Baseband Modulation MCQ || Baseband Modulation Questions and Answers

1. Following type of multiplexing cannot be used for analog signaling?

  1. FDM
  2. TDM
  3. WDM
  4. None of these

Answer.2. TDM

Explanation:-

Multiplexing is the process of combining multiple signals into one signal, over a shared medium.

If the analog signals are multiplexed, then it is called analog multiplexing. Similarly, if the digital signals are multiplexed, then it is called digital multiplexing.

Types of Multiplexing:

(1) Analog Multiplexing

  • Frequency Division Multiplexing
  • Wavelength Division Multiplexing

(2) Digital Multiplexing

  • Time Division Multiplexing
    • Synchronous TDM
    • Asynchronous TDM

Analog Multiplexing:

The signals used in analog multiplexing techniques are analog in nature. The analog signals are multiplexed according to their frequency (FDM) or wavelength (WDM).

Frequency Division Multiplexing

    • In analog multiplexing, the most used technique is Frequency Division Multiplexing (FDM).
    • This technique uses various frequencies to combine streams of data, for sending them on a communication medium, as a single signal.
    • Example − A traditional television transmitter, which sends a number of channels through a single cable uses FDM.

Wavelength Division Multiplexing

    • Wavelength Division Multiplexing (WDM) is an analog technique, in which many data streams of different wavelengths are transmitted in the light spectrum.
    • If the wavelength increases, the frequency of the signal decreases.
    • A prism, which can turn different wavelengths into a single line, can be used at the output of MUX and input of DEMUX.
    • Example − Optical fiber communications use WDM technique, to merge different wavelengths into a single light for communication.

 

2. Determine the number of bits/sec generated by a PCM system if the bandwidth of TV video and audio signal of 4.5 MHz is to be converted to PCM stream with 1024 quantization levels. Assume signal is sampled at 20% above Nyquist rate.

  1. 108 × 106
  2. 18 × 104
  3. 80 × 106
  4. 81 × 105

Answer.1. 108 × 106

Explanation:-

Bit rate in PCM

This indicates how many number of bits are generated by the encoder per second and it is defined as:

Rb = n/Ts = sampling rate × n bits/sec

Rb = n × fs

Calculation:

The bandwidth of the signal is 4.5 MHz and 1024 levels.

1024 levels = 10 bits

Maximum frequency is 4.5 MHz

The bitrate is:

Rb = 10 × (1.2)(2 × 4.5 MHz)

Rb = 10 × 10.8 × 106

Rb = 108 × 106

 

3. Which waveforms are also called as line codes?

  1. Baseband Modulation
  2. PAM
  3. FM
  4. AM

Answer.1. Baseband Modulation

Explanation:-

Baseband modulation represents digital sequences by pulse waveforms that are suitable for baseband transmission. Baseband modulation waveforms are also called line codes or PCM codes.

A line code is a code used for data transmission of a digital signal over a transmission line. This process of coding is chosen so as to avoid overlap and distortion of signals such as inter-symbol interference.

 

4. Which modulation technique does not use past information for modulation?

  1. Delta modulation
  2. Pulse Code Modulation
  3. Adaptive  Differential Pulse Code Modulation
  4. Adaptive Delta Modulation

Answer.2. Pulse Code Modulation

Explanation:-

In PCM, the modulating signal 1s sampled and the amplitude of each sample is rounded off to the nearest one of the finite set of allowable values, then a binary digit is generated, which is proportional to the rounded-off value, and the binary digits are transmitted.

  • It is used in voice, audio, and CD recording.
  • PCM system doesn’t use the previous samples in its processing of information.

