Digital Communication Interview Questions and Answers:

1. What is analog communication ?

Ans. Analog communication is that type of communication in which message or information signal to be transmitted is analog in nature. This means that in analog communication the modu­lating signal is an analog signal. The analog message signal may be had from sources such as speech, video shooting etc.

2. What is digital communication ?

Ans. In digital communication, the message signal to be transmitted is digital in nature. It means that the digital communication involves the transmission of information in digital form.

3. What is sampling ?

Ans. The process of converting an analog signal into a discrete signal (or making an analog or continuous signal to occur at a particular interval of time) is known as sampling.

4. What is meant by sampling period and sampling rate ?

Ans. During sampling, the time period between sample to sample (or time taken by the next sample to occur) is known as sampling period and is denoted by T_s. Sampling rate is reciprocal of sampling period i.e. sampling rate, f_s = 1/T_s.

5. What is sampling theorem?

Ans. The sampling theorem may be stated as below :

A continuous-time signal may be completely represented in its samples and recovered back if the sampling frequency is f_s ≥ 2f_m where f_s is the sampling frequency and f_m is the maximum frequency present in the signal.

6. What is Nyquist rate and Nyquist interval?

Ans. When the sampling rate becomes exactly equal to 2f_m samples per second then it called Nyquist role (or minimum sampling rate). Maximum sampling interval is called the Nyquist interval. It is given by T_s = 1/2f_m seconds.

7. Why instantaneous sampling is possible only in theory?

Ans. The instantaneous or ideal sampling is possible only in theory because it is not possible to have pulse whose width approaches zero.

8. What is pulse amplitude modulation?

Ans. Pulse modulation may be defined as a process of modulation in which continuous waveforms are sampled at regular intervals and information regarding signal at sample times are transmitted together with synchronizing pulses.

9. What is meant by pulse time modulation?

Ans. The modulation technique in which the time or duration of the pulses is varied in accordance with the amplitude of the message signal keeping the amplitude of the pulse constant is referred to as pulse time modulation (PTM).

10. What are the two different types of PTM systems?

Ans. There are two types of pulse-time modulation (PTM) systems viz. (i) pulse duration or pulse width modulation (PDM or PWM) and pulse position modulation (PPM).

11. What is pulse duration modulation?

Ans. Pulse duration modulation (PDM), also known as pulse width modulation (PWM), is the method of modulation in which the samples of message signal are used to vary the duration or width of the individual pulses.

12. What is pulse position modulation?

Ans. In pulse position modulation (PPM), the position of a pulse relative to its unmodulated time of occurrence is varied in accordance with the message signal.

13. What is a pulse digital modulation scheme?

Ans. Pulse digital modulation (PDM) is basically a scheme in which the message and also the carrier are in discrete form. It may be a pulse code modulation (PCM) or a delta modulation.

14. Enumerate the advantages of digital representation of analog signals.

Ans. A digital signal have certain advantages over analog signals which are enumerated below :

1. Immunity to transmission noise and interference.
2. Efficient regeneration of the coded signal along the transmis­sion path.
3. There is a possibility of a uniform format for different types of baseband signals.

15. What is PCM?

Ans. PCM (pulse code modulation) is a process in which the message signal is sampled and the amplitude of each sample is rounded off to the nearest of a finite set of allowable values.

16. Explain noise effect in PCM.

Ans. The performance of a PCM system is influenced by two major sources of noise.

1. Transmission noise, which is introduced anywhere in transmitter output and the receiver input.
2. Quantizing noise, which is introduced in the transmitter and is carried along to the receiver output.

17. What is delta modulation?

Ans. Delta modulation transmits only one bit per sample. Here, the present sample value is compared with the previous sample value and this results whether the amplitude is increased or decreased.

18. Enumerate the advantages of delta modulation over PCM.

Ans. The delta modulation has certain advantages over PCM as under :

• The signaling rate and transmission channel bandwidth are quite small for delta modulation compared to PCM.
• No ADC is required in delta modulation, therefore the transmission and receiver implementation is very much simple for delta modulation. The delta modulation has the drawbacks of (i) slope overload distortion and (ii) granular or idle noise.

19. What is a facsimile machine ?

Ans. A facsimile or fax machine is an electronic device used for transmitting and receiving printed matter via telephone line.

20. Enumerate the application fields of facsimile machine.

Ans. The facsimile machine finds application in the fields of personal use, business, law enforcement information, weather information transmission and libraries.