Advantages of Digital Communication

1. The effect of distortion, noise and interference is less in a digital communication system. This is because the disturbance must be large enough to change the pulse from one state to the other.

2. Regenerative repeaters can be used at fixed distance along the link, to identify and regenerate a pulse before it is degraded to an ambiguous state.

3. Digital circuits are more reliable and cheaper compared to analog circuits.

4. The Hardware implementation is more flexible than analog hardware because of the use of microprocessors, VLSI chips etc.

5. Signal processing functions like encryption, compression can be employed to maintain the secrecy of the information.

6. Error detecting and Error correcting codes improve the system performance by reducing the probability of error.

7. Combining digital signals using TDM is simpler than combining analog signals using FDM. The different types of signals such as data, telephone, TV can be treated as identical signals in transmission and switching in a digital communication system.

8. We can avoid signal jamming using spread spectrum technique.

Disadvantages of digital communication

1. Quantization error

• Every signal is analog in nature

• Since analog to Digital conversion follows the method described below

• Analog->sampling->quantisation->encoding->Digital signal.

• Quantisation error is introduced while quantisation process and this error cannot be removed. Hence this error poses a problem for Digital signals.

1. High power consumption

• Analog signals need to be converted into analog and vice versa which requires extra circuitry

1. Infinite bandwidth

• Bandwidth is inversely proportional to the time period of the pulses

• As pulse width reduces Bandwidth requirement increases.

• Therefore, if we want to have fast switching i.e. fast speed of communication we will have to reduce the time period of pulses or increase the frequency of pulses, this calls for higher and higher Bandwidth,

• Moreover if we wish to reduce the time period of pulses to almost zero ,we want an infinite Bandwidth to accomplish our task,which is not possible in any practical communication system.

1. Difficult transmission

• Owing to the infinite Bandwidth requirement, transmission of Digital signals becomes very difficult for communication engineers.

• The fundamental and most important pulse digital modulation technique is the pulse code modulation (PCM).This technique is the breakthrough for moving from analog to digital communication.

• PCM technique is essentially the result of the thought process to represent message signal in digital form rather than the original analog form.

• The motivation is the merit of digital signal over analog signal for communication, namely, noise robustness.

• PCM may be treated as an extension of PAM.

• In PAM the time parameter is discretized, but the amplitude still remains continuous.

• That is, within the allowable amplitude limits, the signal value can take on infinite values. However, all these infinite values may not be distinct from the perception point of view.

• For instance, in case of speech signal, all amplitude values may not be important from the auditory perception point of view.

• Therefore, we may not lose information by discretizing the amplitudes to some finite values.

• What is essentially done is to round or approximate a group of nearby amplitude values and represent them by a single discrete amplitude value.

• This process is termed as quantization .The signal with discretized amplitude values is termed as quantized signal.

• There will be error between the original analog signal and its quantized version which is measured and represented in terms of quantization noise.

• What is preferable is minimum quantization noise and hence more closely quantizing signal amplitudes.

• This leads to more number of discrete levels .Hence it is a trade off.