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The PCM Advantage

 Jump to #KeyKoder Technology

Early 1980's  Radio/Control  transmitted each channels servo position as an analog pulse width that ranged from 1-2 milliseconds. The pulses were transmitted as a continuously "on" Radio Frequency energy carrier with their beginning and end marked by Amplitude Modulating the RF carrier "off" for 0.35 ms. The channels were sent sequentially with the .35ms end mark of one serving as the beginning mark of the next channel. A special long "Framing" pulse delineated the beginning of the channel series. The receiver used the marks to determine the channels, otherwise the receiver was dumb. Any noise or interference in the "air" was interpreted as a marker and mis-commanded the servos; the receiver couldn't tell the difference. Later FM versions in the early '90's suffered the same problems.

The Pulse Code Modulation receiver is smart; it contains a microprocessor. By clever structuring of the servo commands using mathematics and precise timing PCM facilitates accurate reception even with severe noise or interference.

It begins in the transmitter by conversion of each joystick position into a 10 bit digital word plus the extra bits to enable the receiver to mathematically verify the word. The next trick of PCM is to compact this data representing 1024 servo positions per channel into the FCC specified radio bandwidth while maintaining responsive "real time" control to fly a helicopter. The PCM data is transmitted synchronously; each bit has a particular position in time, within the Frame. The Frame continuously repeats. A precision crystal controlled clock in the receiver locks onto the transmission to maintain synchronization with the data bit by bit even during major signal drop outs. Thus the receiver can process data immediately after interference instead of waiting for a Framing pulse.

Received data is evaluated channel by channel. When the mathematics indicates an error, previously validated channel data that is stored in the smart microprocessor is substituted. If an error persists then failsafe servo operations previously specified by the operator are initiated until accurate commands are again received. The microprocessor converts the proper data into pulse widths to command the servos. No more servo jitters!

Futaba's PCM1024 scheme gives 8 channels of 10 bit resolution. Other competitive systems lack Futaba's sophisticated data compaction scheme and so must limit the resolution of some channels to 8 bits, which is 1/4 as accurate, as a trade-off to satisfy the "real time" data rates required.


KeyKoder Technologies:

The first add-on channel KeyKoder product was sold by VANTEC in 1981. We have improved the product several times since. In this fresh new design we deciphered Futaba's PCM1024 format to capitalize on it's reliability for secure robot applications. The on-off added-channel KeyKoder described in this paper is fundamentally a 32 channel KeyKoder device. In this new technology up to four KeyKoder type devices can be piggybacked on one PCM1024 servo channel. Four of these devices therefore yields 128 on-off functions through a single servo channel.

Other KeyKoder device functions are available, for example the transmitter ID and high security function is incorporated into the KIK29, KIK44 and other "industrial strength" products, as well as Key-on/Key-off operation.

Two other interesting applications for KeyKoder Technology is the addition of more servo channels with 8 bit resolution, and a slow speed data channel for radio controlled advertising blimps or for "calling" special robot functions implemented by an on board computer.

VANTEC is interested in your special commercial or industrial control application using KeyKoder technology; give us a call at (541) 471-7135

Note: Our products are not safety devices nor for use in life-critical or life support systems. Specifications and price subject to change without notice. Patented. Some trade names, trademarks & content owned by others.

Sales: (800) 882-6832

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