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Why Digital Proportional for R/C was never patented by hobby manufacturers.

Many R/C manufacturers patented their creations, Howard Bonner held numerous patents on his escapements and servos. Why then is it nobody that we know of patented Digital Proportional, as it was a completely new concept in the field of radio controlled models? The actual facts are few and scattered, but fortunately there were enough clues to figure out what actually happened.

We start back in the very early 1960s at JPL in Pasadena Ca. before the formation of NASA. JPL was then top aerospace lab in the country. Jerry Pullen worked at JPL and was an avid R/C flyer and member of the Larks club, as was Doug Spreng. Spreng in his auto biography relates to us how he was interested in where Pullen had gained access to NPN transistors that were used in his proportional servos, and wound up working at JPL with Pullen to gain access to the advanced radio control system technology and components used in space craft.

As a result, Speng found out about what was then called Pulse Duration Modulation, (PDM) which we often now call Pulse Position Modulation (PPM) . This was the way signals were sent to control the many servos and switches in space craft like satellites and missiles. Borrowing the concepts of PDM, Spreng built a small version of a PDM feedback servo amplifier, and then proceeded to make a PDM transmitter and receiver for use in a model plane with the assistance of Don Mathes, who was skilled in RF design. Their joint efforts ultimately resulted in a hobby radio system they called the Digicon, which was the first of it’s type to use the PDM technology they “borrowed” from JPL; pirating would actually be a more appropriate, but unflattering description however.

Although the Digicon prototypes worked well initially, some of them began to experience loss of control. It was eventually determined with the aid of Frank Hoover that the problem was the transmitted RF carrier was only on during the pulses, allowing interference to overcome the receiver during the long “sync” pause between pulse frames. Spreng and Mathes had made one critical mistake, but it was easily rectified by inverting the modulation pulses in the transmitter to keep the RF carrier on most of the time and only shut off during the short 250 microsecond pulses, thus keeping the receiver “locked” onto the signal 95% of the time and not allowing noise in. Unfortunately however, the Digicon never recovered from it’s initial bad reputation in spite of the fact that it was revised and updated, Mathes and Spreng therefore sold the design to Bill Cannon, who re –released it a few years later as the C&S Digicon II.

In the mean time, Frank Hoover of F&M and Larson/Kagele of Bonner Specialties had released their first versions of PDM “digital proportional” as these systems were then so named, but they all used analog type servos. None used the PDM feedback servo Spreng had made, which was superior. Kraft was backing Jerry Pullen in the design of his own proportional system, which oddly did not use PDM technology in the Tx/Rx or servos. Why then did all these manufacturers not utilize the PDM feedback “digital” servos?

Hoover points out that the technology in his F&M was a version of PDM as used for missiles and satellites, providing an important clue: everyone knew this technology had been “borrowed” from JPL, and as such nobody wanted to risk any possible investigations from the government as to how they had gained access to what was then considered classified information. It was the servo technology that presented the problem, it was classified. Spreng did not stay long at JPL, but Pullen stayed on longer, and was not willing to risk his job “borrowing” classified technology for his hobby project proportional radio. This explains why Pullen spent so many years unsuccessfully trying to develop an analog proportional hybrid radio of his own design. By late 1965 however, it became apparent that the government, and then the newly formed NASA was not interested in going after R/C companies that were using the PDM technology, largely due to the fact that many people within the space agency were model aviation enthusiasts themselves and sympathetic towards the development of reliable multi-channel proportional radios for the R/C hobby.

This then led to the flood gates being opened in terms of companies that started producing PDM type feedback servo systems called “Digital Proportional” Kraft, Pullen and others then quickly abandoned all previous designs and “digital” became the standard. Bill Cannon was quick to release his Digicon II ahead of most the of the competition once he saw using PDM servos was not going to be a legal problem.

And so we see nobody could really lay rightful claim to a design which had been smuggled out of JPL, and in fact whoever the did develop it had signed the patent rights over for a buck to the government, as was typical practice when working for them in development projects. If the government paid for the research, they owned the designs that came out of it. Therefore it would not only have not been a crime to pirate this technology, which could be considered espionage, but further more, who would be foolish enough to document that deed by filing a patent application for something the government already owned? In actuality, Spreng and Mathes took a calculated risk; they gambled Jerry would keep quiet, which he did and that they would be under the radar. Plus they knew NASA was going to be soon taking over the space division of JPL and were counting on that shake up preoccupying the two agencies and any focus on them therefore being diverted in the process. But they were a little too quick in releasing the Digicon, and as a result others were able to cash in on the technology once it was determined the “coast was clear” and there would be no investigations from the government. This is why nobody was able to corner the market on digital/DPM, as it was not patentable.