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Alpha Key Module

The Alpha Key Module has two functions: (1) Blend ProPresenter’s content over incoming video and then send that combined content back to a switcher or projector; (2) Generate a key and fill signal that is then sent to a broadcast-quality switcher that is able to accept this type of signal.

When using method one, the actual compositing of the incoming video signal and the content in ProPresenter (text or lower-third graphics normally) is done on the video card itself and then sent back out to a switcher or projector. Choose this method by selecting Internal from the drop-down menu.

The second method sends what are referred to as a “key and fill signals” to your switcher allowing you to have a broadcast-quality signal. Choose this method by selecting External from the drop-down menu.

These are the settings that you should set on the HD-SDI tab in Preferences.

  • Check the box for Enable Output Device and choose the "Blackmagic Video Output".
  • For the Mode setting, choose the appropriate setting whether it be standard definition (NTSC) or High Definition (720p, 1080p).
  • The Compression setting should be left blank.

Finally, you will need to specify what kind of keying you wish to do under the "Alpha Key Settings" section:

  • Disabled: This will output a full video out signal to your program and preview ports on the Decklink card
  • Internal Key: If you are using the Alpha Keyer as a downstream key, with a video input coming into the Decklink card and one going out of it, then this is an Internal Key (be aware that audio is passed through the Decklink card when using this mode -- you cannot use the Decklink audio outputs for sound out of ProPresenter when this mode is active).
  • External Key: If you are sending separate key and fill outputs to your broadcast video switcher, this is the option you will choose. It sends the key signal through the preview out of the Decklink card and the fill out of the program output.

Set the Blend to specify the maximum opacity of the video overlays.

In order to get full HD output you must have a video display device attached to the computer that is at least the resolution of the HD output. The reason is that everything is rendered in the graphics card before output and we are unable to render out video textures at resolutions higher than the display devices the computer is connected to. For example, if you're trying to render out at 1920x1080 content to your output display, your primary display must also have at least this resolution, if not higher.

In addition to configuring the Alpha Key settings on the Advanced tab, you will also need to ensure a few settings are correct on the Display tab.

In Preferences, click the Display tab and ensure that the Display at Top Most Window Layer is not checked. You also need to manually enter the output resolution for your SDI signal. If your output doesn’t look correct, verify that the Output Screen resolution is set for your SDI signal and not the connected DVI or VGA display.

If you are still having trouble with the signal, open the Blackmagic preference panel in System Preferences and make sure that Remove field jitter when video is paused is not checked and that When not playing video, send is set to Black. Both of these options are on the bottom of the Setting tab in the Blackmagic preferences panel.