[opendtv] Re: Overscanning on LCD TVs
- From: dan.grimes@xxxxxxxx
- To: opendtv@xxxxxxxxxxxxx
- Date: Sat, 14 Nov 2009 11:28:14 -0800
"The thread started on "overscan", but evolved to other stuff that is often
wrong: setup/black level, sample aspect ratio pixel aspect ratio,
cropping, primaries/color temp, matrix, gamut, gamma, progressive encoded
as interlace (with 3:2 pulldown), etc."--Kilroy
When I originated the thread, I started with one of the many locations that
I am having the translation errors. The thread developed into a second
problem, and here am going to introduce a third that I am experiencing.
In order to save money in our new production facility, we used consumer
level monitors where production quality monitors were not critical. We
used AJA Hi5 HD-SDI converters to convert HD-SDI to HDMI to go into the
Sony Bravia monitor. The monitor is overscanning the picture and there
could be some color space issues, too. This was the first issue in the
original post.
The second problem involves trying to get a VGA or DVI (of all types) into
an HD-SDI signal that can be used in the production chain. We have Gefen
equipment that translates VGA to DVI-D and then DVI-D to HD-SDI. There are
problems at each of the translators. I'd like to state for the record that
I did not specify or purchase the equipment.
I have been able to make adjustments to both converter boxes that make for
an HD-SDI signal that is somewhat useable. By adjusting the converter so
that black is at 16 and white at 235, the HD-SDI signal loses the green
banding "noise" and becomes somewhat stable on a display. However, when
looking on an HD-SDI analyzer, the overshoot and noise in the signal is
still translating to noise in the numbers. Here is where my knowledge runs
out: I didn't think it was possible to have numbers below 16 and over 240
within an HD-SDI but the HD-SDI waveform monitor shows these distorted and
overshooting edges (originally from the analog signal) clearly above and
below those numbers and the picture shows green distortions/noise. When to
excess, the HD-SDI signal is not usable to equipment and cuts in and out.
So clearly the Gefen box is not putting out a compliant SMPTE
292M/296M/274M signal.
And now to introduce a similar problem as with the first issue, all is not
perfect in the production world, either. We have Sony production monitors
that have a 1680x1050 display and we are producing in 1280x720. The
monitors have the following settings:
Normal: the full raster is seen but there are black areas above and below.
The picture appears slightly squished vertically to me(the proper test
signal could confirm but I haven't found one in our test signal generator).
Over: The picture is slightly overscanned at the horizontal yet black bars
remain above and below.
Native: 1280x720 is mapped to 1280x720, displayed 1:1, where the picture
is only taking a fraction of the screen with black all around; the image
appears to be natural (not distorted vertically or horizontally).
Full: The picture fills the screen but part of the edges to each side are
cropped off and the full image is top and bottom, no black.
Clearly, the 16:9 picture on a 16:10 production monitor is demanding the
need for the various settings.
While I don't have aspect ratio test signals in our HD-SDI sources, I do
have test signals in my computer and I would love to use them to find out
what is going on with each of these conversions but I don't have much of
chance without being able to measure and normalize the outputs (VGA and
HDMI) to see if they are proper before I use them as a test source. There
is something to be said for having a good test signal generators and
analyzers of each type. Wish I could afford all those items.
I think modern TV production test signal generators need to include test
signals that would help flush out some of these issues. Greyscales and
some standardized test signals are usually available to figure out many of
the color space issues but with the plethora of rasters, aspect ratios and
other issues that Kilroy mentions above, some signals that handle these
issues need to be in the engineer's toolbox, too.
Dan
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