Pulse Modulation In Communication System

Pulse Modulation 1

Pulse Modulation In Communication System Abstract: This paper writing will present about pulse modulation, which is widely used in communication system. Pulse modulation is the heart of technology in communications in today’s digital world. It’s a process in which analog signals are converted to digital form. The analog signal is represented by a series of pulses and non-pulses (1 or 0 respectively). The stream of pulses and non-pulse streams of 1’s and 0’s are not easily affected by interference and noise. Even in the presence of noise, the presence or absence of a pulse can be easily determined. The methodologies that we use to complete this paper are finding the content from reference books, Internet sources and lecturer. As the result, the technique for pulse modulation included three processes, which are sampling, quantization and coding. 1.0 Introduction Pulse modulation includes many different methods of converting information into pulse form for transferring pulses from a source to a destination. The four dominant methods are: 1. Pulse Amplitude Modulation (PAM) 2. Pulse Width Modulation (PWM) 3. Pulse Position Modulation (PAM) 4. Pulse Code Modulation (PCM)

GMB for aquatic studies is the most widely used instrument in this field at the present time and works well. Using the PAM instrument one can measure the maximum (optimal), dark-adapted quantum yield, a measure of whether PS II is working unconstrained –or is working under some kind of stress.

2.0 Content 2.1 Pulse Amplitude Modulation Pulse-amplitude modulation, also called PAM, is a form of signal modulation where the signal is encoded in the amplitude of a series of signal pulses. Example: A two-bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −2 volts, −1 volt, 1 volt, and 2 volts. It is widely used in base band transmission of digital data Figure 2: A Diving PAM Fluorometer

Figure 1: PAM waveform 2.1.1 Application: Fluorometer Diving-PAM The fluorometer uses a range of flashing (pulsed) lights to measure the photosynthesis. These pulses of light are modulated, and the height (or amplitude) of the pulse of light governs its intensity. The Diving-PAM is developed by R Gademann and U. Schreiber and made by Walz

Furthermore, the effective quantum yield can be measured to an absolute rate of photosynthetic electron transport. In effect this yields the rate of photosynthesis, arguably the most sought after parameter in all algae and plant studies. Moreover, it is a measurement that can be achieved in seconds. Compare this with the time of minutes to hours needed to measure photosynthesis by oxygen evolution or carbon dioxide fixation methods. PAM fluorometer has proven exceptionally good at measuring nonphotochemical quenching, that is the measure of the amount of light that is deliberately diverted into heat by a photosynthetic organism, most notably by the xanthophylls cycle. PAM fluorometer is suitable in determining photosynthetic rates of sea-grasses

Pulse Modulation 2 both in the laboratory and in situ. The DivingPAM further allows for such measurements to be done underwater to depths of 50m. Because PAM fluorometry measures only photondriven electron transport (which gives rise to O2 evolution, but also to photo respiration and other possible consuming reactions involving electron flow through PSII), it cannot be applied by itself if energy or gas exchange budgets are to be determined since these depend also on diurnal rates of dark respiration. 2.2 Pulse Width Modulation This method is sometimes called pulse duration modulation (PPM) or pulse length modulation (PLM). Pulse width modulation (PWM) is an active portion of the duty cycle, which is proportional to the amplitude of the analog signal. PWM uses pulse that all have same amplitude. The width of each pulse in a train is made proportional to the instantaneous value of the modulating signal at the instant of the pulse. The duration of each pulse depends on the amplitude of the signals at the time it is sampled. Pulses of various lengths, which are the information signal itself, will be sent at regular intervals at the carrier frequency of the modulation.

Figure 3: Pulse Width Modulation (PWM) 2.2.1 Application: DC motor speed control PWM is used to vary the total amount of powered delivered to a load without resistive waste. An RC filter can be used to smooth the pulse train into a steady analog voltage. This method is commonly used in DC motor speed control. A pulse width modulator (PWM) is a device that may be used as an efficient light dimmer or DC motor speed controller. The circuit described here is for a general purpose device that can control DC devices which draw up to a few amps of current. The circuit may be used in either 12 or 24 Volt systems with only a few minor wiring changes. This device has been used to control the brightness of an automotive tail lamp and as a motor speed control for small

DC fans of the type used in computer power supplies. A PWM circuit works by making a square wave with a variable on-to-off ratio, the average on time may be varied from 0 to 100 percent. In this manner, a variable amount of power is transferred to the load. The main advantage of a PWM circuit over a resistive power controller is the efficiency, at a 50% level, the PWM will use about 50% of full power, almost all of which is transferred to the load, a resistive controller at 50% load power would consume about 71% of full power, 50% of the power goes to the load and the other 21% is wasted heating the series resistor. Load efficiency is almost always a critical factor in solar powered and other alternative energy systems. One additional advantage of pulse width modulation is that the pulses reach the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily. Finally, in a PWM circuit, common small potentiometers may be used to control a wide variety of loads whereas large and expensive high power variable resistors are needed for resistive controllers.

