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tanong
1. Contrast the advantages and disadvantages of digital transmission.
2. What are the four most common methods of pulse modulation?
3. Which method listed in question 10-2 is the only form of pulse modulation that is used in a digital transmission system? Explain.
4. What is the purpose of sample-and-hold circuit?
5. Define aperture and acquisition time.
6. What is the difference between natural and flat-top sampling?
7. Define droop. What causes it?
8. What is the Nyquist sampling rate?
9. Define and state the causes of foldover distortion.
10. Explain the difference between a magnitude-only code and a sign-magnitude code.
11. Explain overload distortion.
12. Explain quantizing.
13. What is quantization range? Quantization error?
14. Define dynamic range.
15. Explain the relationship between dynamic range, resolution, and the number of bits in a PCM code.
16. Explain coding efficiency.
17. What is SQR? What is the relationship between SQR, resolution, dynamic range, and the number of bits in a PCM code.
18. Contrast linear and nonlinear PCM codes.
19. Explain idle channel noise.
20. Contrast midtread and midrise quantization.
21. Define companding.
22. What does the parameter µ determine?
23. Briefly explain the process of digital companding.
24. What is the effect of digital compression on SQR resolution, quantizing interval, and quantization noise?
25. Contrast delta modulation PCM and standard PCM.
26. Define slope overload and granular noise.
27. What is the difference between adaptive delta modulation and conventional delta modulation?
28. Contrast differential and conventional PCM.
Sagot
1. The primary advantage of digital transmission over analog transmission is noise immunity. Digital signals are inherently less susceptible than analog signals to interference caused by noise because it is not necessary to evaluate the precise amplitude, frequency, or phase to ascertain its logic condition.
Disadvantage:
The transmission of digitally encoded analog signals requires significantly more bandwidth than simply transmitting the original analog signal.
2. The four most common methods of pulse modulation are:
Pulse Width Modulation (PWM)
Pulse Position Modulation (PPM)
Pulse Amplitude Modulation (PAM)
Pulse Code Modulation (PCM)
3. PCM is by far the most prevalent form of pulse modulation. With PCM, the analog signal is sampled and then converted to a serial n-bit binary code for transmission. Each code has the same number of bits and requires the same length of time for transmission.
4. The sample-and-hold circuit periodically samples the analog input signal and converts those samples to a multilevel PAM signal.
5. On a sample-and-hold circuit, the FET (Q1) acts as a simple analog switch. When turned on, it provides a low-impedance path to deposit the analog sample voltage across the capacitor. The time that Q1 is on is called the aperture or acquisition time.
6. With natural sampling, the frequency spectrum of the sampled output is different from that of an ideal sample. With flat-top sampling, the input voltage is sampled with a narrow pulse and then held relatively constant until next sample is taken.
7. Droop is the rapid drop in the capacitor voltage immediately following each sample pulse is due to the redistribution of the charge across C1. The inter-electrode capacitance between the gate and drain of the FET is placed in series with C1 when the FET is off, thus acting as a capacitive voltage-divider network. It is caused by the capacitor discharging through its own leakage resistance and the input impedance of the voltage follower Z2.
8. The minimum Nyquist sampling rate must be greater than or equal to twice the maximum analog input frequency.
9. Foldover distortion occurs when the sampling rate is less than twice the maximum analog frequency.
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11. Overload distortion occurs when the magnitude of the sample exceeds the highest quantization interval. Also called peak limiting.
12. Quantization is the process of converting an infinite number of possibilities to a finite number of conditions
13. Quantization range is the range of input voltages that will be converted to a particular code. Quantization error is a round-off error in the transmitted signal that is reproduced when the code is converted back to analog in the receiver.
14. Dynamic range is the ratio of the largest possible magnitude to the smallest possible magnitude (other than 0V) that can be decoded by the digital-to-analog converter in the receiver.
15. The relationship between dynamic range and the number of bits on a PCM code is
2^n - 1 > or = DR
And for a minimum number of bits 2^n = DR + 1
Where n = number of bits in a PCM code, excluding the sign bit
DR = absolute value of dynamic range
Dynamic range is inversely proportional to resolution
16. Coding efficiency is a numerical indication of how efficiently a PCM code is utilized. It is the ratio of the minimum number of bits required to achieve a certain dynamic range to the actual number of PCM bits used.
17. The worst possible signal voltage-to-quantization noise voltage ratio (SQR) occurs when the input signal occurs when input signal is at its minimum amplitude. SQR is directly proportional to resolution.
18. With linear coding, the accuracy (resolution) for the higher-amplitude analog signals is the same as for the lower-amplitude signals, and the SQR for the lower-amplitude signals is less than for the higher-amplitude signals. With nonlinear encoding, the step size increases with the amplitude of the input signal.
19. During times when there is no analog input signal, the only input to the PAM sampler is random, thermal noise. This noise is called idle channel noise.
20. With midtread quantizing, the first quantization interval is made larger in amplitude than the rest of the steps. The advantage of midtread quantization is less idle channel noise. The disadvantage is a larger possible magnitude for quantization error in the lowest quantization interval. The lowest-magnitude positive and negative codes have the same voltage range as all the other codes (+ or - one-half the resolution) is called midrise quantization.
21. Companding is the process of compressing and expanding.
22. The parameter µ determines the range of signal power in which the SQR is relatively constant.
23. With digital companding, the analog signal is first sampled and converted to a linear PCM code and then the linear code is digitally compressed. In the receiver, the compressed PCM code is expanded and then decoded (i.e., converted back to analog).
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25. Delta modulation uses a single-bit PCM code to achieve digital transmission of analog signals. With conventional PCM, each code is a binary representation of both the sign and the magnitude of a particular sample.
26. The slope of the analog signal is greater than the delta modulator can maintain and is called slope overload. Granular noise is analogous to quantization noise in conventional PCM.
27. Adaptive delta modulation is a delta modulation system where the step size of the DAC (digital-to-analog converter) is automatically varied, depending on the amplitude characteristics of the analog input signal.
28. With differential PCM, the difference in the amplitude of two successive samples is transmitted rather than the actual sample. Because the range of sample differences is typically less than the range of individual samples, fewer bits are required for DPCM than conventional PCM.
