It requires a precision and speed of operation that could not be achieved at reasonable cost with the devices then available. In 1948, Claude E. Shannon of Bell Laboratories set forth the information theory that guided engineers along the way to new solid state devices that combine precision, speed, and economy.
Engineers are now available to design systems both reliable and economical.
The Story of Bell Laboratories. Bell Telephone Laboratories, 1975. Experimental 96-channel PCM system, demonstrated by Bell Laboratories in 1947. Photo shows a front view of the terminal equipment with covers removed from a 12-channel group bay. PCM technique can be considered the ground-breaking technique in digital technology.