 

5. Companding is used in PCM to

  1. Reduce Bandwidth
  2. Reduce Power
  3. Increase S/N ratio
  4. Get almost uniform S/N ratio

Answer.4. Get almost uniform S/N ratio

Explanation:-

  • Companding refers to a technique for compressing and then expanding (or decompressing) an analog or digital signal.
  • For digital audio signals, companding is used in pulse code modulation (PCM)
  • The process involves decreasing the number of bits used to record the strongest (loudest) signals
  • In the digital file format, companding improves the signal-to-noise ratio at reduced bit rates, it gets the almost uniform signal to noise ratio

 

6. When pulse code modulation is applied to non-binary symbols we obtain waveform called as

  1. PCM
  2. PAM
  3. M-ary
  4. Line codes

Answer.4. M-ary

Explanation:-

  1. When pulse-modulation is applied to binary symbols, the resulting binary waveform is called a pulse-code modulation (PCM) waveform.
  2. When pulse-modulation is applied to non-binary symbols, the resulting waveform is called an M-ary pulse- modulation waveform.
  3. An M-ary transmission is a type of digital modulation where instead of transmitting one bit at a time, two or more bits are transmitted simultaneously. This type of transmission results in reduced channel bandwidth.

 

7. If the number of bits per sample in a PCM system is increased from 8 to 16, then the bandwidth will be increased by

  1. 2 times
  2. 4 times
  3. 8 times
  4. 16 times

Answer.1. 2 times

Explanation:-

The number of levels for an n-bit PCM system is given by:

L = 2n

Also, the bandwidth of PCM is given by:

BW = nfs

n = number of bits to encode

fs = sampling frequency

Calculation:

For n = 8, the bandwidth will be:

B.W. = 8 fs

Similarly, For n = 16, the bandwidth will be:

B.W. = 16 fs

We observe that the Bandwidth is increased by 2 times.

 

8. In PCM, if the number of bits are changed from n to (n + 8), then SNR changes by:

  1. 8 dB
  2. 48 dB
  3. 16 dB
  4. 10 dB

Answer.2. 48 dB

Explanation:-

The Signal to Noise (SNR) Ratio for a PCM system is given by:

SNR = 1.8 + 6n dB

Where ‘n’ is the number of bits per sample.

Calculation:

For n + 8 number of bits, the Signal to Noise Ratio will be:

1.8 + 6 ×(n + 8) dB

So the SNR would increase by:

1.8 + 6 × n  – [1.8 + 6 × (n + 8)]

= 48 dB

 

9. Examples of PCM waveforms are

  1. Non-return to zero
  2. Phase encoded
  3. Multilevel binary
  4. All of the mentioned

Answer.4. All of the mentioned

Explanation:-

When pulse modulation is applied to a binary symbol, the resulting binary waveform is called a PCM waveform. There are several types of PCM waveforms. In telephony applications, those waveforms are often called /line codes.

When pulse modulation is applied to a non-binary symbol, the resulting waveform is called M- ary pulse modulation waveform.

PCM waveforms fall into the four primary groups:

(i) non- return-to-zero (NRZ) PCM;

(ii) return-to-zero (RZ) PCM;

(iii) phase encoded PCM;

(iv) multilevel binary PCM.

 

10. In a 10-bit PCM system, a message signal having a maximum frequency of 4 kHz is to be transmitted. If the bit rate of this PCM system is 60 Kbits / sec, the appropriate sampling frequency is

  1. 6 kHz
  2. 7 kHz
  3. 8 kHz
  4. 9 kHz

Answer.3. 8 kHz

Explanation:-

The bandwidth of a PCM system for an encoded signal sampled at a frequency of fs is given by:

Bit rate = n fS

fS = Sampling frequency

n = number of bits used for encoding.

n is related to the number of quantization levels (L) as:

L = 2n

n = log2 L

Calculation:

the maximum frequency for the signal will be 4 kHz.

n = 10 bits, Rb = 60 Kbits/sec

Bit rate = n fS

fs = 6 kHz

And fm is given as 4 kHz i.e

fs ≥ 2fm

fs ≥ 8 kHz

So, fs = 6kHz leads to undersampling.

Hence, options 1 and 2 can’t be correct.

Option 4: fs = 9 kHz leads to oversampling

Hence, option 3 is correct fs = 8 kHz

∴ The appropriate value of the sampling frequency (fs) will be 8 kHz

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