What is digital signal?

What is signal

Signals is a physical quantity or an electrical waveform that carries information. It can also be define as a pattern, or changes that convey information from a source to a destination. The purpose of a signal is to transmit this information timely and accurately. Signal is a physical and electrical representation of information that is used to transmit data in various communication system.

Type of signal

1. Analog signals: They are continuous waveforms that vary in time and amplitude
2. Digital signals: They are not continuous wave but are discrete in nature i.e their values are typically binary digits (bit) a “0” and a “1”

Characteristics/properties of signals

1. Amplitude: These refers to the magnitude or strength of the signals
2. Frequency: is the number of cycles or oscillation that occurs within a given time period. Measured in hertz (Hz)
3. Phase: Refers to the relative positioning or timing of a signal waveform with respect to a reference
4. Duration: Represent the length of time that a signal persist or exist.
5. Periodicity: Refers to whether a signal exhibits a repetitive pattern or not.
6. Symmetry: These describe the balance or distribution of a signal waveform
7. Bandwidth: it refers to the range of frequency that signal occupies or occupies for transmission
8. Energy and power: are measures of the total or average amount of signal energy overtime. Energy is related to the finite duration of a signal while power is the energy per unit time

Example of analog signals

1. Audio signal: In electronics, an audio signal generator is a piece of electronic test equipment that generates electrical signals in the audio frequency range. These signals are usually created using a voltage-controlled oscillator or a pulse train and then route to an amplifier before being sent to the loudspeakers, whose output makes up the desired sound.
2. Electromagnetic waves: The electromagnetic waves consist of both electric and magnetic fields. Electromagnetic waves can travel long distances in space. The electromagnetic signals are also called radio frequency (RF) waves
3. Voltage signal: a voltage signal refers to an electrical signal that varies in voltage, typically measured in volts. Voltage signals are often used to represent analog data such as sound, temperature, and light intensity

Application of analog signals ( where we can find it)

1. Analog telephone
2. AM/FM radio
3. Television broadcasting

Note: that analog signals are susceptible (likely or affected) to noise and distortion during transmission, which can degrade the quantity of the received signal.

Advantages of digital signals

1. Immunity to noise ( insusceptible)
2. Better signal quantity
3. Efficient use of bandwidth

Digital Modulation

It is the whole process of modifying a digital signal to enable it to be transmitted over a communication channel.

Digital modulation techniques

1. Amplitude shift keying (ASK): In ASK the amplitude of the common signal varied to represent digital “1_s” and “0_s“
2. Frequency shift keying (FSK): It involves shifting the frequency of the carrier signal between two or more predefine frequencies to represent different digital symbols. For example, if we have two sine waves with different frequencies but equal amplitudes, they would be indistinguishable on an oscilloscope unless we normalized their amplitudes first. If you multiply each frequency by its corresponding normalized amplitude measurement, you can use frequency as an independent variable.
3. Phase shift keying (PSK): It changes the phase of the carrier signals to represents digital symbols ( 0⁰, 90⁰, 180⁰, 290⁰ )
4. Quadrature amplitude modulation (QAM): Is a complex modulation scheme that combines amplitude and phase modulation. (QAM represents digital data in varying both the amplitude and phase of the carrier)
5. Quadrature phase shift keying (QPSK): is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11), and consequently we need four phase offsets. Again, we want maximum separation between the phase options, which in this case is 90°
6. Orthogonal frequency division multiplexing (OFDM): Orthogonal frequency division multiplexing (OFDM) takes a digital information signal with bit rate R_b, maps n-bit words on to M = 2ⁿ symbols (each symbol being a complex number representing the amplitude and phase of an M-ary modulation scheme), splits the resulting symbol stream (rate R_s = R_b/n) into N parallel streams (each with rate ROFDM = R_s/N) and modulates each stream onto one of N different carriers [12]. The N frequencies chosen for the carriers are such that the carriers are mutually orthogonal over one OFDM symbol period, T_OFDM = 1/R_OFDM, allowing independent recovery of each parallel information stream.

Digital modulation principle

1. Pulse amplitude modulation (PAM): Converts a discrete time signal into a variable amplitude continuous time signal that is good for high-speed wired communication system.
2. Pulse position modulation (PPM): Is a modulation method that only makes every pulse in the carrier pulse sequence change within time but without changing shape and amplitude of pulse signal.
3. Pulse Code modulation (PCM): is a digital scheme for transmitting analog data. It converts an analog signal into digital form. Using PCM, it is possible to digitize all forms of analog data, including full-motion video, voice, music, telemetry, etc.
4. Pulse Width modulation (PWM): is a modulation technique that generates variable-width pulses to represent the amplitude of an analog input signal. The output switching transistor is on more of the time for a high-amplitude signal and off more of the time for a low-amplitude signal.