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Arri Alexa’s output

The Arri Alexa is the only other camera with the ability to record one signal format internally and output another format through its MON/HD-SDI OUT outputs.  It is very important to always check that, unless purposefully doing so, the HD-SDI output matches the internal recording format.

For instance, the camera can record a 29.97 PsF signal internally and output a 29.97p signal through it’s MON output.  Since Previzion can use either signal, as far as Previzion is concerned, there is no conflict with this.

The easiest way to check that all of your frame rates are correct is to push Info>FPS INFO

FPS Info Page

Alexa Plus/LDS Data

In the Alexa Plus, Arri LDS lens positions can be read directly out of HD-SDI into Previzion.  In other words, when using LDS protocol-enabled lenses on an Alexa Plus, Previzion is able to read the lens data from the same SDI line video comes in.

Basic Requirements:

  • After Arri Alexa Plus firmware 7.0.2 (release notes), Lens Metadata is now recorded/output as both raw lens encoder information and interpreted LDS information.
  • Previzion uses the raw lens data.  For the latest version of Arri Alexa Plus’ firmwares, please visit Arri’s website.
  • Previzion build 7224 or later.
  • In previzion you can choose to read data from a lens’s internal LDS encoders or Arri motors if they are attached to the camera.
  • Make sure the lens pins are aligned properly with the camera’s pins to establish a connection.

Previzion in the Electroinc Viewfinder

The are Alexa has the ability to display an incoming video signal on the internal viewfinder, as well as an attached monitor.  This works really well with Previzion, and helps to immerse the camera department in the virtual environment.

  • Connect a composite signal from your DA to the Alexa’s “RET/SYNC IN” using the “Monitor Out” bnc on the tether
  • On the Alexa go to Menu>Monitoring>RET IN path>EVF+MON
  • Press the “USER” button on the Alexa, then press the jog wheel to edit user buttons
  • Select which button you want to toggle the Previzion video, then scroll down and select “Return in active”

 

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ARRI Alexa’s Sensor

The image sensor in the Alexa, Alexa Plus and Alexa Studio all have a pixel pitch of 8.25 µm and share the same sensor: the ALEV III CMOS sensor.  At a pixel recording size of 2880 by 1620 pixels, the physical Alexa sensor width, which is ultimately the number you input into Previzion and any post-tracking software, is 23.76 mm.

The Alexa has a look-around mode they call “Surround View”, but this uses a slightly larger range of the sensor; the actual imaged size is the size shown above.  In order to ensure that the image size registered in Previzion and the  image size being recorded by the camera match, tun off  “Surround view” in “MENU>MONITORING>MON OUT or simply plug Previzion’s output into HD-SDI out 1 or 2.  The output of the Alexa needs to be set to 1:1 of the actual imaged size, without the look-around, to maintain tracking accuracy.

MonOut-noSurround

 

Status info should also be turned off, this will cause the sensor size to be inaccurate with our default sensor size.

“Menu>Monitoring>Mon Out>Frame lines +status info>Status info>Off”

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Arri D20/D21 sensor width

On Arri D20 and D21 models, the sensor has the same effective sensor width.  When shooting 1920 by 1080 at 16:9 format it ends up being 23.76 mm wide.