Stay in sync with HD cameras

Sync With HD Cameras (from Location Sound Inc.)

Everyday, my phone rings and the first words I hear are, “What’s with this 23-frame
time code?” followed by “I am about to start a job that is using a 24P HD camera
(usually the Sony HDW-F900). How do I stay in sync?” Just as we were getting used
to having to contend with pull-ups and downs when dealing with a 24-frame film shoot,
we get another wrinkle. The actual time code rate is 23.976 fps. It is also referred
to as 23.98 fps when rounded up. Since there is no 23.976 frame rate selection on
most available time code audio recorders in the market used for production, including
all DAT recorders, Nagras and digital multi-tracks, you can see why there is major concern.

The situation is not as bleak as you would think; and, in fact, may simplify things.
The following will mainly deal with interfacing with the Sony HDW-F900 camera since
that is the one I have tested with at Panavision in Woodland Hills, CA. Whenever I
write 29.97 fps, I am always referring to “non-drop frame”.

The F900 (shown on left) is being widely used in the industry, and as of this printing,
Panavision had 50 in their rental stock. The F900 is an HD camera capable of running at
24fps progressively scanned. This is a different video standard than the NTSC interlaced
29.97 fps we have come to know and love. Because most video workstations and edit bays
perform in the NTSC 29.97 fps format, the HD picture must be down-converted to NTSC.
This is where the problem lies. The converters of today cannot properly handle the
conversion of 24fps progressive to 29.97 fps NTSC. However, there is no problem converting
23.976 fps to 29.97 fps. These two frame rates are closely related to each other, and
the conversion works perfectly. Since 23.976 is a workable solution, it also means
there is no .1% slow down (that we have all come to know and hate) from shooting
with 24-frame film. Did you hear that? No speed change for picture!

So what about the time code for sound? If you are running a DAT recorder and want
to have the same time code on audio as you have running in the camera in a “Free Run”
(time of day) situation, you can cross jam 23.976 to 29.97 fps from camera to audio.
In this scenario, the camera acts as the master time code. To do this, you will have
to use a Denecke SB-2 Synch Box, a Denecke GR-1 generator/reader, or an Ambient ACL202C
time code Lockit box. Whichever one you choose to use, set it to 29.97 fps and jam sync
it from the 23.976 fps of the camera. The camera will have a BNC connector for time code
output. Then feed the 29.97 fps into your DAT recorder from the Synch or Lockit Box.
Have your DAT recorder set to 29.97 fps.

If you are still running a time code slate, make sure that it is set to 29.97 fps as
well when you jam it from the DAT recorder. The reason why this works is because the
Denecke and Ambient time code products grab the “00” frame of the incoming time code.
When this happens, the first frame of each second of the 29.97 code will match the
first frame of every second of the 23.976 code it was jammed from. There will be a
drift within the second until the first frame arrives and then they match again.
Because of this relationship in “real time”, the two time codes will match and stay
in sync with no drift. The HHB PDR1000-TC DAT recorder does not jam from the “00”
frame of incoming time code. If you try to cross jam into the HHB, it will grab a
different frame every time and give you a different offset each time you jam. The
Fostex PD-4 Version II will not recognize 23.976 when fed into it for cross jam.
Cross jamming with the Denecke or Ambient Boxes can also be performed when using a
Nagra IV-STC recorder or a digital multitrack recorder like a TASCAM DA-98.

Now that sound is running 29.97 fps and camera at 23.976, what happens at the end
of the day? As stated earlier, the picture must be down converted if it is to be
edited in video (NTSC) on a non-linear system like the AVID. During the down conversion
process, the 23.976 fps is converted to 29.97 fps and a window burn-in of 23.976 fps
is created for reference. Now the 29.97 picture time code is frame accurate with the
production audio time code and is easily synched for editing.

When using this method in the field, there are a few things to watch out for. Nothing
is perfect! When the F900 camera changes batteries or is powered down in any way, you
must jam sync again! Changing the batteries creates a time code skip and suddenly you
will have a possible 6 frame offset between sound and picture. There must be excellent
communication between camera and sound on this. The F900 will not lose frames when put
on standby.

