Introduction
Within the telecommunications industry, modulation is an essential and creative technique used to carry invisible signals across the airways. The art of modifying waves to transmit information is a process that we frequently take for granted in our day-to-day interactions with contemporary technology. We set out to demystify modulation in this blog, covering its importance, different varieties, and crucial function in the smooth transfer of data.
Modulation means changing some characteristic of a carrier, such as amplitude, frequency, or phase, using the signal (audio or video). Modulation involves modification, variation, or change. You modify the carrier according to the signal, which is why we call it modulation. This process plays an important role in wireless communication.
Understanding the Basics
NEED FOR MODULATION
- Modulation uses high-frequency carriers to send weak, low-frequency signals over long distances. At the receiver, the signal is separated, and the carrier is discarded.
- Just as a man can deliver a message farther and faster using a horse, a signal can travel longer distances when placed on a high-frequency carrier. At the destination, the message is taken, and the carrier is discarded.
- Here, the message is the signal, the horse is the carrier, and the receiver is the radio or TV. This illustrates the principle of radio transmission and reception.
- The next reason describes the height of the antenna needed.The transmitting antenna should have a height equal to the wavelength.This condition gives best results. We know that
- If the frequency of the signal is 20 kHz, the length of the antenna
we need an antenna of 15 km height, which is totally impractical.
- If f = 1 MHz
now length of the antenna l = λ = `frac{3times10^8}{1times10^6}`=300 m
- Modulation enables wireless transmission, letting us receive audio/video signals globally. Without it, watching a match in France from home would require impractically long wires.
Challenges and Innovations
TYPES OF MODULATIONS
- Amplitude modulation: By changing amplitude of the carrier.
- Frequency modulation: By changing frequency of the carrier.
- Phase modulation: By changing phase of the carrier.
However, the complete classification of modulation processes are given below:
- Single sideband AM (SSBAM)
- Double sideband AM (DSBAM)
- Frequency division multiplexing (FDM)
In India, for sound, amplitude modulation is used
In India, for television signals, frequency modulation is used.
3.Phase modulation.
Other modulation processes are:
- Pulse amplitude modulation (PAM)
- Time division multiplexing (TDM)–used in long play records
- Pulse time modulation (PTM)
- Pulse division multiplexing (PDM)
- Pulse code modulation (PCM)
2.Digital modulation (DM)–They may be:
- Differential PCM (DPCM)
- Adoptive PCM (ADPCM)
- Data modulation (DM)
- Adoptive data modulation (ADM)
Note: 1. The modulations may also be:
- analog modulation
- Digital modulation
Amplitude Modulation
- Amplitude Modulation (AM) is a modulation technique in which the instantaneous amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the analog modulating signal to be transmitted.
- The modulating signal is an analog baseband signal which is random and has a low frequency, while the carrier signal is always a sinusoidal wave with high frequency.
- The variations in amplitude of carrier signal represent the information carried.
- The amplitude of the carrier wave is varied in accordance with the modulating signal while
the frequency and phase of the carrier signal remains unchanged. - The modulating signal seems to be superimposed on the carrier signal.
- The amplitude variations in the peak values of the carrier signal exactly replicate the
modulating signal at different points of time which is known as an envelope. - Modulation Index is given by `mu=frac{A_m}{A_c}`
Frequency Modulation
- Frequency Modulation (FM) is a modulation technique in which the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the analog modulating signal to be transmitted.
- Only the frequency of the carrier signal is varied while the amplitude and phase of the carrier are kept constant.
- The original frequency of the carrier signal is called the center or resting frequency and denoted as 𝑓𝑐.
- The amount by which the frequency of the carrier wave changes or shifts above or below the resting frequency is called frequency deviation ∆f. This means ∆𝑓 ∝ 𝑚(𝑡).
- The total variation of frequency of FM wave from the lowest to highest is termed as carrier
swing (CS),
- Modulation Index
Phase Modulation
- Phase Modulation (PM) varies the carrier signal’s phase according to the instantaneous amplitude of the analog modulating signal you want to transmit.
