ed radiolink systems used in transmitting video must consider response down to 10 Hz. Modern radiolink equipment used to transport video operates in the 2-GHz band and above. The 525-line video requires a baseband in excess of 4.2 MHz plus available base- band above the video for the aural channel. Desirable characteristics for 525-line video then would be a baseband at least 6 MHz wide. For 625-line TV, 8 MHz would be required, assuming that the aural channel would follow the channelization recommended by CCIR Rec. 402-2 (Ref. 9). 16.7.2 Preemphasis Preemphasis is commonly used on wideband radio systems that employ FM modulation. After demodulation in an FM receiver, thermal noise has a ramp-like characteristic with increasing noise per unit bandwidth toward band edges and decreasing noise toward band center. Preemphasis, along with its companion deemphasis, makes thermal noise more uniform across the demodulated baseband. Preemphasis-deemphasis characteristics for television are described in CCIR Rec. 405-1 (Ref. 11). 16.7.3 Differential Gain Differential gain is the difference in gain of the radio relay system as measured by a low-amplitude, high-frequency (chrominance) signal at any two levels of a low-frequency (luminance) signal on which it is superimposed. It is expressed in percentage of maximum 16.7 THE TRANSMISSION OF VIDEO OVER LOS MICROWAVE gain. Differential gain shall not exceed the amounts indicated below at any value of APL (average picture level) between 10% and 90%: Short haul Medium haul Satellite Long haul End-to-end Based on ANSI/EIA/TIA-250C (Ref. 6). Also see CCIR Rec. 567-3 (Ref. 12). 16.7.4 Differential Phase Differential phase is the difference in phase shift through the radio relay system exhibited by a low-amplitude, high-frequency (chrominance) signal at any two levels of a low- frequency (luminance) signal on which it is superimposed. Differential phase is expressed as the maximum phase change between any two levels. Differential phase, expressed in degrees of the high-frequency sine