Vibroplex - understanding the semi-automatic timing

A Vibroplex keyer is a mechanical keyer ("bug") invented in the early 1900s (patent). Bugs are known to be difficult to master, and the simple "bug emulation mode" in many electronic keyers don't reproduce the nuances of bug timing. The slow motion simulations here explore the quirky interactions of a bug's semi-automatic dits with operator timing.

The dah arm of a bug acts like a simple straight key, and is not considered in the rest of this document.

The dit arm has a flat spring (15) after the hinge, allowing that section of the arm to oscillate. The position of a weight at the end of the arm controls the frequency of oscillation.

The core of the dit mechanism can be simplified to an oscillating rod with a fixed end, and a free end with a weight.

In the neutral position, a damper is also placed to one side of the center of oscillation. This stops any movement beyond that point (ideally.)

Pressing the dit paddle rotates the dit arm by a small angle. This moves the fixed base of the oscillating section to a new position, causing the weighted arm to start oscillating around the new position.

Explore this slow motion simulation of the dynamics of the arm. Press the dit button to simulate pressing the dit paddle. You should hear perfectly spaced dits and spaces if you keep it pressed. Release it at various points of the oscillation to see how it affects the dit space timing.

• A light U-shaped spring contact on the arm forms a connection when the arm is on one side of the center of oscillation, resulting a stream of evenly spaced dits and spaces.
• Unlike an electronic keyer, releasing a Vibroplex paddle too early can cause the last dit to be too short, and releasing it too late can cause an extra short dit.
• There is a quarter-dit phase lag between pressing the paddle and the first dit being generated.
• This simulation ignores the effect of the light U-shaped spring contact (19). It effectively increases the spring constant during one half of the oscillation while it makes contact.
• Unlike the main flat spring however, the U spring is not damped. The faster natural frequency of the U spring can continue to strike the contact as the dit arm swings away or towards the dit stop. This may cause a scratchy or "mushy" dit sound. Mechanisms like the dot stabilizer are usually stops analogous to the main damper, but for the U-spring.