The change in amplitude from 5v to 0v is the gate. The gate is set at 4 Hz, meaning it will flash on and off 4 times a second.
This was implemented for a few reasons after the initial release of plasma.
1. The tube produces the most energy when it is first ignited. By adding the 4 Hz gate, you are actually getting 4 stronger pulses of energy from the tube.
2. Heat control. When the tube is on 100% of the time, under certain conditions it could overheat and cause issue. Generally this was restricted to those who lived in high temperature areas and or did not provide adequate ventilation to the tube.
The gate is now standard implementation for all plasma programs and has been since shortly after release.
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