1-3-1 Modulation Method

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1-3 Basic Techniques of a Network

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Modulation refers to the conversion of digital signals to analog signals, or analog signals to digital signals. (Returning converted signals to its original form is called demodulation.) The types of methods used to modulate digital signals to analog signals with a modem in order to transmit data on an analog line are as shown below.

Original Modulation Method source table or explanation box, preserving its complete original arrangement, labels and values. Source text: • AM (Amplitude Modulation) 1 and 0 of the digital signal are made to correspond to ON/OFF (presence or absence of waves) of the analog signal. Although this method is easy to implement, its disadvantage is that it is sensitive to noise. — p. 293 · Enlarge

p. 294

Original Modulation Method source table or explanation box, preserving its complete original arrangement, labels and values. Source text: • FM (Frequency Modulation) 0 and 1 of the digital signal are made to correspond to the low frequency and high frequency of the analog signal. This method is the next easiest to implement after the amplitude modulation method, and is also highly resistant to noise. • PM (Phase Modulation) The digital signal is represented on the basis of shifting of the waveform. It includes the binary phase shift keying method, in which 0 of the digital signal is represented by the standard waveform and 1 is represented by a 180-degree shift, and the quadrature phase shift keying method in which ‘00’ is represented by the standard waveform, ’01’ is represented by a 90-degree shift, ‘10’ by a 180-degree shift, and ’11’ by a 270-degree shift. The advantage of this method is that several bits can be transmitted through a single waveform by finely and minutely shifting the waveform. As a result, the modulation speed (unit: baud) that represents the number of times that modulation is performed in one second is not necessarily the same as the signal speed (or line speed) that represents the number of bits that can be transmitted in one second. The differential binary phase shift keying method, in which the waveform is reversed when 1 is transmitted, is also included. — p. 294 · Enlarge

On the other hand, the modulation method by which voice signals (i.e., analog signals) are encoded (i.e., converted to digital signals) includes PCM (Pulse Code Modulation). In PCM, analog signals are modulated to digital signals through the procedures of sampling, quantization, and encoding.

Original Modulation Method source table or explanation box, preserving its complete original arrangement, labels and values. Source text: [Encoding procedure according to PCM (excerpt from pp.170-171)] 1) Sampling The analog signals to be encoded are sampled at regular intervals. This sampling interval is shown as the sampling frequency. 2) Quantization The sampled sampling values are rounded to the nearest integer numbers. 3) Encoding A quantized integer number is encoded by representing it as a binary number. (The number of bits used as a code is called the quantization bit rate.) — p. 294 · Enlarge

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