Chester and his partner Red Pearce had heard rumors that KTRB was monitoring the KLOC signal while we were testing on the air with the antenna proof and during the after midnight program material sequences, so we purposely kept the modulation level low at these times. When the telegram came from Washington authorizing program test, we hit the air full force with modulation off the peg at 130 and 140 percent. Word came back that the KTRB windows were rattling. Chester and Red with their little 500 watter were in orbit.

FCC regulations in effect at that time looked at transmitter performance largely in terms of measured audio distortion, specifying that an audio proof of performance be taken annually. I was never quite sure whether our super modulation was in violation of a 100% rule, but measured audio distortion was well within limits. Several years later the FCC initiated a proposed rulemaking inquiry to define levels of modulation.

Meantime, I had revised our concept of positive/negative waveform processing, removing it from the high level, high voltage section of the transmitter to the low level audio input. We decided to introduce our system as a presentation into this study, using a Gates transmitter we had recently installed at KPOP in Roseville. Our study was accepted, along with others, by the FCC and referenced in the Decision setting a value of 125% as the positive peak limit. Industry manufacturers took up this or similar ideas in the design of limiting amplifiers currently in use.

Our device was simple in design, with no active components. When viewed on an oscilloscope a sine wave continues above the base line without blemish while the lower portion dwarfs noticeably. The KPOP transmitter was especially clean and we were able to achieve 140% modulation before exceeding the distortion constraints.





 
KLOC and the Magic of Super Modulation

By Cecil Lynch, Consulting Engineer
Cecil Lynch  in May 2008
(Photo courtesy of Frank Azevedo)