Analog communication block diagram; modulation and its reasons

Modulation

Modulation is the process of varying some characteristic (amplitude, frequency or phase) of a high-frequency carrier wave in accordance with the instantaneous value of the message (modulating) signal. With carrier $c(t) = A_c\cos(2\pi f_c t + \phi)$, varying $A_c$ gives AM, varying $f_c$ gives FM and varying $\phi$ gives PM. The reverse process at the receiver is demodulation.

Reasons (needs) for modulation

1. Practical antenna height: an antenna must be about $\lambda/4$ long to radiate efficiently. For a 1 kHz audio signal $\lambda = c/f = 3\times10^8/10^3 = 300$ km, so $\lambda/4 = 75$ km, which is impossible. With a 1 MHz carrier, $\lambda/4 = 75$ m, which is practical.
2. Multiplexing: many messages occupy the same baseband (e.g. 0–4 kHz for speech). Modulating them onto different carriers places them in different frequency slots, so many signals share one channel (FDM), e.g. many radio stations.
3. Reduce noise and interference: wideband modulation such as FM trades bandwidth for better signal-to-noise ratio.
4. Overcome equipment limitations / narrowbanding: the ratio of highest to lowest frequency of audio (20 Hz–20 kHz) is 1000; after shifting to a 1 MHz carrier the ratio is about 1.04, so one antenna and amplifier design works well for the whole band.
5. Match the channel: shift the signal to a frequency band where the channel passes signals well (e.g. optical fibre, satellite bands) and assign frequencies by regulation.
6. Long-distance propagation: low-frequency baseband signals are absorbed quickly; high-frequency carriers travel far by ground, sky or line-of-sight waves.

Functional block diagram of analog communication system

+------+  +-------+  +---------+  +-------+
|Source|->| Input |->|  Trans- |->|Channel|<-- Noise
|      |  |transd.|  |  mitter |  |       |
+------+  +-------+  +---------+  +-------+
                                      |
+------+  +-------+  +---------+      |
| Dest.|<-|Output |<-| Receiver|<-----+
|      |  |transd.|  |         |
+------+  +-------+  +---------+

• Information source: produces the message (speech, music, picture).
• Input transducer: converts it into an electrical baseband signal $m(t)$, e.g. microphone.
• Transmitter: modulates $m(t)$ onto a carrier, filters and amplifies it, and feeds it to the antenna or line.
• Channel: medium (free space, cable, fibre) that carries the signal; it attenuates and distorts the signal, and noise is added here.
• Receiver: amplifies and filters the received signal and demodulates it to recover $m(t)$.
• Output transducer: converts the electrical signal back to the original form, e.g. loudspeaker.
• Destination: the user of the message.