Communication system block diagram; need for modulation

Block diagram of a communication system

A communication system carries information from a source to a destination through a channel.

+--------+  +--------+  +-----------+
| Source |->| Input  |->|Transmitter|
|        |  | transd.|  |           |
+--------+  +--------+  +-----+-----+
                              |
             Noise,     +-----v-----+
             interf. -->|  Channel  |
                        +-----+-----+
                              |
+--------+  +--------+  +-----v-----+
| Desti- |<-| Output |<-| Receiver  |
| nation |  | transd.|  |           |
+--------+  +--------+  +-----------+

• Information source: produces the message, e.g. voice, text, picture, data.
• Input transducer: converts the message into an electrical signal (microphone, camera, keyboard).
• Transmitter: makes the signal suitable for the channel: filtering, encoding (digital), modulation onto a carrier, and power amplification.
• Channel: the physical path (wire, cable, optical fibre, free space). It attenuates and distorts the signal; noise and interference are added here.
• Receiver: picks up the weak signal, amplifies and filters it, demodulates/decodes it, and reconstructs the message as closely as possible.
• Output transducer: converts the electrical signal back into the original form (loudspeaker, display, printer).
• Destination: the person or machine for whom the message is meant.

Example: in a mobile phone call, the microphone, the phone's transmitter, the radio path, the other phone's receiver and its earpiece form the chain.

Needs for modulation

1. Reasonable antenna height: antenna length $\approx \lambda/4$; a 1 kHz signal would need a 75 km antenna, but at 1 MHz only 75 m.
2. Multiplexing: several signals can share one channel by using different carrier frequencies.
3. Better noise immunity: FM and other wideband methods improve output SNR.
4. Long-distance propagation and placing the signal in the frequency band permitted for the service.
5. Narrowbanding: easier design of antennas, filters and amplifiers.