2.3 FUNDAMENTALS OF DIGITAL SIGNALS

In many communication systems, the information source produces analog signals, since the physical world is inherently continuous. In recent decades, however, this situation has changed: many sources now produce digital signals directly because the originator of the message is often an electronic device such as a computer, sensor, or controller. Yet, even when the original information is analog, it is frequently converted into digital form, since the transmission of digital signals offers several important advantages.

All digital signals share a common structure: they consist of only two logical states, conventionally denoted ‘1’ and ‘0’. This uniformity allows the same basic circuitry to be used throughout a system, simplifying implementation and enabling high levels of integration. Because of their binary nature, digital signals are easier to store, multiplex, switch, and route, and they are generally less susceptible to noise and distortion than analog signals. Importantly, digital signals can be regenerated repeatedly without cumulative degradation, allowing reliable transmission over very long distances. Techniques such as encryption, compression, and error control are also simpler and more robust in digital systems than in their analog counterparts.

Moreover, digital transmission supports explicit error detection and correction, enabling reliable communication even in the presence of significant noise. Analog signals, by contrast, cannot be perfectly reconstructed once distorted and cannot easily be protected against errors. Although digital signaling typically requires more complex waveforms and greater bandwidth than an equivalent analog channel, these disadvantages are overwhelmingly outweighed by the benefits. As a result, almost all modern communication systems—whether wired, wireless, or optical—employ digital transmission.

There are two primary ways to transmit digital information from source to sink:

Baseband transmission, in which the digital signal is conveyed directly as varying voltage or current levels;
Modulated (bandpass) transmission, in which a digital baseband signal is used to modulate an analog carrier for longer-distance communication.

Digital modulation techniques are addressed in Chapter 6. The remainder of this chapter focuses on baseband digital signals and their transmission.