Figure 4: PWM Motor Speed Controller Circuit The main disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference (RFI). RFI may be minimized by locating the controller near the load, using short leads, and in some cases, using additional filtering on the power supply leads. This circuit has some RFI bypassing and produced minimal interference with an AM radio that was located under a foot away. If additional filtering is needed, a car radio line choke may be placed in series with the DC power input, be sure not to exceed the current rating of the choke. The majority of the RFI will come from the high current path involving the power source, the load, and the switching FET, Q1.

Pulse Modulation 3 Other than that, PWM also has its communications uses; for instance it is often used in the high powered audio amplifiers used to modulate AM transmitter. It also used for telemetry systems. Though still an analog mode, it is more robust than PAM because it is insensitive to amplitude changes due to noise and distortion. Like PAM, PWM also can be demodulated using low pass filter. 2.3 Pulse Position Modulation Pulse position modulation (PPM) is another type of pulse modulation and it is a time domain method. In general, PPM is defined as the position of individual pulse is varied accordance to the instantaneous amplitude of the modulating signal within a prescribed time while its amplitude and width is kept constant. A synchronizing pulse is sent out from transmitter to operate the time circuits in the receiver and receiver will be properly synchronized to align the local clock with the beginning of each symbol. Thus, receiver would able to measure the difference in the arrival time of pulses and received signals more accurately as well as minimize the error. PPM has the advantages of requiring constant transmitter power, more immune to noise interference since amplitude of modulating signal is held constant and electronic circuits used to decode signals are very simple which lead to low weight modulation devices. Disadvantages of PPM include complex design of transmitter and receiver due to synchronization consideration. Figure 1 shows the PPM waveforms. 2.3.1 Application: Narrowband Radio Frequency (RF) PPM may be used to transmit analog information such as continuous speech or data. PPM is primarily useful for optical communication system due to its advantages of little or no multipart interference. PPM is usually applied in narrowband radio frequency (RF) channels with low power and long wavelengths. This modulation technique is normally found widely application in the radio control of model aircraft, boats and cars. A complete radio control system would consist of transmitter, receiver, servo, and battery. Servo is used to convert the pulse signal to the desired angular motor position. PPM is applied to these systems with position of each pulse representing the angular position of an analogue control signal

from the transmitter. The number of pulses per frame will determine the number of controllable channels available as shown in figure 2. The gap between each pulse is different depending on the amplitude of modulating signals. There is a synchronization period between the sample frames and it is determined by encoding each frame position relative to the previous frame. Receiver will recognize the arrival time of each frame and then avoiding of signal collisions. The position of channel 1 is varying according to the amplitude of modulating signal. It is read and a pulse will be generated to be sent out towards receiver. At the same time, the process of reading position of channel 2 begins and this repeats very fast for all channels. Then, receiver will recognize number of pulse or channel with different gap times and receiver will recognize it and it will determine which servo they heading to. The first pulse will be sent to servo 1, and then second pulse will be sent to servo 2 and so on. Electronic devices used to decode the signal are very simple and thus lead to a small and light in weight decoder. This is extremely suitable to be applied in modern aircraft since weight is an important issue for the aircraft.

Figure 5: PPM Waveforms

Figure 6: Six channel of PPM signal 2.4 Pulse Code Modulation (PCM) The practical implementation of PCM makes use of other processes, which are sampling, quantization and coding process. Sampling of a waveform means determining

Pulse Modulation 4 instantaneous amplitudes of a signal at fixed intervals. While for quantization is a process of allocating levels to the infinite range of amplitudes of sample values of the analog signal. Encoding process is the process of each step level is assigned a number. The numbers start with zero at the lowest level. These assigned numbers are then expressed in binary form (in terms of 0’s and 1’s). This will be the last part of the conversion and the PCM signal will be transmitted or sent. The principal advantage of Pulse Code Modulation (PCM) is the noise immunity. One disadvantage of PCM is that the signal accuracy is reduced because of the quantizing of the samples. Regarding the increased bandwidth requirements by PCM the problems is no longer a serious one because of the advent of large bandwidth fiber optic systems. PCM also finds use in space communications. Way back in 1965 mariners to transmit back pictures of Mars used PCM. The figure 7 below shows the process that happen in Pulse Code Modulation (PCM). Vm(t)

SAMPLING QUANTIZATION CODING VPCM(t)

