FWIW: Been here on the who, when and why of PCM's creation. A little research divulged the following information.

It is true that Multiplex was the first RC Company to put PCM technology to work. However, Futaba brought it the masses...

PCM Pulse Code Modulation
There seems to be some confusion as to the actual date PCM was invented, I have found numerous refernces all fall into 1936,1937 or 1939. Suffice it say the late 30's would be a safe bet... PCM as we know it today is the same technology as when invented in the late 30's by Alec Reeve's.

The difference between then and now [modern day] are the methods used to encode and decode the digital data. The application first envisoned for PCM was telephony. [telephone data comunications] Vacuum tubes, relays and amplifiers were used to convert analog data signals to digital data prior to transmission.

An "A/D" [Analog to Digital Converter] is used to convert the analog data to digital data, in the first PCM system's these devices were seperate devices. As we know PCM today of our models, the A/D or encoder is a microprocessor imbedded into the TX and an D/A [Digital to Analog Converter] or decoder is a microprocessor imbedded into the RX.

In the early days of Radio Control the encoding was done with discreet transistors, later these were replaced with Integrated Circuits and later again these were replaced wih computers on a chip [IC] known as micro-processors.

I beleive in almost all stages of the evolution of Radio Control systems, superior encoding schemes were understood long before the electronics required to implement them were either available or practical.

Historical time line of PCM:

1939
Pulse Code Modulation

Invented by Alec Reeves. Pulse Code Modulation is used to turn an analog signal into a digital signal. Digital signals tolerate large amounts of noise, interference, and distortion. Repeaters were used to regenerate the original signal instead of amplifiers. Amplifiers strengthen a signal, but they also strengthen the noise. Repeaters will detect the original digital signal and retransmit it removing the noise. This increases the quality of the received signal when traveling very long distances

1939
First Digital Computer

George Stibitz built the first digital electrical relay computer. It is the first such machine to use binary logic, it was reffered to as a "Binary Adder". This machine was the first step towards what is today called the "Computer Revolution."

1940
The first relay computer was put into operation by Bell Labs, known as the Complex Number Calculator.

1947
Transistor

The first solid state amplifier known as a transistor was developed at Bell Labs by Bardeen, Brattain and Shockley. Effectively replacing vacuum tubes

1948
Digital High-Speed Digital Transmission System [PCM]

The first Digital High-Speed Digital Transmission System was developed by Shannon, Pierce and Oliver based on coded electronic pulses. Using PCM, they find they can transmit multiple phone conversations over the same set of wires

Early 1980's
The technological revoulution made it practical [affordable] for wide spread use of PCM technolgy.