---
title: "Analog communication block diagram; modulation and its reasons"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:analog-comm-modulation-reasons"
source: https://www.bctengineer.com/q/semester_6/communication_systems/analog-comm-modulation-reasons
site: BCT Engineering
---

# Analog communication block diagram; modulation and its reasons

Communication Systems · Chapter 1: Introduction

- Asked in: 2068 Jestha (old course), 4+4+2 marks; 2067 Shrawan (CS I), 2+4+4 marks (asked 2 times)
- Video: [What is Modulation ? Why Modulation is Required ? Types of Modulation Explained.](https://www.youtube.com/watch?v=mHvV_Tv8HDQ) — ALL ABOUT ELECTRONICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/analog-comm-modulation-reasons
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Define modulation. Explain the reasons for modulation. Draw the functional block diagram of analog communication system and briefly explain each block.

## Short answer

Modulation varies a high-frequency carrier’s amplitude or angle with the message signal. Reasons: reduce antenna size to practical lengths, enable frequency-division multiplexing, improve noise immunity through wideband techniques, match channel frequency bands, and ease equipment design. The analog system includes source, transducer, transmitter, channel, receiver, output transducer, and destination blocks.

## Answer

#### 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

```text
+------+  +-------+  +---------+  +-------+
|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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Modulation is best defined as:
   A. Varying amplitude, frequency or phase of a high-frequency carrier according to the message signal
   B. Amplifying the message signal to a higher power
   C. Converting the carrier into a digital signal
   D. Filtering the message to a narrower band
   Answer: A. Varying A_c gives AM, f_c gives FM and phase gives PM, all in accordance with the message. (Key point: Definition of modulation)

2. Why can a 1 kHz audio signal not be radiated directly, while a 1 MHz carrier can?
   A. Audio signals cannot travel in free space
   B. Audio has a lower power than a carrier
   C. The required antenna (lambda/4) would be about 75 km for 1 kHz but only 75 m for 1 MHz
   D. The required antenna would be 75 m for 1 kHz and 75 km for 1 MHz
   Answer: C. lambda = c/f, so 1 kHz gives 300 km and lambda/4 = 75 km, while 1 MHz gives 300 m and lambda/4 = 75 m. (Key point: Antenna size reason)

3. How does modulation allow many messages to share one channel?
   A. It compresses each message to zero bandwidth
   B. Each message is put on a different carrier frequency slot (FDM)
   C. It removes the overlap by making all carriers equal
   D. It converts all messages into noise
   Answer: B. Different carriers shift messages that share the same baseband into different frequency slots, as with radio stations. (Key point: Multiplexing)

4. What is the narrowbanding benefit of shifting audio (20 Hz to 20 kHz) onto a 1 MHz carrier?
   A. The ratio of highest to lowest frequency falls from 1000 to about 1.04
   B. The bandwidth of the audio falls from 20 kHz to 20 Hz
   C. The ratio of highest to lowest frequency rises to 1000
   D. The noise power falls to zero
   Answer: A. A small frequency ratio lets one antenna and amplifier design work well across the whole band. (Key point: Narrowbanding)

## Close variants set in other exams

- [Modulation; analog communication system block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-analog-block-diagram) — Communication Systems, chapter 1, asked 3×

## Related in Communication Systems

- [Reasons for modulation; digital communication block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-digital-adv-duplex) — Communication Systems, chapter 1
- [Block diagram of analog communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-comm-block-diagram) — Communication Systems, chapter 1, asked 2×
- [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1
- [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1
- [Analog communication block diagram; FDM of three signals](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-block-fdm-three-signals) — Communication Systems, chapter 5
- [Duplex digital communication system vs analog](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-comm-duplex-vs-analog) — Communication Systems, chapter 1

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