Components of Digital Communication System

When you talk to a friend, your voice reaches them as sound. Sound changes smoothly and continuously, so it is an analogue signal. An old landline telephone uses the same idea. The telephone changes your voice into an analogue electrical signal. That signal travels along the wire to the other telephone, and there it becomes sound again. The wire is the path, or channel, that carries the signal from one place to the other.

A signal does not have to be sound. A message can also travel as a changing potential difference along a wire, or as electromagnetic waves through the air, the way radio and mobile phone signals do. For a long time, the traditional methods of communication used analogue signals for all of this.

Radio broadcasts on medium wave and short wave are examples of analogue signal transmission. On its way from the station to your radio, the signal meets other waves and passes through a changing atmosphere. These mix with the signal and change its shape. An analogue signal carries its message in its exact shape, so when the shape changes, the message changes too. You hear this as hissing, crackling, or a voice that fades in and out, especially during a thunderstorm. This spoiling of a signal is called distortion.

There is a second problem. A telephone conversation sent as an analogue signal can be recorded illegally by someone who connects to the line, and the recording is easy to listen to. An analogue signal gives the message very little protection.

• Distortion: outside effects such as other waves and the atmosphere make the transmitted signal unclear.
• Poor protection: a conversation sent as an analogue signal can be recorded illegally.

To avoid distortion and to protect the signal, analogue signals are digitised using various technologies. A digitised signal carries exactly the same data, but it is clearer and more distinct. In a digital system, all data is written as a group of 0s and 1s. In this form the data can be stored, transmitted and recreated. Processing it is easier, and the chance of an error during processing is very low.

Why does a digital signal survive the journey better? Picture a signal that uses only two levels: 0 V stands for 0 and 5 V stands for 1. On the way, noise may pull a 5 V pulse down to 4.6 V, or lift a 0 V pulse up to 0.3 V. The receiver does not need the exact value. It only has to decide whether each pulse is high or low, and 4.6 V is still clearly high. So the original 0s and 1s are read correctly. An analogue signal has no fixed levels like this, so any noise added to it stays in the message.

A digital communication system is a chain of parts. Each part does one job and then passes the signal on to the next. The first parts are on the sending side and prepare the message for its journey. The channel carries it. The last parts are on the receiving side and turn it back into the original message. The block diagram shows the whole chain in order.

• Source: the message to be sent.
• Input transducer: changes the message into an electrical signal.
• Encoder: compresses the data into as few bits as possible.
• Modulator: loads the data onto a carrier wave as an analogue signal.
• Channel: carries the signal to the receiver.
• Demodulator: takes the data back off the carrier wave.
• Decoder: recovers the digital data and removes errors.
• Output transducer: changes the electrical signal back into the original physical form.
• Output signal: the final result that reaches the receiver.

Notice that the receiving side mirrors the sending side. The demodulator undoes the work of the modulator, the decoder undoes the work of the encoder, and the output transducer undoes the work of the input transducer. Learning the parts in these pairs makes the order easy to remember.

A transducer is a device that changes energy from one form into another. The input transducer is needed because sound cannot pass through the electronic parts of the system; only an electrical signal can. At this stage the electrical signal is still analogue. In a real system it is then changed into 0s and 1s by an analogue to digital converter, called an ADC, so that the encoder can work on it.

Compressing means writing the same information with fewer bits. It is like packing a school bag carefully so that the same books take up less space. Fewer bits use less of the bandwidth, so the channel is used efficiently and more messages can share it.

A carrier is a strong, high frequency wave that can travel a long way through the channel. The string of 0s and 1s cannot make that journey well by itself, so the modulator uses it to change the carrier wave. In this way an analogue signal is produced from the digital sequence, and that analogue signal is what goes into the channel.

A channel can be a copper wire, an optical fibre cable, or the open air through which radio waves travel from one tower to the next. The channel is where noise and other waves are most likely to reach the signal. That is exactly why the message is sent in digital form: whatever the channel adds, the 0s and 1s can still be recovered at the other end.

The decoder is where the digital form pays off. Because the data is made only of 0s and 1s, small changes picked up in the channel can be recognised and corrected here. The decoded data is then changed back into an analogue electrical signal by a digital to analogue converter, called a DAC, before it reaches the output transducer.

Here is the whole chain followed with one real message. Sita lives in Pokhara. She sends a voice message on her mobile phone to her grandfather in Dhangadhi.

• Step 1, Source: Sita speaks, and her voice produces sound waves.
• Step 2, Input transducer: the phone's microphone changes the sound waves into an electrical signal, which is then turned into 0s and 1s.
• Step 3, Encoder: the phone compresses those 0s and 1s into as few bits as possible, so the message uses little bandwidth.
• Step 4, Modulator: the bits are loaded onto a high frequency carrier wave, giving an analogue signal ready to send.
• Step 5, Channel: the signal travels as radio waves to a nearby tower, and on through the network to a tower near Dhangadhi.
• Step 6, Demodulator: the grandfather's phone receives the signal and takes the data off the carrier wave.
• Step 7, Decoder: the phone recovers the original 0s and 1s and removes errors picked up on the way.
• Step 8, Output transducer: the data is changed into an electrical signal, and the loudspeaker turns that current into sound.
• Step 9, Output signal: the grandfather hears Sita's voice as sound waves, clear and unchanged.

Both kinds of communication begin with an analogue input, such as a voice, and end with an output a person can understand. The difference lies in between. An analogue system has an input transducer, a modulator, the channel, a demodulator and an output transducer. A digital system adds an encoder before the modulator and a decoder after the demodulator, so the message travels as coded 0s and 1s.

Coding is what makes digital communication clearer and safer. Coded data can be compressed, checked for errors and protected, and an analogue signal allows none of this. The cost is bandwidth. Sending a message as a long string of 0s and 1s needs a wider range of frequencies than sending one smooth wave.