<P> Early electrical communications started to sample signals in order to multiplex samples from multiple telegraphy sources and to convey them over a single telegraph cable . The American inventor Moses G. Farmer conveyed telegraph time - division multiplexing (TDM) as early as 1853 . Electrical engineer W.M. Miner, in 1903, used an electro - mechanical commutator for time - division multiplexing multiple telegraph signals; he also applied this technology to telephony . He obtained intelligible speech from channels sampled at a rate above 3500--4300 Hz; lower rates proved unsatisfactory . </P> <P> In 1920, the Bartlane cable picture transmission system used telegraph signaling of characters punched in paper tape to send samples of images quantized to 5 levels . In 1926, Paul M. Rainey of Western Electric patented a facsimile machine which transmitted its signal using 5 - bit PCM, encoded by an opto - mechanical analog - to - digital converter . The machine did not go into production . </P> <P> British engineer Alec Reeves, unaware of previous work, conceived the use of PCM for voice communication in 1937 while working for International Telephone and Telegraph in France . He described the theory and advantages, but no practical application resulted . Reeves filed for a French patent in 1938, and his US patent was granted in 1943 . By this time Reeves had started working at the Telecommunications Research Establishment . </P> <P> The first transmission of speech by digital techniques, the SIGSALY encryption equipment, conveyed high - level Allied communications during World War II . In 1943 the Bell Labs researchers who designed the SIGSALY system became aware of the use of PCM binary coding as already proposed by Alec Reeves . In 1949, for the Canadian Navy's DATAR system, Ferranti Canada built a working PCM radio system that was able to transmit digitized radar data over long distances . </P>

What is the significance of using pcm in digital transmission