Elements of communication system; modulation is a must

Major elements of a communication system

 [Info source]->[Input transducer]->[Transmitter]
                                         |
                    Noise, interference->[Channel]
                                         |
 [Destination]<-[Output transducer]<-[Receiver]

1. Information source: generates the message (voice, video, data).
2. Input transducer: converts the message to an electrical baseband signal, e.g. microphone.
3. Transmitter: processes the signal (filtering, encoding), modulates a carrier and amplifies it for the channel.
4. Channel: the physical medium (free space, wire, fibre); it attenuates and distorts the signal and adds noise and interference.
5. Receiver: selects, amplifies and demodulates the received signal to recover the message.
6. Output transducer and destination: converts the electrical signal back to the original form (loudspeaker, display) for the user.

Justification: modulation is a must for long-distance communication

Baseband signals (audio 20 Hz–20 kHz, video up to a few MHz) are low-frequency. Sending them directly over long distances by radio faces several constraints that modulation removes:

1. Antenna height: an efficient antenna is about $\lambda/4$. For $f=10$ kHz, $\lambda=c/f=30$ km, so the antenna would be 7.5 km tall, which is impossible. With a 1 MHz carrier, $\lambda/4=75$ m, and at 100 MHz only 0.75 m.
2. Poor radiation and propagation: radiated power from an antenna rises with $(l/\lambda)^2$; low frequencies radiate very little and die out quickly. High-frequency carriers can use sky-wave or line-of-sight paths and reach far.
3. Mixing of signals: all speech and music occupy the same baseband. Without modulation, every transmitter would interfere with every other. Modulation places each one on its own carrier (FDM), allowing many stations at once.
4. Narrowbanding: a baseband signal 20 Hz–20 kHz has a ratio of 1000:1, needing very wideband antennas and amplifiers. After modulation onto 1 MHz the ratio is about 1.04:1, so a simple tuned design works.
5. Noise and interference reduction: modulation methods like FM, PCM and spread spectrum trade bandwidth for a better signal-to-noise ratio, essential when signals are weak over long paths.
6. Channel matching: each medium has a band where loss is low (e.g. optical fibre near 1550 nm); modulation shifts the signal into that band.

Hence long-distance (especially wireless) communication is practically impossible without modulation.