The Importance of Genlock when shooting 3D:
The Side by Side 3D Camera Rig pictured above is a Quasar Rig. While the manufacturer of this rig makes excellent Side by Side as well as Beam splitter 3D Rigs (also known as Mirror rigs or Mirror box 3D rigs), the intent of this article is to discuss the little up side down box that is strapped to the camera rig. This little box – The AJA Gen10 unit, is what contributes in a big way to headache free 3D viewing, and will soon shoot to fame as more indie and big name Hollywood and international movie makers start shooting Digital 3D.
Whether shooting with a beam splitter or side by side 3D Camera Rig, one thing of prime importance is that both cameras shoot in sync. Sync can mean a lot of things and is used very loosely to mean both the left and right video streams recorded by 3d Cameras, should display the same frames of video at any given time. Achieving camera sync can be done in a number of ways, with varying results. The crude way is to use an infra red remote to start both cameras at the same time (really not recommended).
Another method of Sync usually employed with a better degree of precision is by using Professional and prosumer Cameras such as from Sony and Canon that have a LANC remote port and support the LANC protocol. There are sync devices that utilize this unique feature and connect physically to such cameras to start-stop and even synchronize Zoom, focus and other camera settings. Yet, even this method of sync does not guarantee 100% sync between the two cameras on a stereoscopic 3D rig and the cameras can drift apart in a couple of minutes.
Some movie-makers would argue that sync can be achieved in post via a non linear editing (NLE) system. Just throw the two video streams on an NLE system’s time line and line up the shots. All these methods described thus far will give seemingly good results depending a lot on the footage being shot and… luck.
However none of these above methods allow for true guaranteed left and right Camera sync. For that to happen we need to “Genlock” the cameras.
Tri level Genlock Sync, Jam Sync or Bi-level Genlock Sync:
(video credit: unknow. Will update when I can)
Above: The very affordable Aja Gen10 Trilevel genlock box. Other trilevel sync boxes such as the popular “Lockit” are from Manufacturers such as Ambient.
The term Genlock is not new, and while it is being increasing heard today due to the popularity of 3D shooting, and the lack of any integrated Stereoscopic 3D Cameras, (barring the excellent Si2k 3D rig) genlocking cameras is done on a daily basis on any live production shoot, or in Television Studio and broadcast situation. Without getting into deeper details, genlocking cameras is necessary so that in a multi camera shoot, when an online director cuts to or “switches” to another camera, there is no hic-cup or “rolling” of the image, which would occur if the cameras are not all in frame-sync with each other.
Most professional Digital Cameras today have a Genlock in-out port. One of the simplest ways to synchronize two cameras for the purposes of Stereoscopic 3D shooting is to take the video output of one of the cameras and feed it to the Genlock input port of the other. This will JAM sync or give you Bi-level sync.
Importance of Tri-level Sync:
A very good explanation of what Tri level Sync and Bi Level Sync is, is written in this article. (the web domain seems to be down, but it’s saved in the Internet Archives here ) Taking an important point from that article that relates to Stereoscopic 3D:
- HD cameras (also crystal-synced film cameras) can start their recording cycles slightly out of step with one another (i.e. some fraction of a frame). This is not a sync issue per se, just a relationship issue between multiple cameras that will be become apparent later in post. It is a small refinement, but the use of Tri-level genlock ensures all camera start scanning the exact same line at the exact some moment in time. They will be in perfect sync and also in perfect step with each other.
Temporal Mis-match in Stereoscopic 3D:
The importance of Tri level sync for HD Cameras, versus simple Time Code (TC) or Jam sync. While the latter two methods of syncing a 3D Camera pair will keep the Cameras in frame-sync, they will not guarantee a camera sensor scan line level sync at *all* frame rates that HD Cameras can shoot at. Some other reasons for gel-locking 3D cameras the right way, borrow from the genlocking article above…
“The deployment of Tri-level sync is the basis for achieving frame-accurate, identical timecode-number stamping across multiple camcorders. If you just simply jam Timecode you can get up to a one-frame difference between the camcorders in a multi-camera set up. Or in other words, this means that the Timecode numbers might be slightly out of step with each other, by up to one whole frame, in any multi-camera set up.
