Double System Sound with the Panasonic VariCam

There has been a certain lack of "resolution" on how best to provide time code sync for shooting double-system sound with the popular Panasonic Varicam high-definition camcorder. The VariCam is the commonly used name for Panasonic’s AJ-HDC27 variable frame-rate high-definition video camera.

The VariCam is made up of two parts, the camera head and a video tape recorder (VTR). The camera head is capable of shooting at variable frame rates ranging from 4 fps to 60 fps—well-suited to high-def and film-like 24 fps images as well as overcranked/undercranked shooting for slow/fast motion effects. This technical note aims to clarify some of the facts and to provide some suggestions for an effective, repeatable time code setup.

Regardless of the camera head frame rate, the VTR section always records at 60 fps or 59.94 fps (fps = frames per second). Frames are scanned and processed in the DVCPRO HD format with a resolution of 720p i.e. every frame consists of 720 horizontal lines each with a resolution of 1280 pixels.

Because the VariCam always records to tape at 60 fps, each captured frame is recorded once when the camera head is set to capture at 60 fps. However, if the camera head frame rate is set lower than 60 fps, the VTR duplicates frames according to a predetermined pulldown sequence. For example, if the camera is set to 30 fps, the VTR records each frame twice. For 24 fps acquisition, a 2:3 pulldown is used—one frame is recorded twice, the next three times, the next twice and so on as shown below.

For all other frame rates, VariCam merely alters the pulldown pattern to provide the 60 fps result required for recording to tape.

To distinguish which of the 60 frames on tape have new information and which are duplicates, the VariCam flags all the frames containing new information. Playback decks, such as the Panasonic AJ-HD1400/1700, and software, such as AVID and Final Cut Pro, detect these flagged frames to convert the 60 fps source material on tape to the originally selected frame rate.

On its own, the VariCam can only play back at its native 60P rate.

VariCam Time Code and System Frequency

The VariCam provides time code input and output connections, as well as tri-level sync input (GENLOCK IN). Because of this connectivity, the camera can run as either a source device or sync device. When operated as a sync device and set to F-RUN (free-run) mode, the VariCam automatically locks to the received (external) time code signal connected to its TC input. About ten seconds after connection, external sync is retained, even if the time code source is disconnected i.e. the VariCam’s internal generator is jammed. A Tri-level sync signal connected to the VariCam’s GENLOCK IN maintains synchronous lock with external devices. To re-jam a VariCam, it is necessary to remove the external time code feed, switch the VariCam to R-RUN (rec-run), then back to F-RUN, then reconnect the external time code source.

When setting up time code, it is important to consider the VariCam’s System Frequency parameter. These parameters can be set to either 59.94 Hz or 60 Hz and determines how many frames are recorded to tape per second (59.94P or 60P). The first VariCam model, the V-M1, had System Frequency fixed at 59.94 Hz. It wasn’t until the later ‘F’ and subsequent ‘H’ models that the System Frequency could be set to either 59.94 Hz or 60 Hz.

When set to 59.94 Hz (for NTSC workflows), VariCam’s time code generator and time code output run at 29.97 fps. For 60 Hz, time code output runs at 30 fps. In sync mode, the official stance from Panasonic is that VariCam should be fed with 29.97 fps for the 59.94 Hz setting and 30 fps for the 60 Hz setting. However, a number of reliable sources have confirmed compatibility with other rates, with the proviso that integer frames rates (24, 25, 30 fps) are used with 60 Hz and non-integer rates (23.976, 29.97 fps) be used with 59.94 Hz. Although, officially unconfirmed, it would appear that VariCam cross-jams its internal generator to external TC on the ‘00’ frame, then free runs.

VariCam will accept 29.97 drop frame, but it is generally recommended that you do not use it unless specifically requested by production.

Practical Suggestions

1. 744T Source, VariCam synced to Ambient Lockit – Plesiochronous Sync. Recommended.

Jam an Ambient Lockit (or Denecke equivalent) to the free run or Time-of-Day time code from a 702T/744T/788T, then feed its synchronous time code and tri-level sync outputs to Varicam. The advantage of this system is that you do not need to jam more than once during an average duration working day because the Lockit and 744T feature exceptionally low drift time code generators. The following table summarizes recommended settings for standard NTSC and PAL production.

As an insurance policy when working with ‘unofficially supported’ TC input frame rates (23.976, 24, 25 fps), you might consider also feeding TC to one of VariCam’s audio tracks. Consider this example where 29.97 ND is fed to VariCam’s TC IN and 23.976 fps is fed to a spare audio input.

  1. Set a Lockit to output 29.97ND timecode and 720/59.94P tri-level video syncs. Note: When set to output 720 tri-level syncs, the Lockit is designed such that integer TC frame rates (24,25,30) will select 720/60P and pull-down frame rates (23.976 or 29.97) will select 720/59.94P.
  2. Cross-jam the Lockit from a 744T source (set to 23.976 fps). The Lockit will cross-jam on the ‘00’ frame.
  3. Jam the VariCam from the Lockit’s 29.97ND time code output.
  4. Change the Lockit’s frame rate to 23.976 fps and re-jam it from the 744T, then feed the Lockit’s 720/59.94P tri-level sync and 23.976fps TC to the VariCam’s genlock in and audio input respectively. The DVCPRO tape will then contain videotape TC at 29.97 fps and audio track TC at 23.976 fps, both locked and phase correct on the ‘00’ frame.

2. VariCam Jammed From a 744T; No GENLOCK IN Connection – Asynchronous Sync

If a Lockit is not available, you can jam a VariCam directly from the 744T provided you are prepared to re-jam regularly. Once every 1-2 hours should suffice.

3. 744T Synced to a VariCam Source – Asynchronous Sync

When set to either Free-run or Free-run Jam Once mode, you can jam a 744T directly to a VariCam as long as you are prepared to re-jam every 1-2 hours. The 744T is able to cross-jam differing frame rates at the ‘00’ frame. Consequently, if Varicam is set to output time code at 30 fps (System Freq = 60 Hz) you can set the 744T frame rate to 30, 25 or 24 fps depending on production requirements; if VariCam is set to output 29.97 ND, you can set the 744T frame rate to either 29.97 ND or 23.976 fps.

As an alternative to frequent jamming, you can connect the VariCam TC out to 744T TC in via cable or RF link. The 744T must be set to one of its Ext TC modes. If VariCam is set to R-RUN (rec run), consider setting the 744T to Ext TC–Auto Record mode to have recorder enter record automatically when the VariCam begins rolling. Because the 744T only grabs the external time code at the start of recording, rather than continually recording it, there is no need to worry about providing sync lock between the two devices, with the one proviso that takes are not too long. Bearing in mind that DVCPRO tapes have a maximum duration of 31 minutes, TC drift is unlikely to be problematic.