Telecommunication

Consists of three main parts:

1. Transmitter: Converts the message into a transmittable signal.
2. Channel/Medium: Carries the signal (e.g., air, optical fiber).
3. Receiver: Converts received signal back into original information.

1. Transmitter:
   • Uses a transducer (e.g., microphone) to convert information to an electrical signal.
   • Modulates the signal for transmission.
2. Channel (Medium): Physical (e.g., cable, fiber) or non-physical (e.g., air).
3. Receiver:
   • Demodulates and amplifies the signal.
   • Uses an output transducer (e.g., speaker).
4. Repeater:
   • Combines a receiver and transmitter.
   • Amplifies and retransmits the signal.
   • Often used in satellite communication.

1. Point-to-Point: One transmitter to one receiver (e.g., telephone).
2. Broadcast: One transmitter to many receivers (e.g., TV/radio broadcast).

Modulation is the process of changing characteristics of a carrier wave (amplitude, frequency, or phase) according to the message signal.

Purpose: To enable long-distance signal transmission without loss (attenuation).

Types of Modulation

a. Amplitude Modulation (AM)

In amplitude modulation, the amplitude of the carrier wave is varied according to the input signal, while the frequency of the carrier remains constant.

Modulation Index (M):

$M = \frac{V_{\text{max}} - V_{\text{min}}}{V_{\text{max}} + V_{\text{min}}}$

Range: $0 \leq M \leq 1$

Bandwidth (BW):

$BW = 2f_m$

where $f_m$ is the maximum frequency of the message signal.

Disadvantages:

1. High noise
   AM signals are highly susceptible to electrical noise and interference, since noise mainly affects amplitude, which carries the signal in AM.
2. Low efficiency
   A large portion of transmitted power in AM is wasted in the carrier and sidebands, making it less power-efficient.
3. Limited range
   AM has shorter transmission ranges, especially during the day, due to higher attenuation and lower signal strength.
4. Poor audio quality
   The sound quality is inferior compared to FM, with less fidelity and narrower bandwidth, making it unsuitable for high-quality audio.

b. Frequency Modulation (FM)

In frequency modulation, the frequency of the carrier wave is varied according to the amplitude of the input signal, while the amplitude of the carrier remains constant.

Advantages of FM:

1. High signal-to-noise ratio
   FM signals are less affected by noise and interference because most noise affects amplitude, which remains constant in FM transmission.
2. High fidelity
   FM provides better sound quality due to its ability to carry more information, making it ideal for music and speech.
3. Efficient transmission
   FM requires less power to transmit signals over long distances and is more resistant to signal degradation.
4. Suitable for stereo sound
   FM supports stereo broadcasting, enabling dual-channel transmission for enhanced listening experiences in radios and audio systems.

c. Phase Modulation (PM)

In phase modulation, the phase of the carrier wave is varied according to the instantaneous value of the modulating signal, while the amplitude and frequency remain constant.

Applications and Features of PM:

1. Used in digital communication
   PM is commonly employed in digital communication systems, including Wi-Fi, Bluetooth, and satellite communication, due to its ability to encode binary data efficiently.
2. Resistant to amplitude noise
   Since PM does not depend on amplitude variations, it is less prone to amplitude-related noise and interference.
3. Related to frequency modulation
   PM and FM are closely related, and both belong to the broader category of angle modulation, often offering comparable noise immunity.
4. Suitable for data transmission
   PM supports high data rates and secure transmission, making it ideal for modern communication protocols and modulation schemes like PSK (Phase Shift Keying).

a. Antenna Size

Ideal antenna length: $l = \frac{\lambda}{4}$

• Low-frequency signals have huge wavelengths, leading to impractical antenna sizes.
• Modulation increases frequency → smaller, practical antennas.

b. Radiated Power

$P \propto \left( \frac{l}{\lambda} \right)^2$

• Lower wavelength (higher frequency) increases radiated power → efficient transmission.

c. Signal Mixing

• Without modulation, baseband signals from different sources may interfere.
• Modulation allocates distinct frequency bands.

Application in Broadcasting
Broadcasting systems rely on modulated high-frequency waves to transmit information over long distances. These waves carry audio and visual data to a wide audience.

Key Points:

1. A central transmitter sends high-frequency modulated waves
   A broadcasting station uses a central transmitter to send modulated radio-frequency signals through antennas, covering large geographical areas.
2. Receivers demodulate the wave to extract audio/visual information
   Radios and televisions contain demodulators that extract the original sound or video signals from the received modulated waves.
3. Common in TV, radio, and satellite systems
   Modulation is used in AM and FM radio, analog and digital TV, and satellite communication systems, enabling efficient and reliable mass communication.