ude of some characteristic(s) of the signal such as amplitude, frequency, or phase of a voltage, the amplitude or duration of a pulse, the angular position of a shaft, or the pressure of a fluid. Typical analog transmission are the signals we hear on AM and FM radio and what we see (and hear) on television. In fact, television is rather unique. The video itself uses amplitude modulation; the sound subcarrier uses frequency modulation, and the color subcarrier employs phase modulation. All are in analog formats. 2.4.2.2 Digital Transmission. The information content of a digital signal is concerned with discrete states of the signal, such as the presence or absence of a voltage (see Section 2.3.2); a contact is the open or closed position, or a hole or no hole in certain positions on a card or paper tape. The signal is given meaning by assigning numerical values or other information to the various combinations of the discrete states of the signal. We will be dealing extensively with digital transmission as the argument in this text proceeds. 2.4.3 Binary Digital Signals In Section 2.4.1, we defined a digital waveform as one that displayed discreteness. Sup- pose we consider the numbers 0 through 9. In one case, only integer values are permitted in this range, no in-between values such as 3.761 or 8.07. This is digital where we can only SIGNALS CONVEY INTELLIGENCE Amplitude modulation Frequency modulation Phase modulation Figure 2.9 Illustration of amplitude, frequency, and phase modulation, where the modulating signal is the binary digital sequence 00110100010, an electrical baseband signal. assign integer values between 0 and 9. These are discrete values. On the other hand, if we can assign any number value between 0 and 9, there could be an infinite number of values such as 7.01648754372100. This, then, is analog. We have continuity, no discreteness. Consider now how neat it would be if we had only two values in our digital system. Arbitrarily, we'll call them a 1 and a 0. This is indeed a binary system, just two