In analog electronics, voltage is a continuous variable. This is useful because most physical quantities we encounter are continuous: sound levels, light intensity, temperature, pressure, etc. Digital electronics, in contrast, is characterized by only two distinguishable voltages. These two states are called by various names: on/off, true/false, high/low, and 1/0. In practice, these two states are defined by the circuit voltage being above or below a certain value. For example, in TTL logic circuits, a high state corresponds to a voltage above 2.0 V, while a low state is defined as a voltage below 0.8 V.
The virtue of this system is illustrated in Fig. 8.1. We plot the voltage level versus time for some electronic signal. If this was part of an analog circuit, we would say that the voltage was averaging about 3 V, but that it had, roughly, a 20% noise level, rather large for most applications and thus unacceptable. For a TTL digital circuit, however, this signal is always above 2.0 V and is thus always in the high state. There is no uncertainty about the digital state of this voltage, so the digital signal has zero noise. This is the primary advantage of digital electronics: it is relatively immune to the noise that is ubiquitous in electronic circuits. Of course, if the fluctuations in Fig. 8.1 became so large that the voltage dipped below 2.0 V, then even a digital circuit would have problems.