vp 5000 and provides synchronizing signals removed from an external reference video signal to which the vp 5000 is then servo locked to enable vertical switching between the vp 5000 output and the reference signal application_number US 53686583 A description referring now to fig1 the video input 12 to the interface circuit is first taken from the output of the video cassette player 14 through a conventional terminal jack 16 next through a 4 7k ohm resistor 18 and then through a 33 micro farad capacitor 20 the output of the capacitor 20 feeds three parallel lines one to a two meg ohm resistor 22 tied to a positive 8 volt dc potential a second to a one meg ohm resistor 24 tied to ground potential and the third to the control gate base 26 of an fet transistor 28 an example of this transistor would be a 2n5949 or equivalent the source connection 30 of the fet transistor 28 is connected to a positive 8 volt dc potential the drain connection 32 to the fet transistor 28 is connected through two paths one path through a 4 7k ohm resistor 34 to ground and another path through a 0 01 micro farad capacitor 36 to the negative terminal 38 of a signal level comparator 40 this signal level comparator consists of an lm.