Doorway Archive

Open original thread

door1914×1024 342 KB

Over, Under, Thru

Key generators are the bread and butter of creativity in video and imaging systems. From the alpha channel compositing and matte control of modern software packages back to the primitive wipe generators of broadcast consoles, keys are the thing. Doorway is very exciting for me because it is tuned specifically for fully controllable shape and line generation, and complex combinatorial joining of multiple keys together. Through multiple Doorways, a complex 2D figure can be generated and composited together from simple horizontal and vertical ramp generators.
My focus going into designing the Expedition series modules has been to embrace what makes analog video processing unique and irreplaceable: continuous, linear functions. It only makes sense that the series’ signature key generator module should be a soft keyer.
Doorway’s key is generated through high gain amplification of the input signal around a threshold point. Unlike a hard key generator, there is no comparator, digital logic, or on/off state in the binary sense. The edge of the key itself, and its softness/width, are composed of the input signal itself with it’s contrast blown way out of proportion to the typical brightness scale. Likewise, rather than an inversion/negative key function or output, this positive and negative spaces of the key can be set to any grayscale value, or external sources can be inserted. This allows the joining of multiple keys together in various alpha compositing modes: over, under, in, XOR, etc.
Doorway is a linear video key generator and compositor, also known as a soft keyer. A typical video key is generated using a comparator circuit, where the ouput is a binary state: on or off. Doorway instead achieves key generation via high gain amplification and clipping of the input source. The edge of the resulting key is therefore composed of a variable percentage of the input signal, centered around the threshold point.

• Voltage controlled threshold across the full input range. Video rate modulation paths allow displacement of the threshold by another shape or image source.
• Voltage controlled gain amplifies input signal up to a gain of X64, enabling a continuous sweep from fully soft to fully hard edge.
• Foreground and background level controls and signal inputs allow the negative and positive space of the resulting key to be set to any grayscale level, or be filled with external signals. This aids the compositing of multiple shapes and patterns.
• Outline mode switches a full wave rectifier into the signal path. In this mode, gain controls the width of the outline rather than the softness of the key’s edge.
• AC/DC input coupling switches and inverting level attenuators on voltage control inputs.

Specifications

• Width, 10HP
• Mounting Depth, 32mm
• Power +12V @ 80mA
• Power -12V @ 80mA

User Reference

door2303×552 34.2 KB

1. Background offset control. This control sets a grayscale level to be applied to the negative space of the output key. Fully counter-clockwise, the level is set to 0V (black.) Fully clockwise, the level is set to 1V (white.)
2. Foreground offset control. Functions as (1) but for the positive space of the output key.
3. Threshold offset control. This control sets the center of the grayscale level at which the input source is amplified into the output key. Fully counter-clockwise, the threshold value is -0.1V. Fully clockwise, the threshold value is 1.1V.
4. Gain offset control. This controls the gain of the input source around its threshold value, effectively setting the width or softness of the output key’s edges. Fully counter-clockwise, the gain is 1X (fully soft edge). Fully clockwise, the gain is 64X (very hard edge.)
5. Inverting level controls. These controls set the depth of external voltage control modulation applied to the associated parameter. In their center positions, the output is 0. Adjusted clockwise from center, the signal is added to the associated parameter. Adjusted counter-clockwise, the
   signal is subtracted.
6. Voltage control AC/DC coupling switches. In AC mode, slow moving voltages are removed from the input signal and only high frequency content remains.
7. Outline/Solid mode switch. When in its downward position, the key generator functions normally. When in its upward position, key values above white will be inverted. This creates a window around the threshold voltage, allowing just the outline of the key to pass through.
8. Threshold external voltage control input. 0-1V DC full scale. The depth of modulation is set by the associated inverting level control (4).
9. Primary source signal input for frequency multiplier. 0-1V DC expected.
10. Gain external voltage control input. 0-1V DC full scale. The depth of modulation is set by the associated inverting level control (4).
11. Background external input. The signal input into this jack will add to the brightness set by the background offset control (1). This allows an external key, pattern, or other source, to be Inserted into the negative space of the output key. 0-1V DC expected.
12. Key output. 0-1V DC level.
13. Foreground external input. Functions as (11) but for the positive space of the output key. 0-1V DC expected.

Videos

Patching Tips

• Try different ramp switch settings on the Visual Cortex to generate a circle, diamond, star, or triangle.
• Instead of the H+V ramp mix, use external video as the input source to Doorway to generate a luma key.
• Apply voltage control to threshold. Alternate ramp generator outputs (from Cortex) and external VCOs make great sources.
• Apply voltage control to gain. Alternate ramp generator outputs (from Cortex) and external VCOs make great sources.
• Flip the outline mode switch and see how it effects the output with and without modulation.
• Play with the foreground/background level controls. Set them opposite the image shown to generate a negative key. Insert different ramps, mixes, and video sources into the foreground and background input jacks.
• Instead of patching the key into one channel of the Cortex, use it to control the whole Cortex compositor by patching it into the Colorizer & Compositor section’s VC input jack. Use the Cortex’s animation control section’s output to control the Threshold of Doorway.
• Use a vertical ramp as a source (Cortex switches centered.) Switch Doorway to outline mode. Turn up gain. You will see a single vertical line. Modulate threshold of Doorway with an external audio source to get an oscilloscope style audiovisualization display.

Downloads

• Doorway Owners Manual [PDF]

Other Resources

• LZX Community Forum posts tagged #doorway

Related Archived Discussions

A Keying Dictionary

Open original thread

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.

Thread Replies

Lars Larsen posted #8 in reply to #5 on Oct 7, 2022

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

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

image920×404 25.3 KB

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.