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Genlock

Now that we can sync the individual frames via timecode, we need to synchronize the moment where we measure the motion and position of the camera to the moment when the camera captures an image on the sensor. This is sometimes a bit tricky.

Some cameras offer a “genlock” function, where their sensors will synchronize to an external analog sync signal. The sync signal used in all of the high definition video workflows is called “Tri-level sync” (http://www.extron.com/company/article.aspx?id=ta062002). There are small sync generator boxes that can create these signals (the AJA GEN10), so one way of doing this is to run a sync signal (called genlock in industry terms) to both the camera and the Airtrack, which is the part of Previzion that drives the synchronization for the rest of the system.

This can work, but it means that the camera is being externally driven by a little sync box, which is frequently problematic in terms of organizing shoots. (You want to be as minimally invasive as possible.) To get around this, and still get a good sync signal, we use an Evertz digital to analog converter, the 2430DAC-HD. (http://www.evertz.com/products/2430DAC-HD).

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Creating Sync From HDSDI

This converter takes in a standard 4:2:2 HDSDI signal, and does a D-A conversion on it. It also passes the original HDSDI digital signal out untouched, and we then connect this output to the Previzion HDSDI video input. The D-A conversion results in a Y’CbCr analog HD video signal, along with a pure HD genlock output. This HD genlock is what we then route to the Airtrack genlock input, and it works pretty well.

This basically depends on there being a very short delay between the point where the camera triggers the sensor, and when the resulting frame comes out the camera’s HDSDI port. In practice, it actually works pretty well. If you really yanked the camera around, you might see a tiny temporal lag between the virtual background and live video, but in practice it almost never happens, and it makes connecting to an existing camera much, much easier. Some cameras, like the Arri D21, don’t have a genlock input, so this is the only way to sync to these cameras.

Some cameras that have a 2:3 pulldown in their output (The RED, the Varicam, etc.) require the use of an external genlock signal to sync their sensor to the Airtrack, as otherwise the uneven cadence of the 2:3 frames in the output causes problems.