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A technique is described whereby a stable-noiseless vertical sync pulse is produced so as to synchronize computer data overlay on top of the special effects video of a video cassette recording (VCR).
English (United States)
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Stable Vertical Sync Generation for VCR Special Effects Video
A technique is described whereby a stable-noiseless vertical sync pulse is
produced so as to synchronize computer data overlay on top of the special
effects video of a video cassette recording (VCR).
To accomplish this, a monostable circuit is added to filter out random noise in
the special effects vertical sync and another monostable circuit is used to
produce a 200 usec vertical sync pulse. When using special effects video
features in VCR equipment, so as to obtain slow motion, freeze frame or search
modes, the composite video produces noisy and unstable vertical sync pulses,
as shown in Fig. 1.
This unstable condition is unsatisfactory when synchronized computer data is
overlayed for use with a VCR. The normal use of this technique is shown in Fig. 2. Using monostable filter 10, as shown in Fig. 3, to filter out the noise pulses
which are present in the vertical sync of the special effects video, and a 200 usec
monostable circuit 11 to produce a new vertical sync, allows the stability required
to incorporate computer data overlay in the VCR. The timing diagram, as shown
in Fig. 4, shows the pulse duration of the output of the filtering monostable circuit
to be 14 msec. However, the circuit provides adjustment of this duration. Should
the filtering pulse duration be too long, the filtering could interfere with the next
vertical sync pulse of the special effects mode. Although the design utilizes