OT: video signal question

[Don Y]

Bert Thomas wrote:
> Appearently something like video printers exist. They have "normal" video
> inputs and are somehow able to extract a command to make a hardcopy from
> the video signal. In my case the command is triggered by a button on an
> edoscope control panel. Can anyone tell me how the trigger signal is
> embedded in the video signal?

Hmmm... the only video printers I have seen/used were
independant of the video source.  That was the whole
point of their design (e.g., to grab a copy of the
image on a ComputerVision CAD system without having to
tinker with their software in any way).

I worked on such a device.  The "video module" had a
high speed PLL (~200MHz... fast for 1985!) that would
sync to the scan rate and, from which, the sampling clock was
derived.  So, you just waited for a vertical sync, vertical
blanking to go away and then start grabbing scan lines
(and repeat if the signal was interlaced).

The medical instruments I have seen work similarly -- they
run 4BNC or 5BNC to the "printer" and the printer just
grabs the image presented to it.

I'd be curious if there is a "standard" way to encode a
"capture command" in a video signal...

--don