Block diagram of analog communication system

An analog communication system sends a continuously varying message (voice, music, video) from a source to a destination through a channel, usually using analog modulation such as AM or FM.

+--------+  +----------+  +-----------+
| Info   |->|  Input   |->|Transmitter|
| source |  |transducer|  |(modulator)|
+--------+  +----------+  +-----------+
                                |
                                v
                         +-----------+
              Noise ---->|  Channel  |
                         +-----------+
                                |
                                v
+--------+  +----------+  +-----------+
| Desti- |<-|  Output  |<-| Receiver  |
| nation |  |transducer|  |(demod.)   |
+--------+  +----------+  +-----------+

1. Information source

Produces the message to be sent, e.g. human speech, music, a picture or a sensor reading. In analog systems the message takes continuous values.

2. Input transducer

Converts the non-electrical message into an electrical signal called the baseband or message signal $m(t)$. Examples: microphone (sound to voltage), camera (light to voltage).

3. Transmitter

Makes the message suitable for the channel. Main parts:

• Modulator: varies a parameter (amplitude, frequency or phase) of a high-frequency carrier $A_c\cos 2\pi f_c t$ according to $m(t)$, giving AM, FM or PM.
• Filters and amplifiers: limit the bandwidth and raise the power to the required level.
• Antenna (in radio systems): radiates the signal.

4. Channel

The physical medium between transmitter and receiver: free space, coaxial cable, twisted pair, optical fibre, waveguide. While passing through the channel the signal is:

• attenuated (loses power),
• distorted (by limited bandwidth, non-linearity, multipath),
• corrupted by noise and interference, added mostly here and at the receiver front end.

5. Receiver

Recovers the message from the weak, noisy received signal. It includes an RF amplifier, mixer and IF amplifier (in superheterodyne receivers), filters that reject noise and adjacent channels, and the demodulator/detector, which extracts $m(t)$ from the carrier.

6. Output transducer

Converts the recovered electrical signal back into its original form, e.g. loudspeaker (voltage to sound), picture tube/LCD (voltage to image).

7. Destination

The person or device that uses the message.

Example: in AM radio broadcasting, the studio microphone is the input transducer, the AM transmitter and antenna form the transmitter, free space is the channel, and the home radio (superheterodyne receiver with envelope detector and loudspeaker) is the receiver and output transducer.

The quality of an analog system is measured mainly by the output signal-to-noise ratio and the fidelity (how closely the output follows the original message).