TABLE OF CONTENTS Analog signals are electrical replicas of the original signals such as audio and video. Baseband signals are those signals which occupy the lowest, or base, band of frequencies, 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 analog telephony signals may be combined into one baseband signal by the process known as frequency-division multiplexing (FDM). Other common types of baseband signals are the multiplexed video and audio signals which originate in the TV studio. In forming the multiplexed baseband signals, the information signals are modulated onto subcarriers. This modulation step must be distinguished from the modulation process, which places the multiplexed signal onto the microwave carrier for transmission to the satellite.
In this chapter, the characteristics of the more common types of analog baseband signals are described, along with representative methods of analog modulation.
Natural speech, including that of female and male voices, covers a frequency range of about 80 to 8000 Hz. The somewhat unnatural quality associated with telephone speech results from the fact that a considerably smaller band of frequencies is used for normal telephone transmission. The range of 300 to 3400 Hz is accepted internationally as the standard for telephone quality speech, and this is termed the speech baseband. In practice, some variations occur in the basebands used by different telephone companies. The telephone channel is often referred to as...
