An alternative form of signal transmission is that of sequence of numbers, each number representing the signal magnitude at instant of time.
The resulting signal is called a digital signal.
Digital messages are constructed with a finite numbers of symbols, since a digital signal is represents only by digits.
However generally we use binary number system to represent a digital signal so we have only two possible value 0 and 1.
Below figure shows a digital signal observe that the waveform is a pulse train with ()V representing a "0" signal or logic "0' signal or logic "0" and "±5V" representing logic "1".
Bellow Figure show the model of a digital communication transmission system. The overall purpose of the system is to transmit the message sequence of symbols coming out
of a sources to a destination point at as high a rate and accuracy as possible. The source and the destination point are physically separated in space and a communication channel connects the source to the destination point,
Discrete Information Source: Discrete information sources are characterized by the following parameters.
(a) Source Alphabet: These are the letters, digits or special characters available from the infoimation source.
(b) Symbol Rate: It is the rate at which the information source generates source alphabet
Source Encoder and Decoder: Symbols produced by the source generator the firstly converted into digital form by the source encoder and at the receiver and some sort of decoder is used to perform the reverse operation to that of source encoder.
It converts the binary output of the channel decoder into a symbol sequence
Channel Encoder and Decoder
After converting the message signal in the form of binary sequence by source encoder the signal is transmitted through the channel.
The communication channel adds noise and interference to the signal being transmitted.
Hence error are introduced in the binary sequence received at the receiver end. So channel encoder and decoder serve to increase the reliability of a received signal.
How ever the extra bits which are added bv the channel encoders carries no information.
Digital modulator and demodulator: If the modulating signals is digital in nature then digital modulation techniques are used.
at the receiver side , the digital demodulator converts the input modulated signals into the sequence of binary bits.
Communication Channel: The communication can takes place through wire lines, wireless or fiber optical cable.
Bellow Figure show the model of a digital communication transmission system. The overall purpose of the system is to transmit the message sequence of symbols coming outof a sources to a destination point at as high a rate and accuracy as possible. The source and the destination point are physically separated in space and a communication channel connects the source to the destination point,
Discrete Information Source: Discrete information sources are characterized by the following parameters.(a) Source Alphabet: These are the letters, digits or special characters available from the infoimation source.(b) Symbol Rate: It is the rate at which the information source generates source alphabetSource Encoder and Decoder: Symbols produced by the source generator the firstly converted into digital form by the source encoder and at the receiver and some sort of decoder is used to perform the reverse operation to that of source encoder.It converts the binary output of the channel decoder into a symbol sequenceChannel Encoder and DecoderAfter converting the message signal in the form of binary sequence by source encoder the signal is transmitted through the channel.The communication channel adds noise and interference to the signal being transmitted.Hence error are introduced in the binary sequence received at the receiver end. So channel encoder and decoder serve to increase the reliability of a received signal.How ever the extra bits which are added bv the channel encoders carries no information.Digital modulator and demodulator: If the modulating signals is digital in nature then digital modulation techniques are used.at the receiver side , the digital demodulator converts the input modulated signals into the sequence of binary bits.Communication Channel: The communication can takes place through wire lines, wireless or fiber optical cable.