If the production requires rec/run time code where the camera time code only runs when
in record mode, a cross jam will have to occur before each take. To do this you will
need an RF wireless feed transmitting time code from the camera to one of the Denecke
or Ambient time code boxes which, in turn, will feed your recorder. The sound mixer
will then have to remember to jam code from the camera as soon as the camera starts
to roll before a take. From your recorder, you can do an RF wireless feed of time
code to your slate. In my view, time code to the slate is redundant since you have
the same time code running on camera and audio. That goes for “Free Run” mode as well.
Just a standard clap slate would be sufficient. Also, one other note: If you don’t
want to deal with matching time code on picture and audio, you can just treat it as
a film shoot and just run master “time of day” time code on the audio recorder with
a jammed time code slate.

Cross jam can also be performed going to the camera as well with the sound recorder
being the time code master. This is especially useful if you are running multiple
cameras that you want to stay in sync. The Denecke SB-2 and Ambient ACL202C can be
used for this purpose. The latest model of these two devices will generate 23.976 fps.
Set them to 23.976 fps and jam sync them from your 29.97 fps audio time code. Then
walk the ACL202C or SB-2 over to the camera, hang it on the side, and feed the time
code into the BNC time code input of the camera. Repeat this for each camera you have
running. You will need a Denecke or Ambient box for each camera. These boxes have very
accurate crystals and are very stable. One problem with this procedure at the moment
is that the F900 wants to be Genlocked to Tri-Level Sync video when receiving an external
time code source. Tri-Level sync is related to the HD picture and is not a standard NTSC
video sync source. You cannot Genlock a 24P camera with standard NTSC video sync.

Denecke and Ambient will soon have versions of their boxes available with Tri-Level
Sync built in. Without the TriSync, there is the possibility of a 1 frame offset, no
more than that. The F900 camera will occasionally miss the first frame of incoming
time code. In fact, when I tested this, it happened everytime I sent time code into
the camera. A 1-frame offset is easy to deal with; with TriSync there will be no offset.

If you don’t mind having cables going to your cameras, a company called Evertz makes
a host of products that handle HD video and sync issues, including down converters,
Tri-Level Sync generators, and time code generators. Evertz products are used frequently
on sets for TV production using 24P cameras. Usually, the amount of cables running to
camera is not as much an issue on a TV set. Evertz have been getting a lot of requests
from the film world lately to start making some product that is more conducive to being
used out in the field where there is no AC power and flexibility is of the essence.
Hopefully, we will see some of that in the near future.

The Evertz HD9150 down converter, also known as the Afterburner, is being used on sets
to send an NTSC signal to video village for playback. You will also need a down converter
on set if you want a picture feed from camera to your video monitor on your sound cart.
The HD signal is sent directly from the camera to the converter. Be aware that the picture
and time code is delayed about 4 frames when sent through the down converter. Audio that
was recorded on the camera does not get converted but must be delayed to match picture
after the conversion. If the sound mixer is sending video village an audio feed as well,
it must be delayed to match picture in NTSC format. The other option is to have HD Monitors
and VTR’s on set to avoid some of the down converting. This would be an expensive prospect
for some.

As you can see, there are many issues to pay attention to regarding sync sound for an HD
production. As usual, communication is the key, but with everybody on the same page, things
will go smoothly. Just remember the close relationship that 23.976 fps has to 29.97 fps,
and that there is no .1% pulldown. Without a pull-down to worry about, we are really only
talking about the possibility of time code offsets which is easily compensated for, as
opposed to time code drifts.

Resources:

Panavision, Woodland Hills, CA (818) 316-1000

Nolan Murdock, Product Specialist

Evertz Microsystems Ltd., Ontario, Canada

Alan Lambhead, Vice President of Engineering www.evertz.com

Ambient Recording, Germany

Chris Price, Sales Manager

www.ambient.de

Denecke, Inc., North Hollywood, CA

Charlie Parra, Engineering