Digital communications

Abstract

The function of modulation has its origin in radio-electric communications. An emitter can radiate an electromagnetic wave only in a limited, and generally narrow, portion of the spectrum, which can roughly be described as a frequency “window” with Δf centred on a frequency f₀, with Δf 《 f₀. The messages to be transmitted, that can be either analogue (for example speech) or digital (for example Morse code), are represented by signals that occupy only the bottom of the frequency spectrum. The spectrum of the signal, coming from a microphone in the case of speech, does not extend beyond a few kilohertz. The same thing applies for a signal that represents the two short (Tit) or long (Tat) elements of Morse “code”, since the speed of handling several dozen signs per second is very small compared to the frequency f₀ that is measured in hundreds of kilohertz or in megahertz. Another use for modulation is frequency multiplexing which enables several simultaneous communications on the same wideband (cable or optical fibre) support, that are easily separated due to the fact that they each occupy a specific bandwidth, not connected to that of any other.