Module 6: Pulse Code Modulation PDF

DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering...

DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 82 Module 6: PULSE CODE MODULATION Objectives At the end of the chapter, the students would be able to: Gain knowledge and understanding to Pulse Code Modulation (PCM). Understand the PCM Multiplexers and Demultiplexers. Learn the benefits of Pulse Code Modulation. INTRODUCTION As stated previously, digital transmission is the transmittal of digital signals between two or more points in a communications system. The signals can be binary or any other form of discrete-level digital pulses. The original source information may be in digital form, or it could be analog signals that have been converted to digital pulses prior to transmission and converted back to analog signals in the receiver. With digital transmission systems, a phys- ical facility, such as a pair of wires, coaxial cable, or an optical fiber cable, is required to interconnect the various points within the system. ADVANTAGE OF DIGITAL TRANSMISSION 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 with digital signals it is not necessary to evaluate the precise amplitude, frequency, or phase to ascertain its logic condition. Instead, pulses are evaluated during a precise time interval, and a simple determination is made whether the pulse is above or below a prescribed reference level. Digital signals are also better suited than analog signals for processing and combining using a technique called multiplexing. Digital signal processing (DSP) is the processing of analog signals using digital methods and includes band limiting the signal with filters, amplitude equalization, and phase shifting. It is much simpler to store digital signals than analog signals, and the transmission rate of digital signals can be easily changed to adapt to different environments and to interface with different types of equipment. In addition, digital transmission systems are more resistant to analog systems to additive noise because they use signal regeneration rather than signal amplification. Noise produced in electronic circuits is additive (i.e., it accumulates); therefore, the signal-to-noise ratio CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 82  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 83 deteriorates each time an analog signal is amplified. Consequently, the number of circuits the signal must pass through limits the total distance analog signals can be transported. However, digital regenerators sample noisy signals and then reproduce an entirely new digital signal with the same signal-to-noise ratio as the original transmitted signal. Therefore, digital signals can be transported longer distances than analog signals. Finally, digital signals are simpler to measure and evaluate than analog signals. Therefore, it is easier to compare the error performance of one digital system to another digital system. Also, with digital signals, transmission errors can be detected and corrected more easily and more accurately than is possible with analog signals. DISADVANTAGE OF DIGITAL TRANSMISSION The transmission of digitally encoded analog signals requires significantly more bandwidth than simply transmitting the original analog signal. Bandwidth is one of the most important aspects of any communications system because it is costly and limited. Also, analog signals must be converted to digital pulses prior to transmission and converted back to their original analog form at the receiver, thus necessitating additional encoding and decoding circuitry. In addition, digital transmission requires precise time synchronization between the clocks in the transmitters and receivers. Finally, digital transmission systems are incompatible with older analog transmission systems. PULSE CODE MODULATION Pulse modulation consists essentially of sampling analog information signals and then converting those samples into discrete pulses and transporting the pulses from a source to a destination over a physical transmission medium. The four predominant methods of pulse modulation include pulse width modulation (PWM), pulse position modulation (PPM), pulse amplitude modulation (PAM), and pulse code modulation (PCM). Pulse width modulation (PWM) is sometimes called pulse duration modulation (PDM) or pulse length modulation (PLM), as the width (active portion of the duty cycle) of a constant amplitude pulse is varied proportional to the amplitude of the analog signal at the time the signal is sampled. PWM is shown in Figure 1c. As the figure shows, the amplitude of sample 1 is lower than the amplitude of sample 2. Thus, pulse 1 is narrower than pulse 2. The maximum analog signal amplitude produces the widest pulse, and the minimum analog signal amplitude produces the narrowest pulse. Note, however, that all pulses have the same amplitude. CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 83  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 84 With Pulse Position Modulation (PPM), the position of a constant width pulse within a prescribed time slot is varied according to the amplitude of the sample of the analog signal. PPM is shown in Figure 1d. As the figure shows, the higher the amplitude of the sample, the farther to the right the pulse is positioned within the prescribed time slot. The highest amplitude sample produces a pulse to the far right, and the lowest amplitude sample produces a pulse to the far left. With Pulse Amplitude Modulation (PAM), the amplitude of a constant width, constant- position pulse is varied ac- cording to the amplitude of the sample of the analog signal. PAM is shown in Figure 1e, where it can be seen that the amplitude of a pulse coincides with the amplitude of the analog signal. PAM waveforms resemble the original analog signal more than the waveforms for PWM or PPM. With Pulse Code Modulation (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. PCM is shown in Figure 1f. PAM is used as an intermediate form of modulation with PSK, QAM, and PCM, al- though it is seldom used by itself. PWM and PPM are used in special-purpose communications systems mainly for the military but are seldom used for commercial digital trans- mission systems. PCM is by far the most prevalent form of pulse modulation and, consequently, will be discussed in more detail in subsequent sections of this chapter. CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 84  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 85 Alex H. Reeves is credited with inventing PCM in 1937 while working for AT&T at its Paris laboratories. Although the merits of PCM were recognized early in its development, it was not CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 85  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 86 until the mid-1960s, with the advent of solid-state electronics, that PCM became prevalent. In the United States today, PCM is the preferred method of communications within the public switched telephone network because with PCM it is easy to combine digitized voice and digital data into a single, high-speed digital signal and propagate it over either metallic or optical fiber cables. PCM is the only digitally encoded modulation technique shown in Figure 1 that is commonly used for digital transmission. The term pulse code modulation is somewhat of a misnomer, as it is not really a type of modulation but rather a form of digitally coding analog signals. With PCM, the pulses are of fixed length and fixed amplitude. PCM is a binary system where a pulse or lack of a pulse within a prescribed time slot represents either logic 1 or a logic 0 condition. PWM, PPM, and PAM are digital but seldom binary, as a pulse does not represent a single binary digit (bit). Figure 2 shows a simplified block diagram of a single-channel, simplex (one-way only) PCM system. The band pass filter limits the frequency of the analog input signal to the standard voice-band frequency range of 300 Hz to 3000 Hz. The sample-and-hold ciRcuit periodically samples the analog input signal and converts those samples to a multilevel PAM signal. The analog-to-digital converter (ADC) converts the PAM samples to parallel PCM codes, which are converted to serial binary data in the parallel-to-serial converter and then outputted onto the transmission line as serial digital pulses. The transmission line re- peaters are placed at prescribed distances to regenerate the digital pulses. In the receiver, the serial-to-parallel converter converts serial pulses received from the transmission line to parallel PCM codes. The digital-to-analog converter (DAC) con- verts the parallel PCM codes to multilevel PAM signals. The hold circuit is basically a low- pass filter that converts the PAM signals back to its original analog form. Figure 2 also shows several clock signals and sample pulses that will be explained in CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 86  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 87 later sections of this chapter. An integrated circuit that performs the PCM encoding and de- coding functions is called a codec (coder/decoder). PCM SAMPLING The function of a sampling circuit in a PCM transmitter is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant- amplitude pulses that can more easily be converted to binary PCM code. For the ADC to accurately convert a voltage to a binary code, the voltage must be relatively constant so that the ADC can complete the conversion before the voltage level changes. If not, the ADC would be continually attempting to follow the changes and may never stabilize on any PCM code. Essentially, there are two basic techniques used to perform the sampling function: natural sampling and flat-top sampling. Natural sampling is shown in Figure 3. Natural sampling is when tops of the sample pulses retain their natural shape during the sample interval, making it difficult for an ADC to convert the sample to a PCM code. With natural sampling, the frequency spectrum of the sampled output is different from that of an ideal sample. The amplitude of the frequency components produced from narrow, finite-width sample pulses decreases for the higher harmonics in a (sin x)/x manner. This alters the in- formation frequency spectrum requiring the use of frequency equalizers (compensation filters) before recovery by a low-pass filter. The most common method used for sampling voice signals in PCM systems is flat-top sampling, which is accomplished in a sample-and-hold circuit. The purpose of a sample- and- CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 87  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 88 hold circuit is to periodically sample the continually changing analog input voltage and convert those samples to a series of constant-amplitude PAM voltage levels. With flat-top sampling, the input voltage is sampled with a narrow pulse and then held relatively constant until the next sample is taken. Figure 4 shows flat-top sampling. As the figure shows, the sampling process alters the frequency spectrum and introduces an error called aperture error, which is when the amplitude of the sampled signal changes during the sample pulse time. This prevents the recovery circuit in the PCM receiver from exactly reproducing the original analog signal voltage. The magnitude of error depends on how much the analog signal voltage changes while the sample is being taken and the width (duration) of the sample pulse. Flat- top sampling, however, introduces less aperture distortion than natural sampling and can operate with a slower analog-to-digital converter. Figure 5a shows the schematic diagram of a sample-and-hold circuit. The FET acts as a simple analog switch. When turned on, Q1 provides a low-impedance path to deposit the analog sample voltage across capacitor C1. The time