Describing the diverse range of activities in a video synthesizer that involve the word key can be a challenge. Here’s a list of definitions that we’ve used – this is certainly not the only way to categorize or apply these terms, but I hope it makes the intentions behind our own usage of them in relation to LZX modules clear.
- A key is any video signal used to control a transition between two or more other video signals. As such, any signal patched to a key input becomes a key.
– A hard key is a boolean logic signal which switches between two signals.
– A soft key is an analog voltage representing the mix ratio between two video signals.
- A key generator is a module designed to condition an input for the purpose of keying operations.
– A hard key generator is typically implemented as a differential analog comparator.
– A soft key generator is typically implemented as a high gain differential amplifier with black and white level clipping.
– A chroma key generator processes the chroma components (PbPr) of a component video signal, allowing key extraction based on Hue and Saturation of the video source.
– A luma key generator processes the luma component (Y) of a component video signal, allowing key extraction based on the overall brightness of the video source.
– A component key generator acts on one color channel at a time, but still includes the entire colorspace in its output function. For example – a red component key is active when areas of red appear in the image in the absence of any green or blue.
– A multi level key generator produces multiple key signals from a single source, and are often used as the frontend for colorizers and sequencers.
– A window key generator has dual threshold controls, either Upper/Lower or Span/Center
- A fader or switcher is a module which performs a transition between two video sources. Typically it has a direct key input, without much local control over the key itself.
- A keyer is a module which has both a key generator and a fader or switcher. Typically the output of the key generation function is tied directly to the driving circuitry for the fading or switching function.
- A linear colorizer or multi level keyer combines multiple faders with a multi level key generator function to produce a transition across more than two inputs. If we feed static color offsets to the inputs, and drive the key input with a video source, this is a linear colorizer. If other video sources are used instead, it becomes a sequencer or compositor.
To drill down on some of the relational terms:
- A keyer is both a key generator and a fader.
- Keying is always fading, but fading is not always keying.
- Any signal can be a key, even those not processed by a key generator.
- A key is created whenever any connection is made to a key input.
- Keying is what is happening to a fader’s input signals when the fader is controlled by a key. At the fader’s output, are the keyed results.
And, to apply these to various LZX modules:
– Triple Video Fader & Key Generator (Visionary Series) is a triple hard key generator + triple fader. Due to its semi-modular signal flow, it can be also used as a single hard keyer for RGB signals.
– Cadet VIII Hard Key (Cadet Series) is a single hard key generator.
– Doorway (Expedition Series) is a single soft keyer.
– FKG Keyer (Gen3 Series) is a single RGB/Luma soft keyer.
– Topogram (Expedition Series) is a five band multi-level key generator.
– Visual Cortex (Expedition Series) contains an integrated single hard keyer / output encoder.
– Polar Fringe (Expedition Series) is a soft chroma key generator
– Memory Palace (Orion Series) contains a discrete digital implementation of a soft window keyer
Please let me know if I can help clarify or further define the language we’ve used in this context.
If this seems overwhelming I wouldn’t worry about memorizing all these terms – these are complicated ways to describe something that’s already probably intuitive to you if you patch the system. This post is here to serve as a disambiguation guide, so that it’s clear what’s meant across a history of different modules, different series, and different approaches to keying.