OBJECTIVES:

To explain digital modulation
To identify the different digital modulation schemes
To discuss the operations and features of the different modules

KEY TERMS:

Digital Transmission is the transmittal of digital pulses between two or more points in a communication system
Digital Radio is the transmittal of digitally modulated analog carrier between two or more points in a communication system.
Bit Rate is the rate of change in the input of modulator.
Baud Rate is the rate of change in the output of modulator.

Before anything else, let us first recall or understand the difference between Analog Signals and Digital Signals; and the difference between Analog Radio and Digital Radio.

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The Comparison between Analog Signals and Digital Signals:

ADVANTAGES of Digital Signals

Ease of Processing. Rather than being sinusoidal or having a continuous value of signals, the digital signal has a sequence of two discrete values. It can either be 1 or 0 but cannot be in between. These look like square waves.
Ease of Multiplexing. Multiplexing is a method of combining signals so it is easier to combine discrete data than continuous.
Noise Immunity. In [Digital Communication #1: Information Capacity] I explained noise. They are the unwanted signals that interfere with our signals. Because of the type of wave the digital signal possess, it is hard for the noise to penetrate the signal.

DISADVANTAGES of Digital Signals
Circuit Complexity.
Higher Cost.

The Comparison between Analog Radio and Digital Radio:

In digital radio, the modulating and demodulating signals are digital pulses rather than analog waveforms.
Digital Radios used Digital Modulation Schemes.

What is Digital Modulation?

Digital Modulation is the transmittal of digitally modulated analog signals (carrier) between two or more points in a communication system. It is sometimes called as Digital Radio – Tomasi.

Have you ever wonder how is it even possible to send information without physical contact? Just like what I did, I typed this information and sent it here but how? Is it a magic that the messages we send and post online just appear from nowhere? Of course not! And everything we believe as magic can be explained through science. So to correct you and make you understand how things in our electronic communication been possible keep on reading and let us untangle the curiosity in ourselves.

Whenever we send a message from a transmitter, there is what we call carriers. These are the high-frequency signals that carry our information. They undergo a process called modulation. Modulation is where the carrier signal is changed and serves as the envelope or capsule of the information. Why is it that the carrier must be changed? Well that is the nature of modulation because if ever the information is the one being changed then the text you sent will also be changed. For instance you texted your partner the words’ LOVE YOU” but because of the unwanted changes of information occurred, then he/she may receive” YOU ARE NOT THE ONLY ONE” instead. It is impractical to use analog modulation so nowadays, we use digital modulation. In the type of modulation we are currently using, the information signal is also digital, which could be encoded or computer-generated but still uses an analog carrier. An Analog-to-Digital Converter is use resulting to a more complex circuit than an Analog Modulation Technique.
Digital Modulation uses bit rate frequency or bf, which is basically the rate of change to the input of the modulator. It is represented by bits per second or bps.





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What are the Kinds of Digital Modulation?

1. Amplitude Shift Keying

It is simply a double sideband, full carrier amplitude modulation where the input modulating signal is in binary waveform. The output signal is always dependent on its input signal. It is basically an equivalent of an A.M. (Amplitude Modulation) wherein the amplitude of the signal is what being change. It is sometimes called as continuous wave modulation or on-off keying since because it is in binary form, it can only be shifted as on or off.

ASK Operation

If the INPUT logic is HIGH then the OUTPUT is the maximum peak amplitude of the carrier. While if the INPUT is LOW then the OUTPUT is ZERO. It only follows multiplication.

2. Frequency Shift Keying

It is a form of constant amplitude angle modulation that was proved by Emily Baudot, it similar to conventional FM or Frequency Modulation except that the modulating signal is a binary signal. This signal varies between two discrete levels. These levels can either be logic 1 known as Mark Frequency or logic 0 known as * Space Frequency*.

FSK Operation

If the INPUT logic is HIGH then the OUTPUT is the Mark Frequency. While if the INPUT is LOW then the OUTPUT is the Space Frequency.

3. Phase Shift Keying

This is another form of angle modulated, constant amplitude digital modulation --Tomasi

This is basically an equivalent of PM or Phase Modulation but the difference is that its input signal is in the form of binary.

Types of PSK

Binary PSK
There are two possible output phases for a single carrier frequency which is either logic 1 or logic 0.
When the digital signal changes it state then the phase of the output carrier shifts between two angles that are 180 degrees out of phase of the carrier.
It is a form of suppressed carrier, square wave modulation of a continuous wave signal.
It is sometimes called as Phase Reversal Keying or Biphase Modulation

Quaternary PSK
The angle is being modulated while there is constant amplitude.
From the its name “Quaternary”, it basically means that there are 4 possible output phases in a single carrier frequency with different input conditions

8-PSK
The incoming bits are considered in groups of 3 or known as tribits
The incoming serial bit stream enters in the bit splitter
The I (in- phase) or Q (quadrature) bit determines the polarity while the C (control channels) bit provides the magnitude.

16-PSK
The incoming bits are considered in groups of 4 or known as quad bits
The output does not change until 4 bits have been inputted into the modulator.

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Source: https://steemit.com/steemiteducation/@sissyjill/digital-communication-2-digital-modulation