2.4.1 Application: Optimal Pulse Code Modulation (Opcm) and7 Its Application as an Figure Audio Quality Parameter. The introduction of digital audio (Compact Disc) has boosted the audio quality compared to the old analog media. Since then, the term "digital" has become a synonym of quality. This quality is bought at the price of an enormous amount of data - over 700'000 bits per second (700 kbps) are necessary for encoding a mono signal. If we reduce this data rate, the sound quality suffers - because we simply throw away some information. But this reduction is needed for low bandwidth channels as a telephone line, the internet or terrestic broadcasts. Standard digital sound is stored as PCM data (pulse code modulation). The audio signal is

measured at a fixed rate ("sampling rate") with a given precision ("resolution"). On a CD, each stereo channel is measured 44'100 times a second with a precision of 16 bits, resulting in 705'600 bits per second. Decreasing this rate can be achieved by lowering the sampling rate or the resolution. But the best idea is to lower both values. Of course, there is a best compromise between reducing the sampling rate and the resolution. These best combinations are called "optimal PCM" (oPCM). But there are much more intelligent ways in decreasing the data rate with fewer audible quality losses. Special audio formats "codecs" were introduced such as: ADPCM (adaptive differential pulse code modulation), MPEG audio ("layer I, II, III") or RealAudio. Evaluating audio compression is complicated because there is no objective quality parameter. This is why the comparison with uncompressed raw data (PCM files) is introduced. The maximum quality that a PCM file can deliver at a given bit rate is determined first. Since there are two parameters that define a PCM file (sampling rate and resolution) the best combination of these two values has to be worked out. This is how optimal PCM (oPCM) streams are defined: They are the best compromise between sampling rate and resolution at a fixed bit rate. oPCM files depend on a single parameter, the perceptional quality parameter, Q. Sound data compression consists of two steps, which are compression with audible quality loss and compression without audible quality loss ("net compression"). oPCM files can only compress the sound data with quality loss (presumed that the raw data is also oPCM*). They do not offer any net compression. By this means it is possible to evaluate the important net compression of a given codec candidate. oPCM files serve therefore as reference files. For example, Real Audio 3.0 sounds at 40 kbps ("ISDN mono") roughly like oPCM at 96 kbps (Q=6.6). This means a net compression ratio of 1:2.4 (Q gain = +1.3). However, most of the total compression is loss. There is one technical obstacle when generating oPCM files: Standard sound devices do not allow "odd" sampling rates and resolutions. Therefore, we have simulated the oPCM quality by up sampling to a higher rate and resolution.

Pulse Modulation 5 2.4.2 The oPCM Evaluation Method Compared to the Old ISO Methods Sound quality evaluation is not a new topic. Until now, audio quality tests were performed as follows: The "expert listener" was presented with a CD quality file and a test file. Then, the listener had to rate the test item with school notes ranging from 5.0 ("no difference to CD") to 1.0 ("extremely annoying distortions"). The shortcomings of this method (used by ISO) are obvious: The rating is rather subjective and the only reference item is the perfect CD quality. That is why the listener only knows what the rating "5.0" means. The lower ratings are strongly a matter of taste because they are not strictly defined. As a consequence, the results vary from listener to listener. The oPCM method overcomes these problems: The listener knows exactly what a rating of e.g. Q=6.7 means: The test item sounds better than the Q=6.6 oPCM reference file, but worse than the Q=6.8 oPCM file.

Acknowledgement Assalamualaikum Warahmatullahi Wabarakatuh, With Grace and Blessing from Allah, we have succeeded in completed our Paper Writing Assignment for SEE 3533 Session 2007/2008 (02) with successful although we have gone through many obstacles and challenges. As we all know, this Paper Writing Assignment is one of the conditions for Principle Communication subject that we had taken for this semester. We would like to thank all the people who have participate in finishing and completing this project. Especially to PM Abu Samah B. Mohd Supaat our Principle Communication lecturer who have coaching us with advice and guidance in completing this Paper Writing Assignment. Finally, we hope this Paper Writing Assignment that have we completed will perform perfectly so that we can add our marks in the assessment of this Principle Communication subject. Insyaalah. Thank you. References

1. Roy Blake, “Electronic Communication 2. 3. 4. Figure 8: Audio codec quality comparison chart

3.0 Conclusion Although humans are well equipped for analog communications, analog transmission is not particularly efficient. When analog signals become weak because of transmission loss, it is hard to separate the complex analog structure from the structure of random transmission noise. If you amplify analog signals, it also amplifies noise, and eventually analog connections become too noisy to use. Digital signals, having only "one-bit" and "zero-bit" states, are more easily separated from noise. They can be amplified without corruption. Digital coding is more immune to noise corruption on long-distance connections.

5. 6. 7.

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