Elements of a communication system:

As mentioned before, the purpose of a communication system is to transmit intelligence signal from a source to a destination at some point away from the source. Figure 1 is a block diagram of a communication system. This system consists three basic components: transmitter, channel, and receiver.

Fig 1. Communication system.

The transmitter's function is to process the message signal into a form suitable for transmission over the communication channel. This is called modulation. As for the communication channel, its function is to provide a pathway between the transmitter's output and the receiver's input. The job of the receiver is to process the received signal to recover the appropriate message signal. If the different elements do their jobs accordingly, then the output signal should equal to the input message signal. This communication system will be visit again later in this report as we go through the lesson plans for amplitude modulation.

There are two methods in transmitting an intelligent message signal over the communication channel. The two methods are analog or digital. There are both advantages and disadvantages to both methods. For the digital method, its advantages include the following:

  1. Increased immunity to noise in the channel and outside interference which are impossible to prevent in the communication channel.
  2. Offer flexible operation of the system due to the technologies in digital system.
  3. Different types of message signals as voice data, video data, or computer data can be implemented in the same format by the transmitter.
  4. Message signals can be encrypted to provide security for sensitive data.

The disadvantages of the digital method are that the communication system are very complex and can be costly for communication channel such as satellite channels and optical fibers. Analog has two very good benefits. The first is that the communication system of analog method is very simple. The second benefit of analog method is that it is not very expensive because the use of simple technology. It may eventually be eliminated by the digital method as the communication systems become simple by progressive technology and the availability of more communication channel for digital method. There are still many broadcasting systems that use the analog method to transmit messages such as the radio. Some of our telephone networks are still implemented using the analog method. Analog will remain for some time yet so it is a good idea to learn about the analog method as well the digital method.

Before we can send a message signal over the communication channel, we need to modify the message signal into a form that is suitable for transmission over a channel. We called this process of modifying message signal as modulation. This modulation process involves changing some parameters of a carrier wave in accordance with the message signal so the resultant wave will match the communication channel's bandwidth. In order to recover the message signal, the receiver of the communication system will have to go through the demodulation process, which is the inverse of the modulation process.

One may wonder why a message signal must be modified for transmission. One of the reasons was mentioned before which is to change the message signal to match the channel's bandwidth. There are other reasons for modulate a message signal. One of these reasons is that modulation permits multiplexing. Multiplexing means that different message signals can be transmitted in the same channel at the same time. Another reason for modulation is to convert the message signal to be immune to noise and interference. This will allow a good transmission to take place without worrying much about noise.

The modulation process involves encoding the message signal in a carrier wave. This carrier wave is just a sinusoidal wave. We know that this carrier wave or sinusoidal wave has three independent parameters that can be varied with the message signal. These three parameters are amplitude, phase, and frequency. The variation of the amplitude is known as amplitude modulation (AM). Frequency modulation (FM) is to change the frequency accordingly with the message signal and the carrier wave. The last form of modulation is phase modulation. It is done by change the phase of the carrier wave according to the message signal. Most of us are already familiar with AM and FM. The radio communications basically use those in our daily life. In the lesson plan, we will go through the AM process and see how it works.

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