With the release of ProPresenter 7, we no longer have add-on modules that cost extra. We have integrated most of the modules as core features. One of these features is the ability to send an Alpha Key output over SDI (with suitable output hardware) or NDI. Some SDI devices have special settings that are needed to properly work with the Alpha Key option here. These settings (if necessary) are outlined in their own section. This article will outline each output option. Please refer to this article to see which devices support Alpha Key.
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An alpha key is one of three standard keying methods. The other two methods are chroma keying and luminance keying. Most switches support chroma keying. If you're familiar with seeing green screens get replaced with content, then you understand the concept of chroma keying. Most switchers can also remove black and will call it a luminance (or luma) key, but it's actually just a chroma key, but using black instead of green. You can do a chroma key with any solid color. A true luminance key is usually only available on high-end switchers.
The difference with those types of keys and an alpha key is that they generally use a single video input and just remove a color. With an alpha key, you are providing a key and fill signal to your switcher. For NDI, you are only making one connection and the receiving software processes the data and knows what to display. For SDI, you are sending two physical connections. You have to setup your switcher's keying settings to know which is the key and which is the fill signal, and then it handles processing the signal over your other sources.
Before we get into the setup of the alpha key, let's take a look at what's actually being sent for the key and fill signals. The key signal appears as a black and white source and the fill signal looks like your content.
The key signal is actually more complex than that. Any area that appears as black is actually representing no content, or a pixel with zero opacity. The white areas are actually a grayscale from 0-100% representing the opacity of the pixel that exists at a given point in the signal. You can see this best in the example in the upper right corner. The odd number lines are yellow lines set to 25% opacity. The green box with the text is set to 50% opacity. The even lines are blue and have 100% opacity. You can see the difference in the lines outside of the box and with how the yellow lines interact with the green box. Anywhere that is black will show whatever the other video source is that you are using under your key. Any areas that are not solid white will show the content will some amount of opacity.
The fill signal is your actual content. It will look like what's on your screen. If you are showing a full screen video or set a background color for your slide(s), then the key signal will actually be solid white because the entire fill area now has content covering the entire screen at 100% opacity.
An NDI output is simply sent out over your Network and can be picked up by a suitable NDI program/application or a hardware unit. When the Alpha Key is enabled on NDI, ProPresenter will send the rendered image out with an Alpha Key in place of the background color. This can be useful if you want to key your text on top of video, for example. There are no other settings when sending an NDI Alpha Key output, you simply can enable or disable the Key.
You first will need to make sure you have a suitable Blackmagic device connected to your machine that allows for two separate output channels. You will also need to install Blackmagic Desktop Video. As of this article, the current version is 11.6.
If you are a visual learner, check out this tutorial video before reading any further.
Blackmagic Desktop Video Setup
The SDI screen setup in ProPresenter is the same no matter what output device you are using. There are changes that will need to be made for settings in the Blackmagic Desktop Video Setup though.
If you are using the Blackmagic DeckLink 8K Pro, you need to go to the Connectors settings and select SDI 1& 2 In and SDI 3 & 4 Output for the best results.
For the DeckLink Duo 2, open the menu for the first pin and go to Connectors and set it to SDI 1 & SDI 1.
Once this is installed and configured, you can open ProPresenter and begin setting up your Screen.
Open the Screen Configuration from the Screens tab in the menu bar. From there, you will need to create a screen, and then assign this screen to your Blackmagic device. In most cases, you will select the first option. Select the correct output mode for your switcher. Even though you are sending two physical sources, you will only setup on DeckLink screen. This is explained in the next section.
Should your device support Alpha Key, you should see the Alpha Key tab in the linked screen configuration. When the Alpha Key is enabled on an SDI output, you can set what type of Key you wish to use (External or Internal) and also the percentage level of Blend that will be used.
An External key would be used if you are sending separate key and fill outputs to your broadcast video switcher. It sends the key signal through the preview out of the Blackmagic device and the fill out of the program output.
An Internal key would be used if you are using the Alpha Keyer as a downstream key, with a video input coming into the Blackmagic device and one going out of it.