videotaping from the iris

[Dave Martindale]

In article <22219 at sgi.SGI.COM> dunlap at bombadl.SGI.COM (D. Christopher Dunlap) writes:
>
>>   When in RS170 mode, the Iris outputs the three components of RGB
>>   in the correct RS170 timings, un-encoded.   The bottom leftmost rectangle
>>   of 640 by 480 pixels is displayed on the full RS170 monitor screen.
>
>
>Well, I'm not positive about the 4D, but the 2xxx and 3xxx systems
>need to have a Genlock board (CG1) and be sync'd to an external sync
>before the output is "correct RS170 timings", assuming that "correct"
>is taken to mean "Broadcast Quality".

It's not broadcast quality in that the clock oscillator isn't within
the 3ppm accuracy required for broadcast.  On the other hand, all the
funny equalizing pulses and so on are there and in the correct place.

The "right" way to generate broadcast-spec NTSC is to use a standard
sync generator as a timing source for everything, and genlock the IRIS
to it.  That way, the subcarrier and sync are locked together as they
should be.

However, I once tried setting up an IRIS that lacked a genlock board
for videotaping.  I connected the IRIS sync output to the genlock input
of the sync generator (you may need some attenuation here, since separate
sync is usually 4V p-p and the genlock input is probably expecting
composite video with 300 mV sync).  This locked the sync generator's
sync to the IRIS; it worked fine because the number of lines and the
interlace is done correctly in the IRIS.  The sync generator's colour
subcarrier was NOT locked to sync anymore, since there was no colour burst
for it to lock to, so subcarrier just drifted with respect to sync.

This doesn't meet broadcast spec, but is good enough for recording on
a 3/4 or 1/2 inch VCR.  In fact, the drift rate of subcarrier with
respect to sync was only a few Hz, indicating that the oscillator in
the IRIS was only a few Hz off the correct frequency.  The video produced
wasn't broadcast quality, but it was an awful lot better than what you
get coming out of a 3/4 inch or worse VCR.

To make this kludge work, you need a sync generator that will lock its
sync to a B&W signal on its genlock input without getting upset about
the lack of colour burst, and without killing its subcarrier output.
If you have one that will lock subcarrier as well as sync, you're even
better off.

(All of the above was done with a 3000; I haven't tried it with a 4D).