videotaping from the iris

[Dave Martindale]

In article <8811020355.aa14353 at SPARK.BRL.MIL> phil at BRL.MIL (Phil Dykstra) writes:
>
>SGI also sells an RGB -> NTSC encoder board for the 4Ds.  We bought
>one but sadly have not been able to use it.  Why is because the SGI
>board outputs a fully positive video signal (i.e. blanking is around
>+0.3V) rather than a bipolar signal with blanking at 0VDC.  While
>I haven't found anything in the RS170A spec that requires an absolute
>voltage level (it all looks to be AC coupled), much to my surprise our
>Sony BVU850 seems to REQUIRE blanking to be at 0VDC (or at the very
>least average picture level to be at 0VDC).  Black levels get all
>messed up when recorded from the SGI board but come out okay from the
>ENC-VI.

RS-170 video can be capacitively coupled (and often is at the input
of a piece of equipment).  If the equipment depends on blanking being at
a specific DC level, it will have a "DC restorer" circuit that clamps
a reference part of the signal to ground - usually the blanking level
found in the "back porch" after the sync pulse, though the tip
of the sync pulse could conceivably be used too.

If your Sony lacks input coupling capacitors, its DC restorer may be
fighting the SGI board output - capacitively coupling the signal should
fix this.  If the Sony simply lacks its own DC restorer and really does
depend on the absolute voltage levels in its input (unlikely), then
just pass the signal through a video distribution amplifier which does
DC restoration (these are readily available).

If the Sony has its own DC restoration and a capacitively coupled input,
then there has to be something "out-of-spec" in the video signal that you
are feeding it that causes it to be confused - incorrect sync or video
amplitude, incorrect sync pulse width, or the like.