Physics · Ch 15 — Communication System
Elements of a Communication System
Elements of a Communication System
The three elements. Every communication system, however elaborate, is built from exactly three functional blocks, connected in series and shown in this section's block diagram: a transmitter, a channel, and a receiver.
Transmitter. The block that takes the message signal at its input and prepares it for sending. This preparation usually has several stages: if the message does not already exist as an electrical signal (a voice, for instance), a transducer first converts it into one; the electrical signal is then amplified if it is weak, and finally modulated -- superimposed onto a high-frequency carrier wave (Section 10.3 explains why this last step is essential). The transmitter's output, the transmitted signal, is what actually leaves the system and enters the channel.
Channel. The physical medium that carries the transmitted signal from the transmitter's location to the receiver's location. A channel may be a guided medium -- a pair of wires, a coaxial cable, an optical fibre -- or it may simply be free space, used for a wireless (radio) link. Whatever form it takes, a real channel always does two unwanted things to the signal passing through it: it attenuates it (weakens its strength, progressively, the further it travels) and it lets noise -- random, unwanted electrical disturbances from the surroundings or from the receiver's own circuitry -- add onto it, so the signal reaching the far end of the channel is always somewhat weaker and somewhat corrupted compared with what was sent. …
What this figure shows. A simple block diagram is drawn showing the three basic elements of any communication system as connected boxes in a row, with arrows indicating the direction of signal flow. The first box, labelled Transmitter, receives the original message (drawn entering from the left as a separate small arrow) and processes it -- typically including a transducer stage that converts the message into an electrical signal, followed by amplification and modulation onto a carrier -- before passing its output, the transmitted signal, into the second box, the Channel, drawn as the physical medium (a wire, a cable, or simply open space for a wireless link) that carries the signal from the transmitter's location to the receiver's location. A separate arrow labelled Noise is shown entering the Channel box from outside, indicating that unwanted disturbances add to the signal somewhere along this path. The channel's output, the received signal (now generally weaker and somewhat corrupted by the added noise), enters the third box, the Receiver, which extracts the original message from the re …