 TV) systems. Video and audio carriers are "combined" before being fed to the radiating antenna for broadcast. The audio subcarrier is illustrated in Figure 16.6 in the composite mode. For point-to-point transmission on coaxial cable, radiolink, and earth station systems, the audio program channel is generally transmitted separately from its companion video providing the following advantages: It allows for individual channel level control. It provides greater control over crosstalk. It increases guardband between video and audio. It saves separation at broadcast transmitter. TELEVISION TRANSMISSION It leaves TV studio at separate channel. It permits individual program channel preemphasis. 16.7 THE TRANSMISSION OF VIDEO OVER LOS MICROWAVE Video/TV transmission over line-of-sight microwave has two basic applications: For studio-to-transmitter link. This connects a TV studio to its broadcast transmitter. To extend CATV systems to increase local programming content. As covered earlier in the chapter, video transmission requires special consideration. The following paragraphs summarize the special considerations a planner must take into account for video transmission over radiolinks. Raw video baseband modulates the radiolink transmitter. The aural channel is transmit- ted on a subcarrier well above the video portion. The overall subcarriers are themselves frequency-modulated. Recommended subcarrier frequencies may be found in CCIR Rec. 402-2 (Ref. 9) and Rep. 289-4 (Ref. 10). 16.7.1 Bandwidth of the Baseband and Baseband Response One of the most important specifications in any radiolink system transmitting video is frequency response. A system with cascaded hops should have essentially a flat bandpass in each hop. For example, if a single hop is 3 dB down at 6 MHz in the resulting baseband, a system of five such hops would be 15 dB down. A good single hop should be 0.25 dB or less out to 8 MHz. The most critical area in the baseband for video frequency response is in the low-frequency area of 15 kHz and below. Cascad