1. Scanning

Since the inventions of Bell (1847-1922) and Hughes (1831-1900), the principles of telephone speech transmission had not evolved. Thanks to developments in semiconductor technology (transistors and integrated circuits), the digital techniques that had been developed mathematically for over half a century were now applied to telecommunications.

Digitizing an audible signal is based on the application of Shannon's law (1933). This involves periodically analyzing the amplitude of the signal to be transmitted. The sampling frequency is set at a value at least equal to twice the maximum frequency to be digitized. In telephony, voice frequencies range from 300 to 3,400 Hz. Sampling is carried out every 125 µs, i.e. at 8,000 Hz. The amplitude of the medium, measured every 125 µs, is encrypted and converted into an 8-bit binary number. It is these 8-bit elements that are...