By the early 1960s transistors had replaced the tube and electric motors driving control surfaces were more common. The first low cost "proportional" systems did not use servos, but rather employed a bidirectional motor with a proportional pulse train that consisted of two tones, pulse-width modulated (TTPW). This system, and another commonly known as "Kicking Duck/Galloping Ghost", was driven with a pulse train that caused the rudder and elevator to "wag" though a small angle (not affecting flight owing to small excursions and high speed), with the average position determined by the proportions of the pulse train. A more sophisticated and unique proportional system was developed by Hershel Toomin of Electrosolids corporation called the Space Control. This benchmark system used two tones, pulse width and rate modulated to drive 4 fully proportional servos, and was manufactured and refined by Zel Ritchie, who ultimately gave the technology to the Dunhams of Orbit in 1964. The system was widely imitated, and others (Sampey, ACL, DeeBee) tried their hand at developing what was then known as analog proportional. But these early analog proportional radios were very expensive, putting them out of the reach for most modelers. Eventually, single-channel gave way to multi channel devices (at significantly higher cost) with various audio tones driving electromagnets affecting tuned resonant reeds for channel selection.