Your
online will be one field longer or shorter than your cut. Put another way,
with a ratio of 24 film frames to 30 video frames, a 6 frame film edit corresponds exactly to a 7 1/2 frame video
edit. However, video edits cannot include
partial frames (less than two fields,) so the video edit must be 7 or 8
frames long.
Based on durations and timecodes, the digital systems
add or subtract a video frame to match as closely as possible the film cut
list. Therefore, your video online will never be more than 3 fields out of
sync with your cut negative. (Only! Ha!) So if you ended a cut on a close-up
of a dead man - and that actor opened his eyes on the very next frame - that
frame could end up in the movie if you online your show!
Meanwhile, (not to depress you,) the audio is correcting itself independently of the
video because the audio edits are usually
occurring at different points (due to split edits and overlaps).
Unfortunately, this means the picture and audio can be out of sync for a
video edit, although by no more than a frame. To quote the Avid user manual
- "this effect is the natural result of the sync disruption caused when 24
fps is converted to 30 fps."
What Can Be Done About These
Discrepancies
in My Online Versus Cut Negative?
If your producers want to cut negative, the most
accurate and cost efficient way to go is to make a digital output (without
pickups - as previously mentioned) - do not
online. Give the output to your negative cutters
and sound editors. When the negative is cut, it will precisely match your
output. If you must online a film
show, (normally series or rush TV movies) you
must be aware that at any point a cut you made may be up to 1 frames longer
or shorter.
So, plan to view the final online and look for critical
edits that may have changed. You can then correct them in an online bay.
Also, there will be some cuts that go in and out of sync with your video -
there is nothing you can do about this except warn your sound editors!
(Unfortunately, if the dailies were already a frame or so out of sync, and
then the online adds another frame - this can sometimes be quite
noticeable.) Lastly, the Avid has a special "match back" program which lets
you know exactly where each field discrepancy occurs - (it lists (+) or (-)
at each cut in the list) - so that you can manually adjust them.
Additional
Notes:
- An odd field must always cut to an even field
or you have Color Framing problems - a color
shift or bump occurs. Because of this you'll find that if you call around
town, every video house is either field one or field two dominant. This
means that all video edits made, in any bay in that video house, should
always occur on the same field. Keep this in mind if you see an
unexplained color bump on your online master.
- The "3-2 Pull Down" -
(the A-B-C-D // 2-3-2-3 sequencing) - originally started with a 3 field
(3-2-3-2) but the technicians soon discovered that starting with a 2 field
was better, but they never changed the name!
- If you're trying to do a 24 frame capture from a
digital output tape or any tape that
originated from film and it stutters - that's because you started on a
random or 3 field frame. Keep re-trying until you start on the first field
of a 2 field frame and it will digitize properly.
- Video is really 29.97 frames per second
because NTSC standard television operates at
59.94 Hz, therefore 24 fps film is actually
transferred in telecine at 23.976 frames per
second - this allows the film frames to divide evenly into 29.97. (The
sound is also slowed down by .01%.) Upon playback, digital systems speed
them up to play at a true 24 frames per second. Fine. The repercussions to
us as film editors occur only if you need to output audio from the Avid or
Light-works that will be transferred to 35mm mag and expected to play in
sync with a projected film print. (There is a procedure for doing this,
but it is not an aspect this article will go into.)
This article was updated in the November/December
'96 Newsletter.
For more details about timecodes and 59.94 Hz versus 60 Hz, please
read "To Drop
or Not to Drop" in the May/June '96
Newsletter.
