I'm putting together an article on the Wireless Set No. 10, which deserves to be MUCH better known - it was the first TDM and PCM comms system in the world. Sadly there is very little info on it, but I know that "Wireless for the Warrior" had a relatively detailed description including post-war development. Does anyone have a copy?
I just happened on this while making a last scan to look for information to archive on wireless set number 10. I edited the wikipedia page today, it had been on my to-do list for a long time, and I had spare time today. I'm sorry that it looks like I undid some of your edits. You may not see this, but the information maybe be useful for someone else who happens upon this short thread.
Yes, there are references to the device using PCM for the first time. The problem is that this is so unlikely that it's almost certainly not true that it utilized PCM, and also there are better references that say it used PWM, which would be the obvious choice. In fact, part of the wikipedia article that attributed PCM goes on to describe PWM:
"The sine wave was processed in a series of tubes to produce a sawtooth wave, which was then gated by the audio signal. When the voltage of the sawtooth was above the voltage of the audio signal, an output pulse was produced – the higher the voltage of the audio signal, the smaller the width of the sawtooth above that level, and the shorter the pulse. The final output was a train of pulses at the 9 kHz reference frequency, the width of each pulse being inversely proportional to the audio signal voltage."
The one "external reference" that was already in the article links for a very short webpage that says it used PWM (
"Wireless set No. 10...pulse width modulation..."). A much better reference is the paper titled A Brief History of Microwave Radio Fixed Point to Point (Relay) Communications Systems (catchy, huh?). Unfortunately, it's a for-pay paper via IEEE Explorer, but I found an excerpt regarding Wireless Set Number 10, that says it used PWM:
"The first microwave radio relay system radios were the British Wireless Set No. 10 developed by the UK Signals Research and Development Establishment (SRDE). The Pye Company built the RF section and the TMC Company built the multiplex. It was an eight-telephone-channel TDM pulse width modulation 5-GHz radio system designed to operate in tandem as a radio relay. It was demonstrated to the US Signal Corps Labs and Bell Laboratories on September 1942. This spurred the development of similar systems in the United States: the RCA AN/TRC-5 and the Bell Labs AN/TRC-6 (Carl, 1966; Fagen, 1978; Sobol, 1984)."
On the flip-side, there is no simple way to transmit PCM, a digital protocol, in the 1940s. And there is no reason to convert voice to digital in that particular use anyway. PWM fit the bill perfectly, because it's an analog modulation (converting amplitude to pulse width) that could take advantage of the magnetron, the key transmitting device available for the device. The magnetron could only toggle from off to on, quickly, so it was perfectly suited to transmitting pulses, with no synchronization necessary.