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Pulse amplitude modulation
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In pulse amplitude modulation (PAM), the strength of each individual pulse varies according to the modulating waveform. In this respect, PAM is very much like ordinary amplitude modulation. An amplitude-versus-time graph of a hypothetical PAM signal is shown in Fig. 26-12A.
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486 Data transmission Normally, the pulse amplitude increases as the instantaneous modulating-signal level increases. But this can be reversed, so that higher audio levels cause the pulse amplitude to go down. Then the signal pulses are at their strongest when there is no modulation. Either the positive or negative PAM methods provide good results. The transmitter works a little harder if the negative modulation method is used.
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Pulse duration modulation
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In PAM, the pulses all last for the same length of time. The effective transmitted power is varied by changing the actual peak amplitude of the pulses. Another way to change the transmitter output is to vary the duration, or width, of the pulses. This scheme is called pulse duration modulation (PDM) or pulse width modulation (PWM), shown in Fig. 26-12B. Normally, the pulse duration increases as the instantaneous modulating-signal level increases. But, as with PAM, this can be reversed. The transmitter must work harder to accomplish negative PDM. Regardless of whether positive or negative PDM is employed, the peak pulse amplitude remains constant.
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Pulse interval modulation
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Even if all the pulses have the same amplitude and the same duration, modulation can still be accomplished by varying how often they occur. In PAM and PDM, the pulses are always sent at the same time interval, such as 0.0001 second. But in pulse interval modulation (PIM), also called pulse frequency modulation (PFM), pulses might occur more or less frequently than the zero-modulation interval. This is shown in Fig. 26-12C. Every pulse has the same amplitude and the same duration; it is their frequency that changes. When there is no modulation, the pulses are evenly spaced with respect to time. An increase in the instantaneous data amplitude might cause pulses to be sent more often, as is the case in Fig. 26-12C. Or, an increase in instantaneous data level might slow down the rate at which the pulses are sent. Either scheme will work equally well.
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Pulse code modulation
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In recent years, the transmission of data has been done more and more by digital means. In digital communications, the modulating data attains only certain defined states, rather than continuously varying in an analog way. Digital transmission offers better efficiency than analog transmission. With digital modes, the signal-to-noise ratio is better, the bandwidth is narrower, and there are fewer errors. Teleprinter data is always sent digitally, as is Morse code. But voices and video can be sent digitally, too. The only drawback that early digital experimenters faced was somewhat degraded fidelity. But today, that has been overcome to the extent that digitized music recordings and transmissions actually sound better than the best analog reproductions. In pulse-code modulation (PCM), any of the above aspects amplitude, duration, or frequency of a pulse train can be varied. But rather than having infinitely many possible states, there are finitely many. The greater the number of states, the better the fidelity. But the transmitting and receiving equipment must be more and more sophisticated as the
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Image transmission 487 number of digital states increases. An example of eight-level PAM/PCM is shown in Fig. 26-12D.
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Analog-to-digital conversion
The graph of Fig. 26-12D illustrates a method of analog-to-digital (A/D) conversion. A voice signal, or any continuously variable signal, can be digitized, or converted into a string of pulses, whose amplitudes can achieve only a finite number of states.
Resolution
The number of states is always a power of 2, so that it can be represented as a binary-number code. Fidelity gets better as the exponent increases. The number of states is called the sampling resolution, or simply the resolution. You might think that the resolution would have to be very large for good reproduction to be possible. But in fact, a resolution of 23 = 8 (as shown in Fig. 26-12D) is good enough for voice transmission, and is the standard resolution for commercial digital voice circuits. A resolution of 24 = 16 is adequate for compact disks used in advanced hi-fi systems!
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