SIGNAL MODULATION
1.Context
Signal modulation, including AM, FM, and PM, is crucial for transmitting information efficiently by adjusting wave properties like amplitude, frequency, and phase; these modulation techniques help manage signal interference and enable clear communication across different technologies
2.What are 'Waves' and how to Measure them?
- Picture a vast, tranquil lake with perfectly still water. In the center lies a small pile of pebbles. You pick up a pebble and toss it into the water, causing a single ripple to spread outward in a circle, gradually moving across the lake. This shows how a brief disturbance can generate a single wave. Once the wave passes, the lake returns to its calm state.
- If you had tossed a small piece of gravel, the ripple would have been small. If the stone was heavier, it would create a more forceful splash and a taller wave. This wave height is known as the amplitude. In sound waves, higher amplitude means a louder sound, while in light waves, it indicates greater brightness.
- If you drop a pebble every second, a series of waves would spread out in concentric circles. If I were sitting in a boat some distance away and felt the waves hit, the boat would move up and down.
- By observing how much the boat rises, I could estimate the wave's amplitude and, in turn, the weight of the stone. Additionally, by tracking how often the boat moves, I could determine how frequently stones are being dropped.
- The number of wave crests passing a point each second is the wave's frequency, measured in hertz (Hz), where one hertz equals one cycle per second. For instance, Chennai Akashvani radio station transmits waves with 783,000 crests per second. The distance between two successive crests or troughs is called the wavelength, which is measured in meters
2.1. Measuring the Waves
Waves can be measured in several ways, depending on the type of wave (e.g., water waves, sound waves, light waves) and the property being measured. Here are the key parameters used to measure waves:
Wavelength (λ)
- Definition: The distance between two consecutive points that are in phase (e.g., two crests or two troughs) in a wave.
- Measurement: Measured in meters (m) for physical waves like water waves or electromagnetic waves.
- Formula: For a wave with speed vv and frequency ff, the wavelength is given by: λ=v/f
- Application: Used for light, sound, water waves, etc.
Amplitude (A)
- Definition: The maximum displacement of a wave from its equilibrium position.
- Measurement: Measured in meters (for water waves or sound waves), volts (for electrical waves), or other units depending on the wave type.
- Application: Amplitude is directly related to the energy of the wave. For example, a higher amplitude in a sound wave corresponds to a louder sound.
Frequency (f)
- Definition: The number of complete cycles (or oscillations) that pass a given point per second.
- Measurement: Measured in Hertz (Hz), where 1 Hz = 1 cycle per second.
- Formula: Frequency is related to the speed and wavelength of the wave by the equation: f=v/λ
- Application: Used for sound waves (pitch), light waves (color), etc.
Wave Speed (v)
- Definition: The speed at which a wave propagates through a medium.
- Measurement: Measured in meters per second (m/s).
- Formula: v=f×λ
- Application: Different wave types (sound, light, water) have different speeds depending on the medium they travel through.
Period (T)
- Definition: The time it takes for one complete cycle of the wave to pass a given point.
- Measurement: Measured in seconds.
- Formula: The period is the inverse of the frequency: T=1/f
- Application: Used to describe repetitive wave phenomena like sound or light.
Wave Height
- Definition: Specifically for water waves, the vertical distance from the trough (lowest point) to the crest (highest point) of a wave.
- Measurement: Measured in meters.
- Application: Commonly used in oceanography to describe the size of waves at sea.
3. What are FM and AM?
- We can have two waves with identical frequencies but different phases, meaning one wave starts a bit later than the other. It’s similar to a march-past where one group is out of sync with the rest. As a result, the corresponding crests (or troughs) of the waves are slightly misaligned.
- When one crest aligns with another, they combine and become larger. However, when a crest meets a trough, they cancel each other out. In this way, out-of-phase waves can be uniquely combined to create a larger wave.
- The phase difference between two waves of the same frequency is often used to encode information.
- One benefit of Phase Modulation (PM) is that minor changes in the signal's amplitude do not affect the transmission of information. This is because the information is encoded in the phase, not the amplitude of the signal, making PM resistant to noise and interference-related amplitude fluctuations.
- For this reason, PM is preferred over AM or FM for clear digital communication, like in Wi-Fi networks.
4. What are digital and analog?
- Phase Modulation (PM) is not suitable for analog radio or TV broadcasts because it operates in a digital format. When modes like AM, FM, or PM are used to transmit dots and dashes (or 0s and 1s), it constitutes digital transmission.
- In contrast, when AM or FM transmits composite waves, it’s analog transmission. Analog signals are continuous, whereas digital signals are discrete.
- One key difference between water waves and electromagnetic waves, such as radio signals, is that water waves require a medium to travel through.
- Electromagnetic waves, on the other hand, are made up of oscillating electric and magnetic fields in three spatial dimensions.
- Despite this distinction, electromagnetic waves can encode information through their phase, frequency, and amplitude, just like waves on the surface of an ideal lake
5.How is signal modulation used?
- One key advantage of modulation is that multiple signals, modulated in different ways, can share the same channel—such as airwaves in a city or a fiber-optic cable—without interfering with each other.
- Similarly, lightning generates radio waves that can disrupt signals, leading to static and noise. However, digital transmissions are largely unaffected by this interference due to modulation. This is a significant factor behind the development of modern communication technologies like the internet.
- Despite the benefits of digital transmission, operators often prefer analog for television and radio signals because analog receivers are more affordable. As a result, TV and radio broadcasts typically use AM or FM.
- The International Telecommunication Union assigns frequency bands to various services to reduce interference and improve transmission efficiency.
- AM operates in the frequency range of 535 to 1,705 kHz, allowing for long-wavelength signals that can travel over mountains and buildings, offering broader coverage.
- FM, on the other hand, delivers better sound quality since it is less affected by static. However, its frequency range of 88-108 MHz corresponds to shorter wavelengths, limiting its transmission range to line-of-sight. Additionally, FM receivers tend to be more expensive.
- Nonetheless, as the cost of electronic components continues to drop, the global shift from analog to digital broadcasting is accelerating
Source: The Hindu
