Modulated Carrier Waves

Author: J.B. Hoag

It is necessary to change or modulate the carrier wave in some way if we are to transmit a message. Three methods are used today for this purpose. First, the carrier wave may be stopped and started, so as to break it up into telegraphic dots and dashes, as in Fig. 16 B.

Fig. 16 B. The letter "A" of the International Morse Code

Second, the strength of the continuous wave or current may be increased or decreased by sending it through a microphone, or by more elaborate and practical methods which will be described presently. In this so-called amplitude-modulation method, the peaks of the carrier wave rise and fall, as in Fig. 16 C, at the comparatively low frequency of the sound or audio waves, and to an extent proportionate to the intensity of the sound wave.

Fig. 16 C. An amplitude-modulated carrier wave

Third, the amplitude of the carrier wave may be kept constant, but its frequency varied, at a rate dependent upon the frequency of the sounds in the microphone, and by an amount proportional to the loudness of the sounds. This is called frequency modulation and is represented in Fig. 16 D.

Fig. 16 D. A frequency-modulated wave

In this chapter we shall concern ourselves only with a brief explanation of the principles of amplitude modulation.