By Alex B. Best, Scientific-Atlanta, Inc.
The purpose of the modulator in CATV systems is to accept a source of video and audio or 4.5 MHz FM signal and convert these to a standard television signal. This source of information may be a camera, a demodulator, or microwave relay output. Because the CATV modulator performs the same function as a television transmitter, its specifications must conform very closely to those of a transmitter.
The block diagram of a CATV modulator is shown in Figure 1. The video enters the unit through the "video phase equalizer" module. The function of this module is to predistort the phase versus frequency characteristics of the video to conform to the standard FCC predistortion requirements required for color transmission. After leaving this module the video enters the video amplifier module through a second phase equalizer section. This second equalizer, which is an integral part of the video amplifier, predistorts the delay characteristics of the video to compensate for delay errors introduced by the vestigial sideband filter and other filters in the modulator itself.