In debugging digital television head end equipment, mainly to set the output level of the modulator and the modulator adjusting frequency.
(1) The output of the modulator output signal level adjustment in the front end part of the cable television system, the analog and digital signals are mixed outputs. Due to their different modulation scheme, and therefore, their output levels are also different. Carrier modulated analog television is VSB; i.e., residual sideband amplitude modulation system; and digital cable 64 QAMM modulated, double sideband having similar characteristics, their peak power and average power are different, based on calculations and practical experience, typically digital modulator output level lower than the analog modulator output level of 10dB. In addition, the output between analog and analog channels, digital channels and digital level of size and flatness directly affect the system metrics network. On the front, the addition to the output level control circuit design, the high and low have good control of the size difference between the smaller channels, the system flatness level difference tone better. General front-end analog and analog adjacent channel level difference between digital and digital adjacent channel should not exceed ± 0.5dBμV.
(2) the frequency of the modulator settings mentioned above due to the different modulation analog channels and digital channels, and therefore their output frequency settings are different. Analog TV modulator frequency is set by the vision carrier frequency, and the frequency of the digital TV modulator set according to the center frequency of the channel, it is very important.