|Science > Physics > Communication > You are Here|
- Communication is the process of exchange of information between two entities. It involves sending, receiving and processing of information. Thus there is a transmission of information from a source at one place to a receiver located at another place.
- The communication system is a device or setup used in the transmission of the information from one place to another.
Elements of Communications System:
- Primarily communication system consists of three main parts: i) a transmitter ii) a communication channel and iii) a receiver. The transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter, while the channel is the physical medium that connects them. Depending upon the type of communications system, a channel may be in the form of wires or cables connecting the transmitter and the receiver or it may be wireless.
- Example: When a person talks with another person directly, then the speaker is the transmitter conveying the information in the form of sound waves through a communication channel, the intervening air and listener is the receiver. If the distance between the speaker and the listener is large sound waves cannot reach from the speaker to the listener directly and communication is not possible. Now to make communication possible we have to change the communication channel and method.
General Working of Communication System:
- The devices such as microphones, photodetectors and piezoelectric sensors which convert non-electrical signals into electrical signals are called transducers and vice versa. Microphones convert the sound signals into electrical signals. Photodetectors convert light signals into electrical signals, and piezoelectric sensors convert pressure variations in electrical signals.
- The purpose of the transmitter is to convert the message signal produced by the source of information into a form suitable for transmission (generally electric) through the channel. If the output of the information source is a non-electrical signal like a voice signal, a microphone (transducer) is used.
- Next step is the modulation. The information signals in the form of electrical signals cannot be transmitted over large distances, so they are superimposed on a high-frequency wave called the carrier wave. This superimposition of the electrical signal on the high-frequency wave (carrier wave) is called modulation. Thus modulator converts it to transmittable form.
- Then the signal is amplified and fed as an input to the transmitter. Which is projected through communication channel using the antenna.
- Then this signal propagates through the communication channel. Wires, cables, optical fibres or earth’s atmosphere are used as a communication channel. When a transmitted signal propagates along the channel there is a possibility that the signal may get distorted due to channel imperfection, noise, etc. Thus the receiver receives a corrupted or low powered version of the transmitted signal.
- The receiver has the task of operating on the received signal. Antenna retrieves the signal from a communication channel. At the receiver end, the signal is converted back into recognizable form which can be interpreted by the receiver. The demodulator separates the low-frequency audio signal from the modulated signal. The audio amplifier boosts up the signal to compensate the loss of power due to attenuation. the amplified signal is fed to a transducer (loudspeaker) which converts the electrical signals into audio signals.
Types of Communication System:
- In this mode, communication takes place over a link between a single transmitter and a receiver. Example: Telephony.
- Broadcast Communication:
In this mode, there are a large number of receivers corresponding to a single transmitter. Example: radio and television broadcast
Undesirable Effects on Signal During Transmission:
- The loss of strength of a signal while propagating through the communication channel is known as attenuation. Though there are losses at the transmitter end and at the receiver end still it is a standard practice to assume transmitter and receiver as ideal. The losses due to attenuation can be compensated with amplification. Hence it is not a serious problem.
- It is a waveform perturbation caused by the imperfect response of the system to desired signal itself. Distortion disappears when the signal causing distortion is turned off or disappears. If the channel has a linear but distorting response, then distortion may be corrected or at least reduced using filter devices called equalizers.
- Interference is a contamination due to extraneous signals from other transmitters, machinery, power lines, switching circuits and human sources. It is mainly at the receiver end due to intercepting several signals at the same time by the antenna at the receiver end. Proper filtering circuits remove the interference by making the interfering signals to occupy different frequency band than the desired signal.
- Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. These are random and unpredictable electrical signals produced by natural processes. The noise may be internal or external. When such random variations in the electrical signal are superimposed on the information bearing symbol it gets partially or fully corrupted. Using filters noise can be reduced but cannot be eliminated completely.
The Terminology of Communication System:
- Any device that converts one form of energy into another can be termed as a transducer.
- Example: A microphone is a transducer which converts sound energy into electrical energy (electrical signals). A loudspeaker is a transducer which converts electrical energy (electrical signals) into sound energy.
- An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature,
etc) into corresponding variations in the electrical signal at its output.
- Information converted into electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. A message is defined as a physical manifestation of information as produced by the source. The terms signal and message are used interchangeably.
- In an electronic communication system, a signal means time-varying electrical signal obtained from the original signal using transducer. These signals have their own nature, frequency and amplitude. There are two types of electrical signals
- Analog Signals: An analog signals are continuous variations of voltage or current. They are essentially single-valued functions of time. The sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components.Sound and picture signals in TV are analog in nature.
The general equation of such signal is Q = Qo sin ωt here ω = 2π/T
- Digital Signals: Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level and ‘1’ corresponds to a high level of voltage/ current. Generally, such signals are in the form of pulses. There are several coding schemes used for digital communication. The output of a computer, transmission of documents through the internet is done using digital signals. The digits 0 and 1 are called bits. A group of bits is called byte or binary word. There are several coding schemes used for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD). American Standard Code for Information Interchange (ASCII) is a universally popular digital code to represent numbers, letters and certain characters.
- Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system.
- The source generating the noise may be located inside or outside the system. Lightning, turning on or off electrical appliances may cause noise.
- A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception,
- The important component of a transmitter in radio communication are a microphone, audio amplifiers, oscillator, modulator and antenna.
- A receiver extracts the desired message signals from the received signals at the channel output.
- The important components of a receiver in radio communication are an antenna, demodulator, amplifiers, loudspeaker.
