The VG-737 video detector is used to indicate the
presence of a video signal by detecting the vertical
sync component of a composite video signal. The
detection circuitry used ensures that noise alone will
not trigger the detector.
The video input is a high impedance bridging
differential input circuit with the input common
raised above ground, this allows the VG-737 input to
bridge a signal cable without affecting common mode
rejection circuitry that may be incorporated in the
equipment terminating the cable.
The video signal level threshold at which the VG-737
switches is set by an internal control and may be
adjusted for a signal level in the range 0.4 V to
0.7 Vp-p at the input.
The detector circuitry consists of a sync separator and
vertical integrator, which triggers a dual mono stable
circuit that allows the response time of the alarm circuit to be set. |
The response time is set by means of
DIP switches in the RC timing circuit and is
adjustable from 1 to 32 seconds for the VIDEO FAIL
time out and 1 to 8 seconds for the VIDEO ON
response time, the adjustment being made in 1 second
steps.
The alarm circuitry consists of a latching circuit
triggered by the detector circuit, which drives a relay
whose contacts are available at the rear panel of the
VG-737.
The alarm circuit can be reset automatically upon
return of the video signal or by a contact closure from
the front panel or a remote circuit.
Visual indication of the state of the VG-737 detector
and alarm circuits is provided by LED indicators on
the front panel. |
