on predefined channels. The end result of this procedure is to enable cable TV subscribers to-either manually or via a remote controller-tune their CATV boxes to specific channels to watch desired programs. Of course, premium programming will be encrypted prior to entering the mixer shown in Figure 7.5. THE MIXER The mixer takes input signals from different sources and combines them onto the headend. In SO doing, the mixer places each received signal from a satellite or antenna onto a predefined channel for broadcast onto the CATV distribution system. In actual operation, the mixer receives input at different frequencies and moves such input to other predefined frequencies. Because the predefined frequencies represent channels, we can simply state that the mixer moves input signals to their appropriate channel placements. AMPLIFIER Another device physically located within the headend is an amplifier. The amplifier boosts the level of the received signal SO that subscribers can receive a high-quality signal. As we continue our exploration of the original CATV infrastructure, we will note that the headend amplifier is one of many located within the cable network. In addition to an amplifier located at the headend, there are other amplifiers located throughout the CATV operator's serving area. Each amplifier boosts signal power, and its power-in/power-out ratio is referred to as its gain. Amplifier gain is measured in decibel milliwatts (dBmW). Recognizing the physical phenomenon in which high frequen- cies attenuate more rapidly than do low frequencies, CATV amplifiers are designed to provide a higher level of gain at high frequencies. If you are familiar with the manner in which high-speed modems automati- cally generate attenuation equalization, you will note that this nonuni- form level of amplifier gain increases in tandem with frequency. The Cable TV Distribution System Figure 7.6 illustrates the original CATV distribution network. Note that the amplifiers are unidirectional as the original CATV network design