A CRT display device capable of displaying a signal from the existing image signal source, in which a vertical frequency is approximately fixed and a horizontal frequency is widely distributed beyond a ratio of 3:1, on one image screen. The CRT display device includes a scan converter unit and a display unit. The scan converter unit includes an output horizontal frequency unifying circuit, a horizontal blanking period ratio converting circuit, a vertical frequency converting unit and a vertical blanking period ratio converting circuit. The display unit includes a vertical deflection circuit and a circuit for correcting a vertical S-shaped distortion. In a phase synchronous circuit, a lock-out detector is connected to an output of a three state output digital phase detector, and on the basis of an output thereof, a switch is subjected to the 'ON'/'OFF' control. For a period of time when an output of the lock-out detector indicates a lock-out state, a loop gain of the PLL is increased and the pull-in time is shortened. For the remaining periods of time, i.e., in the steady state, the loop gain is decreased and the noise resistance characteristics are improved.
Scalable CRT display device and phase synchronous circuit for use in display device