---
title: "Why modulation is necessary"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:modulation-necessary-channel"
source: https://www.bctengineer.com/q/semester_6/communication_systems/modulation-necessary-channel
site: BCT Engineering
---

# Why modulation is necessary

Communication Systems · Chapter 1: Introduction

- Asked in: 2072 Magh (CS I), 2+4 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/modulation-necessary-channel
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Why modulation is necessary? Describe how channel can be modulated?

## Short answer

Modulation is needed for practical antenna size, multiplexing, noise reduction, and allocating frequency bands. A sinusoidal carrier can be modulated by varying amplitude (AM), frequency (FM), or phase (PM). Digital modulation switches these parameters discretely (ASK, FSK, PSK). The modulated signal occupies a band around f_c matching the channel passband.

## Answer

#### Why modulation is necessary
Baseband message signals (speech, music, data) are low-frequency signals that cannot be sent directly over long distances by radio. Modulation shifts them onto a high-frequency carrier, which is needed for:
1. **Practical antenna size:** antenna length $\approx \lambda/4$. At 1 kHz this is 75 km; on a 1 MHz carrier it is only 75 m.
2. **Multiplexing:** many signals can share one channel by using different carrier frequencies (FDM), e.g. radio and TV stations.
3. **Reducing noise and interference:** wideband modulation such as FM gives a better output SNR.
4. **Narrowbanding:** a smaller ratio of highest to lowest frequency makes amplifiers and antennas easier to design.
5. **Long-distance propagation** and placing the signal in the band allotted to the service.

#### How the channel (carrier) can be modulated
The signal sent into the channel is a carrier $c(t) = A_c\cos(2\pi f_ct + \phi)$. A message $m(t)$ can be impressed on it by varying one of its parameters:

| Type | Parameter varied | Expression |
|---|---|---|
| Amplitude modulation (AM) | Amplitude | $A_c[1 + k_am(t)]\cos 2\pi f_ct$ |
| Frequency modulation (FM) | Instantaneous frequency | $A_c\cos[2\pi f_ct + 2\pi k_f\int m\,dt]$ |
| Phase modulation (PM) | Phase | $A_c\cos[2\pi f_ct + k_pm(t)]$ |

- **Continuous-wave (analog) modulation:** the carrier is a sinusoid (AM and its variants DSB-SC, SSB, VSB; FM; PM).
- **Pulse modulation:** the carrier is a pulse train whose amplitude, width or position is varied (PAM, PWM, PPM), or the samples are coded (PCM, DM).
- **Digital modulation:** a digital message switches the carrier amplitude, frequency or phase between discrete values (ASK, FSK, PSK, QAM).

In each case, the modulated signal occupies a band of frequencies around $f_c$ that matches the passband of the channel.

## Check yourself

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

1. An antenna needs length about lambda/4. Why does modulation help here?
   A. A higher carrier frequency has a shorter wavelength, so a practical antenna length (e.g. 75 m at 1 MHz)
   B. It lengthens the wavelength of the message
   C. It removes the need for any antenna
   D. It lowers the carrier frequency to 1 kHz
   Answer: A. At 1 kHz lambda/4 is 75 km, but a 1 MHz carrier needs only 75 m. (Key point: Antenna size)

2. How does modulation allow many stations to share one channel?
   A. By giving all stations the same carrier
   B. By amplifying each message differently
   C. By placing each on a different carrier frequency (FDM)
   D. By reducing the message bandwidth to zero
   Answer: C. Different carrier frequencies keep the signals separate in frequency, which is FDM. (Key point: Multiplexing)

3. In frequency modulation, which carrier parameter follows the message?
   A. Amplitude
   B. Instantaneous frequency
   C. Initial amplitude and pulse width
   D. Phase only, not frequency
   Answer: B. FM varies the instantaneous frequency; AM varies amplitude and PM varies phase. (Key point: Parameters varied)

4. A pulse train whose width is varied by the message is which type?
   A. Digital modulation, ASK
   B. Continuous-wave modulation
   C. Pulse modulation (PWM)
   D. Phase modulation
   Answer: C. Varying pulse amplitude, width or position gives PAM, PWM, PPM, which are pulse modulation. (Key point: Types of modulation)

## Related in Communication Systems

- [Angle modulated signal 12 sin(10⁶t + 5 sin10⁴t)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-12sin-numerical) — Communication Systems, chapter 3
- [Angle modulated signal 20cos(6×10⁸t + 7sin1250t)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-20cos-6e8t) — Communication Systems, chapter 3
- [Modulated signal Z(t) = 50cos{98.6×10⁶×2πt + 15sin(2×10³×2πt)}](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-50cos-98-6mhz) — Communication Systems, chapter 3
- [ISB modulation; AM vs FM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aisb-am-vs-fm) — Communication Systems, chapter 2
- [Angle modulated voltage 10 sin(10⁸t + 3 sin10⁴t)](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aangle-mod-10sin-1e8t) — Communication Systems, chapter 3
- [Two-tone AM: net modulation index, power, spectrum](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aam-two-tone-50-20-12) — Communication Systems, chapter 2, asked 2×

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