Every sound you hear, which of course includes human speech, is in the form of an analogue signal. Only until just a few decades ago our telephony networks were built on an analogue infrastructure. Whilst an analogue signal is perfect for human communication it is not resilient or competent at improving from line noise. Line noise is usually due to static being present over the voice network. At the beginning of telephony networks amplifiers were used to boost the analogue signals in order to make the signal more audible. The problem with this technique was that the entire signal was amplified, both voice and line noise being increased. The line noise often made the connection inoperative.

Line noise is much less of an issue with digital telephony networks due to the fact that repeaters on the lines do not just amplify the signal but also clean the digital signal back to it’s initial state. The reason this is achievable with digital signals is because these signals are based on 1s and 0s. The repeaters (or digital amplifiers) only need to determine whether or not to restore a 1 or a 0. This system results in a much cleaner sound being sustained even though signals are being repeated along the way.

When the advantages of digital communication signals were fully acknowledged the telephony networks moved over to pulse code modulation (PCM). PCM is the most frequently used process for encoding analogue voice signals into digital signals made up of 1s and 0s.