16-03-2011, 10:31 AM
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Advantages of digital communication system over analog communication system
Communication systems
The block diagram on the top shows the blocks common to all communication systems
Remember the components of a communications system:
Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
Transmitter: The device that sends the transduced signal
Transmission channel: The physical medium on which the signal is carried
Receiver: The device that recovers the transmitted signal from the channel
Output transducer: The device that converts the received signal back into a useful quantity
Analog Modulation
The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel
The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer
For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of 0-3000 Hz
The “hello” wave is a baseband signal:
Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation
Remember the radio spectrum:
For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of 88-108 MHz (VHF band)
Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily
This process is accomplished by a device called a modulator
The transmitter block in any communications system contains the modulator device
The receiver block in any communications system contains the demodulator device
The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum
For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum
For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum)
The demodulator extracts the original baseband signal from the received modulated signal
To Summarize:
Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal
Modulation is done to bring information signals up to the Radio Frequency (or higher) signal
Types of Analog Modulation
Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
Frequency Modulation (FM)
Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Phase Modulation (PM)
Another form of analog modulation technique which we will not discuss
AM vs. FM
AM requires a simple circuit, and is very easy to generate.
It is simple to tune, and is used in almost all short wave broadcasting.
The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?)
However, it is quite inefficient, and is susceptible to static and other forms of electrical noise.
The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals.
The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.
The main disadvantage of FM is the larger bandwidth it requires
Advantages of digital communication system over analog communication system
Communication systems
The block diagram on the top shows the blocks common to all communication systems
Remember the components of a communications system:
Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
Transmitter: The device that sends the transduced signal
Transmission channel: The physical medium on which the signal is carried
Receiver: The device that recovers the transmitted signal from the channel
Output transducer: The device that converts the received signal back into a useful quantity
Analog Modulation
The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel
The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer
For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of 0-3000 Hz
The “hello” wave is a baseband signal:
Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation
Remember the radio spectrum:
For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of 88-108 MHz (VHF band)
Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily
This process is accomplished by a device called a modulator
The transmitter block in any communications system contains the modulator device
The receiver block in any communications system contains the demodulator device
The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum
For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum
For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum)
The demodulator extracts the original baseband signal from the received modulated signal
To Summarize:
Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal
Modulation is done to bring information signals up to the Radio Frequency (or higher) signal
Types of Analog Modulation
Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
Frequency Modulation (FM)
Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Phase Modulation (PM)
Another form of analog modulation technique which we will not discuss
AM vs. FM
AM requires a simple circuit, and is very easy to generate.
It is simple to tune, and is used in almost all short wave broadcasting.
The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?)
However, it is quite inefficient, and is susceptible to static and other forms of electrical noise.
The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals.
The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.
The main disadvantage of FM is the larger bandwidth it requires