FECN Digital Modulation

Digital Modulation

• is a way of sending information using signals. In simple terms, it’s like turning "on" and "off" switches in different patterns to represent data (like the 1s and 0s of computer code). Instead of just sending plain 1s and 0s, digital modulation changes a signal—such as its strength, frequency, or timing—to carry the data.

• helps send data quickly, reliably, and over longer distances. It’s how things like the internet, mobile phones, and Wi-Fi work to transmit information.

Bit Depth

is the number of bits used to represent each sample’s amplitude value.

8 bit

Used for early digital audio, such as old video games, and low quality images.

16 bit

The standard for CD audio and digital photography (JPEG, PNG). This offers a wide dynamic range and is considered “high quality” for many consumer application.

24 bit

The professional standard for recording and production in the music industry.

how is digital data transmitted over cables and wireless links?

The answer lies in broadband communication techniques that use modulation, facilitated by a modem.

• A modem, which includes both a modulator and a demodulator, converts binary signals into analog signals that can be transmitted over telephone and cable TV lines or via radio waves. The modem then demodulates these analog signals to reconstruct the original binary data.

What is the four main types of modulation used in modern modems

amplitude shift-keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK).The fourth type is quadrature amplitude modulation (QAM), which combines both amplitude and phase modulation to increase data rates.

Frequency-shift keying (FSK)

is primarily used in lower-speed modems (under 500 kbps) operating in noisy environments.

PSK (Phase-Shift Keying)

works within narrower bandwidths and supports a wide range of speeds

QAM

combines both amplitude modulation and PSK, enabling very high data rates in limited bandwidths.

M-ary Encoding

is a method of digital communication in which multiple bits are grouped together to form a single symbol, with each symbol representing one of M possible states.

• In this context, "M" represents the number of distinct symbols that can be transmitted, and it can be any power of 2, such as 4, 8, 16, or more.

Binary Coding

each symbol represents one bit, either 0 or 1.

Symbol and State

In M-ary encoding, instead of transmitting one bit at a time (as in binary encoding), multiple bits are combined into a single symbol..

Advantages of M-ary Coding

• Higher Data Rates: M-ary encoding can transmit more data per symbol, leading to increased bandwidth efficiency.

• Better Utilization of Bandwidth: By grouping bits into symbols, fewer symbols are needed to transmit the same amount of information, reducing the bandwidth required.

Trades off of M-ary Coding

• Complexity: As M increases, the system becomes more complex, both in terms of modulation and demodulation.

• Signal-to-Noise Ratio (SNR): Higher M-ary levels require better signal quality (higher SNR) to distinguish between the various symbols, making the system more susceptible to noise.

Baud

is a term that is often misunderstood and commonly confused with bit rate (bps).

Bit Rate

refers to the rate of change of a digital information signal, which is usually binary signal

Baud

like bit rate, is also a rate of change

Baud Rate

• is the rate at which signals can change (pulses/second)

• the unit is the baud (Bd), and it is a measurement of speed

Bit Rate

• is the rate at which data is sent (bits/second)

• the unit is bits per second (bps), and it is also a measurement of speed.

Nyquist Bandwidth/frequency

The minimum theoretical bandwidth necessary to propagate a signal is called

According to H. Nyquist,

According to him binary digital signals can be propagated through an ideal noiseless transmission medium at a rate equal to two times the bandwidth of the medium.

ASK (Amplitude shift-keying)

The simplest modulation technique, where a binary information signal directly modulates the amplitude of an analog carrier.

Digital Amplitude Modulation (DAM).

ASK is similar to standard amplitude modulation except there are only two output amplitudes possible. Also known as ?

FSK (Frequency-Shift Keying

is the oldest and simplest modulation technique used in modems.

Space

In FSK, two different sine wave frequencies represent binary 0s and 1s. For example, in data communication terms, a binary 0? is represented by a frequency of 1070 Hz

Mark

In FSK, two different sine wave frequencies represent binary 0s and 1s. For example, in data communication terms, a binary 1? is represented by a frequency of 1270 Hz

PSK (Phase-Shift Keying)

• the binary signal being transmitted alters the phase of a sine wave, depending on whether a binary 0 or binary 1 is being sent. (Remember that phase shift refers to a time difference between two sine waves with the same frequency.)

phase reversal or phase inversion

A 180° phase shift, which is the largest possible phase difference, is called a?

In Phase

0 degrees

Shift

45 degrees

Shift

90 degrees

Phase Inversion

180 degrees