---
title: "Communication system block diagram; need for modulation"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:comm-block-diagram-needs-modulation"
source: https://www.bctengineer.com/q/semester_6/communication_systems/comm-block-diagram-needs-modulation
site: BCT Engineering
---

# Communication system block diagram; need for modulation

Communication Systems · Chapter 1: Introduction

- Asked in: 2076 Baisakh (CS I), 6+2 marks
- 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/comm-block-diagram-needs-modulation
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Draw the block diagram of communication system and explain briefly. What are the needs for modulation?

## Short answer

A communication system consists of source, input transducer, transmitter, channel, receiver, output transducer, and destination. Transmitter modulates signal onto carrier; channel adds noise. Modulation is needed for practical antenna size (λ/4), multiplexing many signals via different carriers, improved noise immunity, long-range propagation, and narrowbanding for easier filter design.

## Answer

#### Block diagram of a communication system
A communication system carries information from a source to a destination through a channel.

```text
+--------+  +--------+  +-----------+
| Source |->| Input  |->|Transmitter|
|        |  | transd.|  |           |
+--------+  +--------+  +-----+-----+
                              |
             Noise,     +-----v-----+
             interf. -->|  Channel  |
                        +-----+-----+
                              |
+--------+  +--------+  +-----v-----+
| Desti- |<-| Output |<-| Receiver  |
| nation |  | transd.|  |           |
+--------+  +--------+  +-----------+
```

- **Information source:** produces the message, e.g. voice, text, picture, data.
- **Input transducer:** converts the message into an electrical signal (microphone, camera, keyboard).
- **Transmitter:** makes the signal suitable for the channel: filtering, encoding (digital), **modulation** onto a carrier, and power amplification.
- **Channel:** the physical path (wire, cable, optical fibre, free space). It attenuates and distorts the signal; noise and interference are added here.
- **Receiver:** picks up the weak signal, amplifies and filters it, demodulates/decodes it, and reconstructs the message as closely as possible.
- **Output transducer:** converts the electrical signal back into the original form (loudspeaker, display, printer).
- **Destination:** the person or machine for whom the message is meant.

**Example:** in a mobile phone call, the microphone, the phone's transmitter, the radio path, the other phone's receiver and its earpiece form the chain.

#### Needs for modulation
1. **Reasonable antenna height:** antenna length $\approx \lambda/4$; a 1 kHz signal would need a 75 km antenna, but at 1 MHz only 75 m.
2. **Multiplexing:** several signals can share one channel by using different carrier frequencies.
3. **Better noise immunity:** FM and other wideband methods improve output SNR.
4. **Long-distance propagation** and placing the signal in the frequency band permitted for the service.
5. **Narrowbanding:** easier design of antennas, filters and amplifiers.

## Check yourself

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

1. In the communication block diagram, where are noise and interference mainly added?
   A. In the output transducer only
   B. In the destination
   C. In the input transducer
   D. In the channel
   Answer: D. The channel attenuates and distorts the signal, and noise and interference enter there. (Key point: Channel role)

2. Which set lists functions performed by the transmitter?
   A. Demodulation and message reconstruction
   B. Adding noise to the signal
   C. Filtering, encoding, modulation and power amplification
   D. Converting sound to electrical signal
   Answer: C. The transmitter prepares the signal for the channel; demodulation is the receiver's job and transduction is done by transducers. (Key point: Transmitter functions)

3. Why is modulation needed with respect to antenna size?
   A. Antennas only work for digital signals
   B. Antenna length is independent of frequency
   C. Modulation increases wavelength
   D. Antenna length is about lambda/4, so baseband signals (large lambda) would need impractically long antennas; a high-frequency carrier needs short ones
   Answer: D. Antenna ~ lambda/4; a 1 kHz signal needs about 75 km, a 1 MHz carrier only 75 m. (Key point: Antenna size need)

## Related in Communication Systems

- [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×
- [Analog communication block diagram; modulation and its reasons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-comm-modulation-reasons) — Communication Systems, chapter 1, asked 3×
- [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×
- [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
- [Elements of communication system; modulation is a must](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-elements-modulation-justify) — Communication Systems, chapter 1

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