Organization: Queen's University at Kingston
Date: Tue, 26 Jan 1993 19:04:32 EST
From: <HARRAPR@QUCDN.QueensU.CA>
Newsgroups: comp.sys.next.hardware
Subject: Video via the NeXTDimension - Summary



I posted a question about resolution and picture stability during output on the
NeXTDimension board a couple of weeks ago. I've received many responses as
e-mail, and a few have been posted to this discussion group. Here is a summary
both of the private responses and the net postings.

Thanks to everyone who helped out, and expecially to Russ Werner at NeXT for the
posting on the NDs strengths and limitations. BTW, these are not necessarily in
the order they were posted or sent to me.

Peter Eisch <peter@tahiti.umhc.umn.edu> writes...

= In simple words, the NTSC signal is interlaced, meaning that every other
= line isdrawn and thus takes twoscans of the screen to redraw the whole
= screen.  The window border is EXACTLY onone line across and
= thus only gets redrawn every other pass.  This is what causes it to flicker.

Denis Norton <nort@goat.geo.arizona.edu> writes...

= single pixel high horizontal lines will vibrate your eyes out...
=  keep the lines fat, select optimum colors... and experiment.
=  i have not had any problems with interdevice noise, but
=  am using the top of the line JVC editing decks.

David L. Leon <dleon@seas.smu.edu> writes...

= You could use a time base corrector (TBC) between the ND and VCR.  They
= are not cheap however.  The vibrations could also be fixed with a sync
= restorer unit (about $200 US), you would have to fine tune the clamp and
= possibly boost the image.

= Izumi Ohzawa <izumi@mindseye.berkeley.edu> writes...
=
= Just to present a data point, I recorded ND video to a
= standard VHS VCR via composite video out.  (Nothing fancy
= a Toshiba that was less than $400).
=
= It made good video recording just like I expected.  You should
= be able to make good quality video straight from the ND video
= out.
=
= A few things to note off the top of my head.
=
= [1] keep your VCR reasonably far away from your NeXT and
= ND Color monitor, or any other screen.
=
= [2] Check the composite video output first by connecting it
= to a monitor first.  Do you get stable image this way?
=
= [3] Try  to feed the video through VCR's amps and then to
= a video monitor.
=
= [4] If the power plug for your VCR is not keyed for polarity,
= try reversing it.
=
= Just some suggestions.  The bottom line is  that you shouldn't
= need any of expensive time-base correcter or any other
= expensive professional equipment.

===========================================================================

Ross Werner <Ross_Werner@next.com> writes...

One of the unfortunate facts of life with the existing television standards
(NTSC and PAL) is that they are interlaced.  A complete picture, or frame, is
made up of two fields - one containing all the odd lines, the other with all
the even lines.  Drawing of the fields alternates at a 60Hz rate (50 Hz for
PAL.)

This means that any single pixel wide horizontal line will flicker since it is
drawn in only one field.  The NeXT user interface uses lots of single pixel
wide horizontal lines, this looks great on the NeXT monitor but flickers
annoyingly on the NTSC output.

The NTSC standard will also result in some other artifacts, such as "chroma -
crawl" on edges where there is a color change.

I don't know what you mean by "window borders appeared to vibrate wildly (left
to right)."  I have used many different VCRs (everything from a $199 consumer
model to an $8000 professional model) to record the ND video output, and with
the exception of the problems I described above, the video quality has been
excellent.

On a high resolution screen such as the NeXTdimension's, it is very easy to
create an image that looks just great, but will look terrible when encoded to
NTSC.  Reasons for this include the interlace, color encoding, and limited
frequency response of the NTSC and PAL standards.  Some very basic rules of
thumb for creating images that will encode well:

        1) Don't use single pixel wide horizontal lines.
        2) Avoid the use of fully saturated colors.
        3) For practical purposes, the luma (Black/White) bandwidth of NTSC
           is about 4 Mhz.  Simply stated, alternating black and white
           single pixel wide vertical lines will end up as a grey mush.
        4) The chroma (color) bandwidth of NTSC is 3Mhz at most.  Small color
           features (less than a few pixels horizontally) will tend to
           disappear or cause unpleasant artifacts.

Let me emphasize that these rules of thumb relect limitations of the NTSC
television standard, not the ND hardware.

The NTSC encoder on the ND is pretty good, but as a result of deliberate cost
and board space tradeoffs it is not "broadcast" quality.  (Go check the cost of
a broadcast quality encoder.)  If you want the best possible NTSC image, get a
professional RGB -> NTSC encoder and drive it from the RGB video output.

===============================================================================

Rob Harrap                     ==========      ===========      ____________
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