ine's utilisation. Since there is a limit to how fast a telegraph operator may key-in messages, a question arose as to whether it is possible to use the same line by two or more telegraph operators. This was in effect multiplexing two or more telegraph calls on the same circuit and is known as time division multiplexing (TDM). Proposals for TDM of telegraphy are recorded as early as 1848, only 4 years after Morse's telegraph system formally went into operation. A rotating switch (commutator) connected a telegraph line to several operators in turn, and a matching rotating switch at the receiver distributed the incoming multiple stream to listening operators. The commutators 'sampled' the input of operators: and the speed of sampling had to be set to a level to ensure the telegraph data transmitted by each operator was fully captured. The speed setting of the commutators was based on practice, with little theory formulated. The importance of TDM technology grew with the invention of telephony and the need for high capacity lines which connected telephone exchanges. In 1903, William Miner patented a rotating commutator as shown in Figure 5.1. The challenge again was to set the speed of the 'sampling' commutator in order that the reproduced voice was of an acceptable quality, as a slow rotating commutator resulted in distorted voice reproduction. Miner showed through trial and error that 'the best results [were obtained] with a rate of 4300 per second'. Miner concluded that sampling needs to be done at twice the voice's upper frequency component - his voice frequency was filtered at slightly above 2 kHz. This is the first time a report is made of an optimal sampling rate for voice signals. The challenge for formulating a theoretical solution was to determine the necessary sampling rate. At such a rate a receiver would be able to interpolate the received samples and reconstruct the original signal. It was intuitive that the sampling rate must be proportional to the highest frequency of the signal. Furthermore, it had 