All About Doorway

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.


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

User Reference

  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.


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.


Other Resources

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