- It is a link connecting a transmitter and a receiver. It is the physical medium which carries the signal from transmitter to the receiver ideally without any noise or distortion.
- The atmosphere, optical fibres, parallel wires, coaxial cables, etc. are used as communication channels.
- Communication channels are divided into two types.
- Guided communication channel: This is generally used in point to point or in-line communication. Parallel wires, coaxial cables are used for this type of communication.
- Non-guided communication channel: This channel is used for space and satellite communication. Free space is an example of a non-guided communication channel.
- The loss of strength of a signal while propagating through the communication channel is known as attenuation.
- It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier
- Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.
- It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.
- A repeater is a combination of a receiver and a transmitter. A repeater picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Thus repeaters serve as amplifiers.
- Repeaters are used to extend the range of a communications system keeping original form intact. A communication satellite is essentially a repeater station in space. Mobile towers are the repeaters.
- The original low-frequency message/ information signal cannot be transmitted to long distances. Therefore, at the transmitter, information contained in the low-frequency message signal is superimposed on a high-frequency wave, which acts as a carrier of the information. This process is known as modulation.
- There are several types of modulation, abbreviated as amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).
- The process of retrieval of information from the carrier wave at the receiver is termed demodulation. This is the reverse process of modulation. Thus at the end of process low-frequency message is retrieved again.
- An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both the ends i.e. transmitter end and receiver end.
- The same antenna can be used for transmitting and receiving functions.
- Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal.
- An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both the ends i.e. transmitter end and receiver end. At the transmitter end, the antenna converts electrical signals into electromagnetic waves while at the receiver end the antenna converts electromagnetic waves into electrical signals.
- A vertically held transmitting antenna produces vertically polarized electromagnetic waves. The design of an antenna mainly depends on the frequency of the carrier wave and directivity of the beam.
Types of Antenna:
- Hertz Antenna: It is a straight conductor antenna whose length is equal to the half of the wavelength of the radio signals to be transmitted or received.
- Marconi Antenna: It is a straight conductor antenna whose length is equal to the quarter of the wavelength of the radio signals to be transmitted or received. The lower end of Marconi antenna is grounded.
- Doublet or Bipolar Antenna: It is a bilobed (Two rounded structures) conductor antenna whose length is equal to the half of the wavelength of the radio signals to be transmitted or received. The Number of lobes can be increased to obtain longer antenna.
- Omnidirectional Dipole Antenna: It is used for transmission of radio waves
- Dish Type Antenna: Dish antenna is directional antenna because it can receive only those waves which are directed towards it and send waves in a particular direction only. The main component of this antenna is dipole or horn feed. In receiver form, the dish which is in a shape of parabolic reflector collects waves directed towards it and focusses them on the horn feed. In horn feed the electromagnetic waves are converted into electrical signals and are then fed to the amplifier using cables. In the transmission mode, the electrical signals are converted into electromagnetic waves in horn feed and projected on the dish reflector. From reflector, waves are transmitted in the form of a parallel beam.
Size of Antenna or Aerial: :
- An antenna or aerial is a system of elevated conductors which couples the transmitter or receiver to the communication channel. Thus it is required at both the ends i.e. transmitter end and receiver end. At a transmitter end, it converts electrical signals into electromagnetic waves, while at a receiver end it converts electromagnetic waves into electrical signals.
- For the efficient transmission and reception, the length of the antenna or aerial is such that it acts as a resonant circuit at the frequency of operation. If λ is a wavelength of the RF signal employed, then the length of the antenna is generally λ/4. As the wavelength is inversely proportional to the wavelength, the length of the antenna decreases with the increase in the carrier wave frequency.
- Calculate the required length of dipole antennas for following carrier waves
- 40 MHz
c = νλ
∴ λ = c/ν = 3 x 108 / 40 x 106 = 7.5 m
Length of antenna = λ / 4 = 7.5/4 = 1.875 m
- 400 MHz
c = νλ
∴ λ = c/ν = 3 x 108 / 400 x 106 = 0.75 m
Length of antenna = λ / 4 = 0.75/4 = 0.1875 m
A Bandwidth of a Signal:
- In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. 2. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.
|Frequency Range||Bandwidth of signal|
|300 Hz to 3100 Hz||2800 Hz|
|20 Hz to 20000 Hz||
|Video||1500 MHz to 1506 MHz||
|Computer data||2000 MHz to 2600 MHz||
- The important thing to be noted that irrespective of where the band is located in the frequency spectrum, it will carry the same amount of information. For example, Signal will carry the same amount of information in a frequency range from 1 MHz to 2 MHz (bandwidth 1 MHz) or the frequency range from 5 MHz to 6 MHz (Bandwidth 1 MHz)
Bandwidth of Transmission Medium:
- Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space, and fibre optic cable.
- Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services.
- Optical communication using fibres is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fibre can offer a transmission bandwidth in excess of 100 GHz.
- Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.
- Frequency allocation to different services is given below.
|Nature of Broadcast||Frequency Band|
|Amplitude modulated (AM)||540 kHz to 1600 kHz|
|Frequency modulated (FM)||88 MHz to 108 MHz|
|Very high frequency (VHF) T.V.||54-72 MHz to 174-216 MHz|
|Ultra high frequency (UHF) T.V.||420 MHz to 890 MHz|
|Mobile telephony Mobile to base station||896 MHz to 901 MHz|
|Mobile telephony Base station to Mobile||840 MHz to 935 MHz|
|Satellite Communication Uplink||5.925 GHz to 6.425 GHz|
|Satellite Communication Downlink||3.7 GHz to 4.2 GHz|
|Science > Physics > Communication > You are Here|