- After phase modulation, amplitude and frequency of the carrier signal remain unaltered.
- You map the modulating signal to the carrier signal by varying the carrier’s instantaneous phase.
- Phase modulation and frequency modulation are closely related to each other.
- In both the cases, the total phase angle 𝜙 of the modulated signal varies.
Pulse Modulation
- You can use pulse modulation to transmit analog information, such as continuous speech or data.
- You sample continuous waveforms at regular intervals.
- It has the advantage of ability to use constant amplitude pulses.
- You can subdivide pulse modulation into two categories: analog and digital.
- In analog, the indication of sample amplitude may be indefinitely variable.
- In digital pulse modulation, you send a code that indicates the sample’s amplitude to the nearest predetermined level.
- Pulse-amplitude and pulse-time modulation are both analog, while the pulse code and delta modulation systems are both digital.
Pulse Amplitude Modulation
- Pulse Amplitude Modulation (PAM) is the simplest form of pulse modulation.
- You sample the signal at regular intervals and set each sample proportional to the signal’s amplitude at the instant of sampling.
- Disadvantage - PAM does not use constant-amplitude pulses.
- Hence it is not used frequently.
- In PAM, the amplitude of the pulses of the carrier pulse train is varied in accordance with the modulating signal.
Pulse Width Modulation or Pulse
Duration Modulation
- Pulse Width Modulation (PWM), also called Pulse Duration Modulation (PDM), adjusts the width or duration of each pulse to match the instantaneous value of the analog signal.
- The starting time and amplitude of each pulse are constant.
- Disadvantage - Pulses are of varying width and hence of varying power content.
- The transmitter must be powerful enough to handle the maximum-width pulses.
Pulse Position Modulation
- Pulse Position Modulation (PPM), is a system in the position of each pulse in relation to the position of a recurrent reference pulse is varied according to the instantaneous sampled value of the modulating signal.
- The amplitude and width of the pulses are constant.
- Advantage over PWM – Requires constant transmitter power output
- Disadvantage – Dependence of transmitter-receiver synchronization
Pulse Code Modulation
- Pulse Code Modulation (PCM) digitally samples the message and rounds it off to the nearest value within a finite set of allowable values.
- The rounded values are coded.
- The PCM generator produces a series of numbers or digits.
- Each digit in binary code represents the signal sample’s amplitude at that instant.
- Signals are transmitted as binary code.
Digital Modulation Schemes
- In digital communications, the modulating signal consists of binary data or its M-ary version.
- When you need to transmit digital signals, you vary the amplitude, frequency, or phase of the sinusoidal carrier according to the incoming digital data.
- Since digital data exists in discrete steps, you also modulate the bandpass sinusoidal carrier in discrete steps.
- This reason is why you call this type of modulation digital modulation.
- Digital modulation schemes are classified as
- Amplitude Shift Keying (ASK)
- Frequency Shift Keying (FSK)
- Phase Shift Keying (PSK)
Amplitude Shift Keying
- Amplitude Shift Keying (ASK) represents digital data as variations in the amplitude of a carrier wave.
- You can generate an ASK signal by applying the incoming binary data and the sinusoidal carrier to the two inputs of a product modulator.
Frequency Shift Keying
- In Frequency Shift Keying (FSK), you transmit digital information by changing the carrier signal’s frequency in discrete steps.
- The simplest FSK is binary FSK (BFSK).
- BFSK uses a pair of discrete frequencies to transmit binary information (0s and 1s).
Phase Shift Keying
- Phase Shift Keying (PSK) conveys data by changing (modulating) the phase of constant frequency carrier.
- You represent each symbol (pattern of bits) using a specific phase.
- The simplest form of PSK is Binary PSK (BPSK).
- It uses phases 0° and 180°.
- You widely use it for wireless LANs, RFID, and Bluetooth communication.
Why FSK and PSK are preferred over ASK?
- Because of the constant amplitude of FSK or PSK, the effect of non-linearities, noise and interference is minimum on signal detection.
- These effects are more pronounced in ASK.