TABLE OF CONTENTS As already mentioned in connection with analog signals, baseband signals are those signals which occupy the lowest, or base, frequency band in the frequency spectrum used by the telecommunications network. A baseband signal may consist of one or more information signals.
For example, a number of telephony signals in digital form may be combined into one baseband signal by the process known as time-division multiplexing.
Analog signals may be converted into digital signals for transmission. Digital signals also originate in the form of computer and other data. In general, a digital signal is a coded version of the original data or analog signal. In this chapter, the characteristics of the more common types of digital baseband signals are described, along with representative methods of digital modulation.
Digital signals are coded representations of information. Keyboard characters, for example, are usually encoded in binary digital code. A binary code has two symbols, usually denoted as 0 and 1, and these are combined to form binary words to represent the characters. For example, a teleprinter code may use the combination 11000 to represent the letter A.
Analog signals such as speech and video may be converted to a digital form through an analog-to-digital (A/D) converter. A particular form of A/D conversion is employed, known as pulse-code modulation, which will be described in detail later. Some of these sources are illustrated diagrammatically in Fig. 10.1.
In
