Digital Communication System Block Diagram



Digital technology is a branch of Electronics and communication. It makes use of digital signals, which are presented in discrete steps. A continuous signal is an analog signal. The output of a data source is transmitted from one point to another in a digital communication system. A data system can transmit rectangular pulses at speeds ranging from 100 to 500 Kbits per second.

A typical application of this system are:

• Computer to computer communication

• Computer interrogation (either for data storing or data manipulation and calculation)

• Programming

• Data collection

• Telemetry and alarm system

• Financial credit information

• Transfer travel and accommodation booking services

Digital communication may be classified into two categories. They are.

• A web-based system (Online System)

Data is transferred directly to or from a computer in an online system.

The online system can either be real-time or non-real-time. A real-time system demands an immediate reaction. The data transmission speed in an on-real system is significantly slowed.

• Offline System

Data is sent to or from an intermediate storage location, such as a card, a paper tape punch, magnetic tape, or a disc, in an off-line system.

Based on transmission, digital communication can be classified into three categories:

1.  Simplex

Simplex refers to a one-way channel connection. here the communication is possible in only one direction.

Example: Radio, TV, etc.

2. Semi-Duplex

A semi-duplex connection is required for either direction transmission. Communication can be possible only one way at a specified time.

3. Full-Duplex 

A full-duplexer allows data to be transmitted in both ways at the same time.


A digital information source generates a limited number of messages. A nice example of a digital source is a typewriter. A source's ability to emit a finite number of characters (messages) is restricted.

A digital communication system is a system that sends data from a digital source to a digital sink. A digital waveform is a time function with just a discrete set of possible values. Only two values exist in the digital waveform since it is a binary waveform.

Voltages and currents with digital signals are common in electronic digital communication systems.

If an analog signal must be transmitted, it is first converted to a digital signal using an Analog to Digital Converter before being sent over the channels. Because an analog signal has a continuous range of values that is a function of time.

For example, a sine wave of 1000 Hz might be used to represent a binary 1 in a digital line transmission, whereas a sine wave of 500 Hz could be used to represent a binary 0. As a result, analog waveforms produced from an analog source are used to convey digital data.

Block diagram of Digital Communication System

Digital signals are the coded representation of information that is transmitted in digital communication.

Figure: Block diagram of a Digital Communication System

The figure shows a block schematic of the essential parts of a digital communication system.

The block diagram description is:


The message signal to be sent is produced by the information source. In the case of a digital source, the information source generates a discrete and random message signal that does not change over time.

When using an analog source, the data is analog and changes over time. By sampling and quantizing the analog signal, an Analog to Digital Converter may transform it into a digital signal.

A message is created by a source of information, such as a human voice, a television image, teletype data, or the temperature and pressure of the atmosphere.

Because the message in these cases is not electrical, a transducer is needed to transform it into an electrical waveform known as the message signal.

A baseband signal is another term for the waveform. The term "baseband" refers to the frequency range that the message signal generated at the source falls inside.

An analog or digital message signal can be used.

The amplitudes and times of analog signals fluctuate constantly across their intervals. A voice signal, a television signal, and the location of the signal are all examples of signal types.

Both the amplitude and tome of a digital signal are discrete. Digital signals include computer data and telegraph transmissions.

As illustrated in the block diagram, an analog signal may always be transformed into a digital signal by combining three fundamental operations: sampling, quantizing and encoding.

Only sample values of the analog signal at regularly spaced discrete-time instants are maintained during the sampling procedure.

Each sample value is approximated by the nearest discrete level in a limited collection of levels throughout the quantizing processes. The selected level is represented by a code word with a predetermined number of code components throughout the encoding procedure.


The source encoder receives the symbols supplied by the information source. These symbols aren't capable of being directly communicated. The source encoder is responsible for converting them into digital format.

The code words for the symbols are assigned by the source encoder. A unique code word exists for each different symbol. They choose a unique value in the data set to give a series of codes to a specific piece of data. Pulse code modulators, delta modulators, vector quantizers, and other sources encoders are examples of common source encoders.

The encode function in source coding converts the digital signal created at the source output into other digital signals. It is a one-to-one mapping that is used to eliminate or minimize redundancy. As a result of this, we can deliver an effective service.


The channel encoder transforms a binary sequence into a transmittable message or information signal. Noise and interference may be introduced into the signal transmission process. Channel encoding is used to avoid these mistakes.

The input sequence is supplemented by the channel encoder by some superfluous binary bits. With some well-defined logic, these superfluous bits are inserted.

The goal of channel coding is for the encoder to map the incoming digital signal into a channel input and for the decoder to map the channel output into an output digital signal, allowing for reliable communication across a noisy channel.

This is accomplished by adding redundancy in a predetermined manner in the channel encoder and exploiting it in the decoder to precisely recreate the original encoder input.

We reduce redundancy in source coding, but we incorporate controlled redundancy in channel coding to mitigate the channel noise impact.

We can do source coding on its own, channel coding on its own, or both. Naturally, in the latter situation, the source encoding is done as indicated in fig.

As illustrated in fig. 1, we create in reverse order in the receiver: channel decoding comes first, followed by channel encoding in the transmitter.

In the receiver, we create in reverse order channel decoding first, followed by source decoding for any combination, resulting in a gain in system performance at the expense of higher circuit complexity.


The communication channel is a physical medium that is used to transfer signals from a transmitter to a receiver located at a distance. Any digital communication system's information backbone is formed by it. Optical Fiber Cables can also provide a higher data rate.

This channel in a wireless system is made up of the atmosphere. To complete the link, a multi-hop system may include coaxial cables, fiber optic cables, and microwave links. Satellite channels make global communication relatively simple.


The digital demodulator transforms the communication channel's input modulated signal to a binary bit sequence.

The digital modulator converts the input binary sequence into an analog signal waveform that can be sent via a communication channel without being distorted.

The following modulators are used to generate equivalent modulated analog signals appropriate for transmission across a band width-restricted analog transmission line of short-haul or long haul systems, using digital on-off signals as keys.

1 Amplitude Shift Keying (ASK)

2 Frequency Shift Keying (FSK)

3 Phase Shift Keying (PSK)


The channel decoder reconstructs an error-free, precise bit sequence while reducing the impacts of noise and distortion in the channel.


The source decoder reverses the source encoder's action. It transforms the channel encoder's binary output to a symbol sequence. Decoders with variable and fixed lengths are also conceivable. Memory is used by certain decoders to store the code words.

Advantages of digital communication

• Digital circuits that are relatively affordable can be employed.

• Data encryption is used to protect privacy.

• The dynamic range (the difference between the highest and lowest value) can be increased.

• Data from Voice, Video, and Data sources may all be combined and sent via a single digital transmission system.

• Unlike analog systems, noise does not build from repeater to repeater in long-distance networks.

• Errors detected are minor, even when the received signal contains a lot of noise, i.e. the S/N Ratio in Digital Systems is high. • Errors are frequently rectified by using error correction coding.

Disadvantages of digital communication

• More bandwidth is necessary for digital systems than for analog systems and synchronization is required.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

Post a Comment

Previous Post Next Post