19 - Digital modulation

Abstract

The term digital modulation is used in two senses: modulation of an analog carrier by a digital baseband signal and modulation of a digital carrier by an analog baseband signal. A digital signal may modulate an analog carrier by using amplitude modulation, frequency modulation, or phase modulation. An important example of the use of a digital carrier for transmitting an analog baseband signal is the use of pulse code modulation (PCM) for sending voice signals over digital circuits. In a digital transmission system, a modulator lies between the baseband input channel and the transmission path and a demodulator lies between that and the baseband output channel. The principal advantage of PCM and other forms of digital transmission is that it is possible to obtain satisfactory transmission in the presence of very severe crosstalk and noise. In the case of binary transmission, it is only necessary to detect the presence or absence of each pulse. Provided that the interference level is not as high as to cause frequent errors in making this decision, the output signal will be almost noise free. In a digital transmission system, the receiver attempts, at the required instants of time, to determine whether a pulse is present that represents valid information or just noise. Optimum strategies for doing this have been developed. After the digitally encoded signal has been transmitted, it is often necessary to reconstruct an analog signal at the receiver. Conversion from digital to analog is generally easier to achieve than conversion from analog to digital. There are again many techniques that may be used for this conversion, each possessing its own blend of merits and demerits.