The Mystery of Key Effects

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This article is simply reference material

There are currently digital video mixers available priced from $900 to $15,000. Though they provide different levels of video quality and complexity of effects, one of their common denominators is the ability to perform keying effects. The work “key” was derived from a digital mixer’s capability for cutting an electronic keyhole in a video picture. Into this keyhole is placed another video signal or a matte color which results in one image being placed over, or within, another.

Key effects go by a variety of names including Luminance Key, Chroma Key, External Key and Downstream Key. Owners of Panasonic’s digital video mixers (i.e. AVE-5, AVE-7) will recognize Downstream Key on their units as “Superimpose.”

All of the key effects work on the principal of taking an image shape presented from one video source and using it as the cut-out for placing another video source (or matte color) inside of it or around it. And typically this cut-out is defined by its brightness level in contrast to whatever else is in the scene.

As an example, Luminance Key detects the dark portions of a video picture and electronically replaces that with another image, generally from another video source being fed into the digital mixer. Some popular wedding effects using Luminance Key is incorporating pre-recorded animation videos over wedding footage to create some very sophisticated-looking graphics.

Chroma Keying, however, does not obtain its keying cues from the brightness of a video signal, but rather looks for a specific color of hue to be used as the cut-out reference. Such is the case with the TV weatherman standing in front of a blue or green wall as those specific shades of blue or green are replaced by a computer generated weather map. Chroma Key can be used in post-production for placing your bride and groom in otherwise unattainable locations. Or a portable Chroma Key setup can be the life of the party at a reception for guests to experience the voyage to where no man has gone before.

External Key is a feature on a digital mixer that permits the use to “cut” his or her keyhole with an image shape fed into a discrete video input of the mixer. Into that image shape is another video picture. The Panasonic AVE-7 and Panasonic MX-30 utilize the External Camera input to provide to provide the external key image. Units such as the Sony DFS-300 have a specific video input labeled “External Key.” Many wedding videographers use the External Key mode to, let’s say, feed the shape of a heart into the digital mixer by pointing a video camera at a black over white heart-shaped image. This heart pattern now has one video picture occurring on the inside of it and another encompassing the outside. It doesn’t take video producers very long to realize that they now have an unlimited assortment of new wipe patterns available via External Key.

Downstream Key (a.k.a. Superimpose) does just that. It performs its effect downstream of the rest of the digital mixer’s other effects. It does not require one of the digital mixer’s two video channels to achieve its superimposing capabilities. You might even think of the Downstream Key section as a separate video mixer built into your primary mixer. Like External Key, it also requires an image shape (“Key Source”) to cut the keyhole and it requires some form of fill signal (“Key Fill”) to be placed inside of it. Since the Downstream Key’s effect take place after the fact, it is most often used to superimpose titles or graphics over two other video sources that are transitioning between each other.

Slice and Dice

Virtually all of the current digital mixers use the “B” bus as the keying channel for both Luminance Key and Chroma Key. On Sony mixers/switchers it is called the Foreground bus. This is the bus to which the keyable image is assigned, be it an animation video or a subject standing in front of a blue or green backdrop. Whenever the dark portion (Luminance) or specific shade of a color (Chroma) appears, then the video image present on the “A” bus fills it in.

A level control on the mixer that adjusts the amount of separation between the two buses is called the Slice or Clip level control. Once the video signals are present from the two different sources, turning the Slice or Clip control fine tunes the effects by trimming off unwanted edges or “jaggies” from the key image. In the case of Chroma Key, an additional control called the Hue level selects the precise backdrop color on the “B” bus that needs to be filled. Chroma Key often becomes the most difficult key effect to perform primarily because an exact combination blend of the Slice/Clip control and the Hue control must be achieved for perfect separation. This added to the lighting technique necessary for Chroma Keying can make the overall setup procedure somewhat time consuming, not to mention frustrating.

The External Key and Downstream Key modes also have a Slice/Clip control for adjusting the video level during a key effect. The big difference between these two types of key effects and the aforementioned Luminance and Chroma Key is that they frequently depend upon a third video source to be the key fill signal such as an image from a third VCR source or one of the mixer’s built-in background matte colors. However, you can assign your “B” bus’ signal to be the key fill signal if you like when using the External Key mode.

The Panasonic digital mixers’ Downstream Key (Superimpose) relies upon a background matte color as the fill signal after an image shape is presented to the Downstream Key section. The actual image shape can come from either a video camera aimed at a title card or one of the many compatible plug-in character generators (KB-50, KB-15, KB-12).

The Sony DFS-300 can fill the image shape with an actual video signal other than a background matte color. The Sony DFS-300 will allow you to create an image shape from its Downstream Key In connector and fill it with a video signal from the Downstream Key Video In connector. You can actually create a picture-in-picture effect between the two primary input buses and then add another picture-in-picture on top of that image by feeding the shape of a rectangular box into the Key In connector and a third discrete video source into the Video In connector. The result: Two different picture-in-pictures over a full screen video scene.

Since you can use the Luminance Key, Chroma Key, or External Key modes separately from the Downstream Key section, you can conceivably create three or four layers of video imagery simultaneously.

By the way, the advertisement that ran for the Panasonic AVE-7 depicting a bride holding the groom in the palm of her hand is obviously a Chroma Key effect since Luminance Key would cancel out either the bride’s white dress or the groom’s black tuxedo. Surprisingly, the Panasonic WJ-AVE-7 doesn’t not have Chroma Key as one of the features. This somewhat misleading ad photo has sent many a Panasonic WJ-AVE-7 user into frustrating confusion. (A fine-print disclaimer at the bottom of the ad says “simulated picture”).