A Keying Dictionary

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
– Keychain is a triple hard key generator

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.

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I’m now going to have this in my head all morning…
I’ve got the key, I’ve got the secret
I’ve got the key to another way…

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regarding the window key what kind of math is used for the span/center controls?

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what kind of math is used for the span/center controls?

You need a Span/Center processor that generates the Upper/Lower thresholds.
Upper Threshold = Center + (Span / 2)
Lower Threshold = Center - (Span / 2)

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Is there any reference circuit for this implementation? Would be interested in breadboarding something!

Thank you for posting this Lars!!! Luckily we have lots of keying module options but sometimes not all transparent regarding the specifics of each? Wondering if a spreadsheet with all the differences or overlap would help? Nonetheless this post helps clarify differences. I think I have all the ‘keys’ for now so I’m good. :slight_smile:

Can we add to the list above?: “Keychain” (Gen3 Series) is a triple hard key generator.

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I haven’t published a reference circuit, but a block diagram is fairly straightforward.

Here’s how I do it after trying many ways:

I tried lots of specialty high gain parts or integrated high gain VCA parts that were expensive and troublesome over the years. What works best in the end is just doing multiple stages after the VCA block, with 4x - 5x per gain stage, and use a separate op amp per stage. Two 5x stages in series it gives you a total gain of 25x, which gets a decently hard edge without going too crazy (this is about where Topogram sits for example.) Cadet II RGB Encoder is a good reference for a precision Black/White clipping circuit.

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Thanks so much for sharing this, very helpful!