What is the reason for this? It all depends on where the camcorder’s frame-building cycle is in relation to any other cameras shooting at the same time. Imagine three HD camecorders and three operators on a set ready for action. They hear the command to roll cameras and they all press their respective record buttons more or less at the same time. A tiny moment later, all three cameras start recording their first line of video. But they are almost certainly not going to be doing this in the exact same instant in time.
For example, as Camera “A” starts its recording cycle by scanning line one of a frame, camera “B” might start its recording cycle while camera “A” is in the middle of building this same video frame, and camera “C” might start its recording cycle as camera “A” is right at the end of building this frame of video. So in this case, all three cameras are about half a frame out of step with each other, and camera “A” and “C” are virtually a full frame out of step with each other. This is not a sync issue per se, as this relationship will remain fixed throughout the shot. This problem (in as far as it is a problem) can be overcome by feeding all cameras identical timecode that is referenced, or kept in step with an external Tri-level sync generator.
As the genlocked cameras are now recording in perfect step with each other, Timecode stamping will now occur at the exact same moment in time across all cameras, effectively eliminating any random partial-frame offset between cameras. A simple Timecode jamming operation doesn’t establish an absolutely precise relationship between the timecode and camcorder sync across several cameras. With simple jamming, there will always be a random offset within a single frame.”
While the article is obviously written for genlocking Cameras in general, we can see how this is really important for headache free 3D Viewing. Tri-level sync will ensure that Temporal mis-alignment of the left and right views will not occur at any Frame rate that an HD camera is shooting at, thereby making for headache free viewing of scenes such as:
- Water Fountains
- Speeding vehicles in movie chase scenes
- Any moderate horizontal movement of either subjects or Camera
The Water Faucet test for stereoscopic 3D mis-sync:
A simple test is to shoot running water out of a standard Faucet. Make sure that the water stream is in focus. Shoot with tri-level sync, and simpler TC or bi-level sync at different frame rates with an HD camera that accepts external genlock sync. Then put the left right video streams on an NLE time line and try to judge the distance a water drop has traveled, or any noticeable motion artifacts visible between the left and right frames of video.
Why is Tri-level sync not preferred? The short answer is that it can go wrong due to something as simple as a faulty cable. This will lead to a dropped or “green frame” that may go un-noticed on a live shoot or set, and can be un-salvageable even in post.
…”If there is an interruption to the external sync source, then the camcorder re-locks to its own internal sync system. But because there is no relationship between the internal sync system and the external sync system, this means there is a momentary problem while the camcorder re-establishes, or re-locks, to the new sync source, whether it be internal or external. The problem is made worse by say, an intermittent problem in a faulty cable or connector, that causes the system to flip flop back and forth between internal and external sync (this is perhaps the worse case scenario).”…
In all cases, TriLevel sync would come from an external source such as the Aja Gen10 and fed to both Cameras that comprise the 3D rig. Readers are urged to read the very informative article on Gen-locking, and Tri-level Sync here, and apply it when building and Shooting with a stereoscopic 3D Rig.
The author of the article on trilevel sync that has been linked to above, was kind enough to provide a more detailed explanation to additional queries on bi-level (SD sync) and tri-level sync. What was learnt (our conclusions – not the author of the linked to article) is that there are some newer HD cameras (particularly Panasonic HD ones) that are designed to accept bi-level as well as tri-level sync. *Some* HD cameras will accept bi-level sync but usually only when shooting at preset (typical SD) frame rates of say 59.94i and 50i, -or- 29.97 PsF and 25 PsF which are still valid for HD.
In effect it makes sense to setup for externally generated tri-level sync and use genlock units such as the Aja Gen10 or similar for temporally correct, and in-sync stereoscopic HD 3D footage.