# Digital Signal Processing Questions and Answers – Sample and Hold

This set of Digital Signal Processing Multiple Choice Questions & Answers (MCQs) focuses on “Sample and Hold”.

1. What is the main function of (A/D) or ADC converter?
a) Converts Digital to Analog Signal
b) Converts Analog to Digital signal
c) All of the mentioned
d) None of the mentioned

Explanation: The electronic device that performs this conversion from an analog signal to a digital sequence is called an analog-to-digital (A/D) converter (ADC).

2. What is the main function of (D/A) or DAC converter?
a) Converts Digital to Analog Signal
b) Converts Analog to Digital signal
c) All of the mentioned
d) None of the mentioned

Explanation: A digital-to-analog (D/A) converter (DAC) takes a digital sequence and produces at its output a voltage or current proportional to the size of the digital word applied to its input.

3. The S/H is a digitally controlled analog circuit that tracks the analog input signal during the sample mode and then holds it fixed during the hold mode to the instantaneous value of the signal at the time the system is switched from the sample to the hold mode.
a) True
b) False

Explanation: The sampling of an analog signal is performed by a sample-and-hold (S/H) circuit. The sampled signal is then quantized and converted to digital form. Usually, the S/H is integrated into the (A/D) converter. The S/H is a digitally controlled analog circuit that tracks the analog input signal during the sample mode and then holds it fixed during the hold mode to the instantaneous value of the signal at the time the system is switched from the sample mode to the hold mode.

4. The time required to complete the conversion of Analog to Digital is ________ the duration of the hold mode of S/H.
a) Greater than
b) Equals to
c) Less than
d) Greater than or Equals to

Explanation: The A/D converter begins the conversion after it receives a convert command. The time required to complete the conversion should be less than the duration of the hold mode of S/H.

5. In A/D converter, what is the time relation between sampling period T and the duration of the sample mode and the hold mode?
a) Should be larger than the duration of sample mode and hold mode
b) Should be smaller than the duration of sample mode and hold mode
c) Should be equal to the duration of sample mode and hold mode
d) Should be larger than or equals to the duration of sample mode and hold mode

Explanation: The A/D converter begins the conversion after it receives a convert command. The sampling period T should be larger than the duration of the sample mode and the hold mode.
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6. In the practical A/D converters, what are the distortions and time-related degradations occur during the conversion process?
a) Jitter errors
b) Droops
c) Nonlinear variations in the duration of the sampling aperture
d) All of the mentioned

Explanation: An ideal S/H introduces no distortion in the conversion process and is accurately modeled as an ideal sampler. However, time-related degradations such as errors in the periodicity of the sampling process (“jitter”), nonlinear variations in the duration of the sampling aperture, and changes in the voltage held during conversion (“droop”) do occur in practical devices.

7. In the absence of an S/H, the input signal must change by more than one-half of the quantization step during the conversion, which may be an impractical constraint.
a) True
b) False

Explanation: The use of an S/H allows the A /D converter to operate more slowly compared to the time actually used to acquire the sample. In the absence of an S/H, the input signal must not change by more than one-half of the quantization step during the conversion, which may be an impractical constraint.

8. The noise power σn2 can be reduced by increasing the sampling rate to spread the quantization noise power over a larger frequency band (-Fs/2, Fs/2).
a) True
b) False

Explanation: The noise power σn2 can be reduced by increasing the sampling rate to spread the quantization noise power over a larger frequency band (-Fs/2, Fs/2), and then shaping the noise power spectral density by means o f an appropriate filter.

9. What is the process of down sampling called?
a) Decimation
b) Fornication
c) Both Decimation & Fornication
d) None of the mentioned

Explanation: To avoid aliasing, we first filter out the out-of-band (fl, F J 2) noise by processing the wideband signal. The signal is then passed through the low pass filter and re-sampled (down sampled) at the lower rate. The down sampling process is called decimation.

10. If the interpolation factor is I = 256, the A/D converter output can be obtained by averaging successive non-overlapping blocks of 128 bits.
a) True
b) False

Explanation: If the interpolation factor is I = 256, the A/D converter output can be obtained by averaging successive non-overlapping blocks of 128 bits. This averaging would result in a digital signal with a range of values from zero to 256 (b as 8 bits) at the Nyquist rate. The averaging process also provides the required anti-aliasing filtering.

11. The crosshatched areas gives two types of Quantization error in DM, they are?
b) Granular noise
c) Slope-overload distortion & Granular noise
d) None of the mentioned

Explanation: The crosshatched areas illustrate two types of quantization error in DM, slope-overload distortion and granular noise.

12. The slope-overload distortion is avoided, if which of the following conditions satisfy?
a) Min|dx(t)/d(t)| ≤ Δ/T
b) Max|dx(t)/d(t)| ≤ Δ/T
c) |dx(t)/d(t)| ≤Δ/T
d) None of the mentioned

Explanation: The crosshatched areas illustrate two types of quantization error in DM, slope-overload distortion and granular noise. types of quantization error in DM, slope-overload distortion and granular noise. Since the maximum slope A (T in x (n) is limited by the step size, slope-overload distortion can be avoided if max|dx(t)/d(t)|≤Δ/T).

13. In DM, By increasing Δ, reduces the overload distortion but increases the granular noise, and vice versa.
a) True
b) False

Explanation: The granular noise occurs when the DM tracks a relatively flat (slowly changing) input signal. We note that increasing Δ reduces overload distortion but increases the granular noise, and vice versa.

14. Which of the following is the right way to reduce distortion in the DM?
a) By setting up an integrator in front of DM
b) By setting up an integrator behind the DM
c) By setting up an integrator in the middle of DM
d) None of the mentioned

Explanation: We note that increasing Δ reduces overload distortion but increases the granular noise, and vice versa. One way to reduce these two types of distortion is to use an integrator in front of the DM.

15. What are the effects produced by Dm by setting up an integrator at the front of DM?
a) Simplifies the DM decoder
b) Increases correlation of the signal into the DM input
c) Emphasizes the low frequencies of x(t)
d) All of the mentioned

Explanation: One way to reduce these two types of distortion is to use an integrator in front of the DM. This has two effects. First, it emphasizes the low frequencies of x (t) and increases the correlation of the signal into the DM input. Second, it simplifies the DM decoder because the differentiator (inverse system) required at the decoder is canceled by the DM integrator.

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