DIGITAL COMMUNICATION # 1: DIGITAL TRANSMISSION

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KEY TERMS

Digital Transmission – is the sending of information over a physical communications media in the form of digital signals. Analog signals must therefore be digitized first before being transmitted.

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Digital Signals – are described as discrete, their amplitude maintains a constant level for a prescribed period of time and then it changes to another level.

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Analog Signals – the amplitude changes continuously with respect to time with no breaks or discontinuities.

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Pulse Modulation – It is the conversion of information into pulse form for transferring pulses from a source to a destination.

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Key Formula

Resolution = Vmax/Dynamice Range

fs=>2fa : where fs=sampling frequency ,fa= fundamental frequency

Objectives of the Study

•Define key terms in digital transmission
•Familiarize the different pulse modulation techniques
•Study the operations, processes, features of Pulse Code Modulation (PCM)
•Illustrate the highlights of delta modulation
•Elaborate Differential PCM

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Advantages of Digital Transmission

•It is more immune to noise
•Digital pulses are better suited for processing and multiplexing
•Digital signals are simpler to measure and evaluate
•Digital systems are better suited to evaluate error performance.

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DISADVANTAGES OF DIGITAL TRANSMISSION

•The transmission of digitally encoded analog signals requires more bandwidth.
•Analog signals must be converted to digital codes prior transmission and converted back to analog at the receiver.
•Digital transmission requires precise time synchronization between the transmitter and receiver clocks.
•Digital transmission systems are not incompatible with existing analog facilities.

PULSE MODULATION

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Is the conversion of information into pulse form for transferring pulses from the source to a destination.

PULSE MODULATION AND ITS METHODS

Basically pulse modulation has four (4) types of methods this are the following;

PULSE AMPLITUDE MODULATION

Is a method of converting information wherein the amplitude of a constant width, constant position pulse is varied in accordance to the instantaneous amplitude of modulating signal.

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PULSE WIDTH MODULATION

Is the process where the width of a fixed amplitude pulse varies proportionally to the amplitude of the analog signal.

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PULSE POSITION MODULATION

Is a form of pulse modulation where the position of a constant widthin a prescribed timeslot is varied according to the amplitude of the modulating signal. This method is seldom used in the application of the output.

PULSE CODE MODULATION

Is a sampled and converted to a fixed length, serial binary number for transmission. The binary number varies according to the amplitude of the analog signal. This method is the most widely and popularly used.

MODULATION

PULSE CODE MODULATION (PCM) BLOCK DIAGRAM

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FUNCTIONS OF EACH BLOCK

BANDPASS FILTER

It limits the frequency of the analog input signal to the standard voice band frequency range of 300GHz to 3000Hz. (antifoldover filter)

SAMPLE AND HOLD CIRCUIT

It periodically samples the analog input signal and converts the samples to multilevel PAM signal.

ANALOG TO DIGITAL CONVERTER (ADC)

It converts PAM samples to parallel PCM codes.

DIGITAL TO ANALOG CONVERTER (DAC)

It converts PCB codes back to PAM signals.

HOLD CIRCUIT

It converts PAM signals to its original form.

PULSE CODE MODULATION (PCM) PROCESSES

• SAMPLING

Is the conversion of the analog input to PAM signal

QUANTIZING

Is the assignment of the amplitudes to the PAM signals. It is a process of converting an infinite numbers of possibility to a finite number of conditions (rounding off the amplitude of flat – top samples to a manageable number of levels)

• Encoding

It is the assignment of binary codes of the PAM quantized signals ready for transmission.

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Sampling Circuit

For the Analog-to-Digital Converter to accurately convert a voltage to a binary code, the voltage must be relatively constant so that the ADC can complete the conversion before the voltage level changes.

TYPES OF SAMPLING

NATURAL SAMPLING

Is when the tops of the sample pulses retain their natural shape during the sample interval.

FLAT-TOP SAMPLING

It is uses sample and hold circuit, the input voltage is sampled with a narrow pulse and held relatively constant until the next sample is taken.

SAMPLING

The Nyguist Sampling Theorem establishes the minimum sampling rate that can be used for a given PCM System.

BIBLIOGRAPHY

DIGITAL COMMUNICATIONS ENGR. OBISO, PAGE 29-33

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