that Q1 is on is called the aperture or acquisition time. Essentially, C1 is the hold circuit. When Q1 is off, C1 does not have a complete path to discharge through and, therefore, stores the sampled voltage. The storage time of the capacitor is called the A/D conversion time because it is during this time that the ADC converts the sample voltage to a PCM code. The acquisition time should be very short to en- sure that a minimum change occurs in the analog signal while it is being deposited across C1. If the input to the ADC is changing while it is performing the conversion, aperture distortion results. Thus, by having a short aperture time and keeping the input to the ADC relatively constant, the sample-and-hold circuit can reduce aperture distortion. Flat-top sampling introduces less aperture distortion than natural sampling and requires a slower analog- to- digital converter. Figure 5b shows the input analog signal, the sampling pulse, and the waveform developed across C1. It is important that the output impedance of voltage follower Z1 and the on resistance of Q1 be as small as possible. This ensures that the RC charging time constant of the capacitor is kept very short, allowing the capacitor to charge or discharge rap- idly during the short acquisition time. 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. Also, note the gradual discharge across the capacitor during the conversion time. This is called droop and is caused by the capacitor discharging through its own leakage resistance and the input impedance of voltage follower Z2. Therefore, it is important that the input impedance of Z2 and the leakage resistance of C1 be as high as possible. Essentially, voltage followers Z1 and Z2 isolate the sample-and-hold circuit (Q1 and C1) from the input and output circuitry. CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 88  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 89 SYNTHESIS In conclusion, Pulse Code Modulation is a foundational digital signal processing technique that enables the representation of analog signals in a digital form. With applications ranging from telecommunications to multimedia, PCM is a versatile technology that balances the need for accurate signal representation with practical considerations of data storage and transmission. Ongoing advancements in PCM and related technologies contribute to improved signal quality and more efficient use of digital resources. CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 89  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 90 References Admin AfterAcademy. (2020, January 19). What are Data Transmission Modes in a network? [Online Article]. https://afteracademy.com/blog/what-are-the-data-transmission-modes-in-a- network Brain, M., Wilson, T.V., & Johnson B. (2001, April 13). Wireless Networks [Online Image]. https://cdn.hswstatic.com/gif/wireless-network-1a.jpg Bus Topology [Online Image]. 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Data Communication Architecture, Protocols, and Standards [Online Article]. https://www.slideshare.net/desbinwag/datacom-module-2 CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 95  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 96 C P E - D D C 3 1 3 – D A TA A N D D IG ITA L C O M M U N IC A TIO N S Q U IZ N O.6 Name: ___________________________________________ Date: ____________________ Course, Yr., & Sec.: _________________ Score: ___________________ I. True or False. Write TRUE if the following stament is correct and FALSE if it is incorrect in the given space. Any forms of erasures, tamperings, and mispelled are considered wrong. Use black or blue pen in writing your answer. _____________________________1) The primary advantage of digital transmission over analog transmission is noise immunity. TRUE _____________________________2) The processing of analog signals using digital methods and includes band limiting with filters, amplitude equalization, and phase shifting is called multiplexing. FALSE _____________________________3) The digital transmission systems are more resistant to additive noise than analog systems because they use signal amplification rather than signal regeneration. FALSE _____________________________4) A bandwidth is one of the most important aspects of any communications system because it is costly and limited. TRUE _____________________________5) The conversion of sampling analog information signals into discrete pulses and transporting the pulses from a source to a destination over a physical transmission medium is called pulse modulation. TRUE _____________________________6) With pulse duration modulation, the position of a constant width pulse within a prescribed time slot is varied according to the amplitude of the sample of the analog signal. FALSE _____________________________7) In pulse width modulation, the active portion of the duty cycle known as width, of a constant amplitude pulse is varied proportional to the amplitude of the analog signal at the time the signal is sampled. TRUE CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 96  DON HONORIO VENTURA STATE UNIVERSITY Bacolor, Pampanga COLLEGE OF ENGINEERING AND ARCHITECTURE Department of Computer Engineering 97 _____________________________8) With pulse length modulation, the amplitude of a constant width, constant- position pulse is varied according to the amplitude of the sample of the analog signal. FALSE _____________________________9) The process when the analog signal is sampled and then converted to a serial n-bit binary code for transmission is called pulse code modulation. TRUE _____________________________10) Pulse Amplitude Modulation and Pulse Position Modulation are used in special-purpose communications systems for the military and Pulse Code Modulation is by far the most prevalent form of pulse modulation. FALSE CPE-DDC313: DATA AND DIGITAL COMMUNICATIONS PREPARED BY: JUVY C. GRUME, PCPE, MIT 97