videotaping from the iris
Chandlee Harrell
chandlee at alpine.SGI.COM
Fri Nov 18 14:48:40 AEST 1988
Iris 3000s are shipped with two video options. The two defaults
provide video timings for 1280 by 1024 60hz monitors and 640 by 480
RS170/NTSC monitors. A customer can request that the RS170
option be replaced with either PAL/SECAM timings or for a 1280 by
1024 interlaced 30hz monitor.
All Iris 4D systems ship with all four of the above timing options.
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.
An option board may be purchased from Silicon Graphics for Iris 4D
systems which takes the three RGB outputs and color encodes them
into a composite video signal. This signal is appropriate for
connecting directly to any television, or to a VCR. The composite
video output does not match up to broadcast quality; there is some
minor difference that I am not familiar with. This should only be
a concern for the media organizations, not for those of us creating video
presentations. (Otherwise one buys a much more expensive broadcast
quality color encoder.)
This option board is called (internally, at least) the CG2/3. It
also provides the genlocking capability. This is the capability to
sync up and overlay the video from two separate systems (while in
either high res 60hz or NTSC modes).
So option one for video taping on an Iris 4D is to buy a CG2/3.
The bottom left quarter of the screen may be recorded directly into
any recording device that accepts NTSC composite video.
Option two allows you to video tape the full picture on your 1280 by
1024 high resolution display. Pixel averaging is done to reduce
4 pixels down to one giving the appropriate number of pixels (640 - 480)
for NTSC. Pixel averaging provides better images over simply drawing
the image into the bottom leftmost portion of your screen because a
certain amount of anti-aliasing takes place in the pixel averaging.
It also allows full screen video taping. Option two is available
from vendors like RGB Technologies (don't know their pricing). The output
from these systems is, again, standard composite video. Note:
Silicon Graphics has some ongoing development that should help
those desiring the pixel averaged approach.
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