Communication system — block diagram with modulator, channel, demodulator

Question

Draw and explain the block diagram of a generalized communication system. What is the role of each block — transmitter, channel, and receiver? Why is modulation necessary?

Solution — Step by Step

Every communication system has three essential parts:

Transmitter — converts the message signal into a form suitable for transmission
Channel — the physical medium through which the signal travels (wire, optical fibre, free space)
Receiver — extracts the original message from the received signal

graph LR
    A["Information Source"] --> B["Modulator"]
    B --> C["Power Amplifier"]
    C --> D["Transmitting Antenna"]
    D --> E["Communication Channel"]
    E --> F["Receiving Antenna"]
    F --> G["Amplifier"]
    G --> H["Demodulator"]
    H --> I["Output / Speaker"]

The transmitter contains a modulator that superimposes the message signal onto a high-frequency carrier. The receiver contains a demodulator (detector) that separates the message back from the carrier.

The original audio signal (say 20 Hz to 20 kHz) has very low frequency. Transmitting it directly has three problems:

Antenna size: Antenna length should be comparable to wavelength. For 20 kHz, $\lambda = 15$ km — impractical.
Signal mixing: All stations would broadcast in the same frequency range and interfere.
Low radiation efficiency: Power radiated is proportional to $(\ell/\lambda)^2$ — tiny for low frequencies.

Modulation shifts the signal to high frequency, solving all three problems.

Why This Works

Communication is fundamentally about sending information from point A to point B. The message signal (voice, music, data) is always a low-frequency baseband signal. The carrier wave is a high-frequency sinusoid that can actually be radiated efficiently.

Modulation “rides” the message on the carrier. The three types:

AM (Amplitude Modulation): carrier amplitude varies with message
FM (Frequency Modulation): carrier frequency varies with message
PM (Phase Modulation): carrier phase varies with message

The receiver does the reverse — demodulation extracts the original message. For AM, a simple envelope detector (diode + RC filter) works. FM requires a frequency discriminator.

This is a very scoring topic for CBSE boards — the block diagram and “need for modulation” are repeated almost every year. The 3-mark and 5-mark questions follow a predictable pattern. Draw the diagram neatly, label all blocks, and mention all three reasons for modulation. Easy 5 marks.

Alternative Method

For the CBSE board exam, remember the bandwidth allocation concept: each AM station occupies $2f_m$ bandwidth where $f_m$ is the maximum message frequency. If message frequency is 5 kHz, each station needs 10 kHz bandwidth. This is why the AM radio band (530-1710 kHz) can fit many stations.

The modulation index for AM is $\mu = A_m/A_c$ where $A_m$ and $A_c$ are message and carrier amplitudes. For distortion-free transmission, $\mu \leq 1$ . This formula appears in CBSE numericals.

Common Mistake

Thinking the channel only adds noise. Students often write that the channel “transmits the signal with some noise added.” While noise is the main issue, the channel also causes attenuation (signal weakening) and distortion (signal shape change). A complete answer should mention all three: noise, attenuation, and distortion. For CBSE boards, mentioning all three shows deeper understanding and can earn you that extra mark.

AM wave: $s(t) = A_c[1 + \mu\sin(\omega_m t)]\sin(\omega_c t)$

Modulation index: $\mu = A_m/A_c$ (must be $\leq 1$ )

Bandwidth of AM = $2f_m$

Antenna length $\ell \approx \lambda/4$ for effective radiation