Please explian resolution vs scan lines - AVS Forum
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post #1 of 4 Old 02-12-2002, 05:14 PM - Thread Starter
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Hello. I'm really trying to get into HDTV (720P/1080i)

I get confused by the projector stats
800 X 600 SGA
1028 X 768 XGA
12?? X ???? SXGA

How does the above relate to displaying 720P or 1080i without scaling down.

What does native vs. max resolution mean?

Will a XGA projector display 720P or 525 lines without scaling or down converting the signal?

Please forgive my limited knowledge. Thanks
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post #2 of 4 Old 02-12-2002, 10:53 PM
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SXGA usually means 1280x1024.

All of the resolutions you mentioned above
are for 4:3 projectors.

There are many new 16:9 (wide) 720p projectors (LCD, DLP) and Plasma screens that now support 1280x720 or 1280x768 "native" resolution.

1080i "native" is 1920x1080
720p "native" is 1280x720

In all cases with 4:3 projectors (with the possible exception of 720p on an SXGA projector)
some scaling will need to be done to throw away
some of the HDTV data to make it fit into the
actual pixels available on the display device.

"Native" resolution typically means the actual pixel
resolution of the display device. With LCD,DLP,dILA & Plasma this is an exact number.
With CRTs is it an approximation.
"Max" resolution typically refers to the maximum
input signal resolution that can be accepted
by the electronics in the device.
For instance many 17" CRTs will accept a 1600x1200
"max resolution" but their "native" resolution
(or "effective resolution" is closer to 1280x1024).
Many 1024x768 "native" projectors will accept
1280x1024 signals (so you don't have to reset
your laptop just to plug in) and then it will
scale it down to the native res.

An XGA projector will have to throw away some
of the pixel data in a 720p signal since
720p has 1280 across and XGA is only 1024 across.

What you will find in the real world is that
different projectors have different quality
scalers. Some do OK but others introduce all
sorts of ugly artifacts (like "stairstepping"
on horizontal lines). Also mixing and matching
16:9 signals with 4:3 devices can sometimes
be unpleasant. Sometimes the device will make
the image fit the full device so you get
a vertically stretched picture.

Your best bet is to see if you can try your
combination of equipment before you actually
buy it.

I hope this helps and answers some of your
questions.
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post #3 of 4 Old 02-12-2002, 10:55 PM
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BTW:

Many STBs (and PC products like the HiPix card)
have their own scaler to accomodate different
output devices. So you may be able to get
better results having your HDTV receiver device
scale to the "native" res of your projector
rather than trying to get the projector
to sort it out for you.

Unfortunately HDTV receiver devices don't have
optimal scaler settings for all possible types of
display devices.
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post #4 of 4 Old 02-13-2002, 07:49 AM
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With regard to your heading: Scan lines involving CRTs refers to electron beam(s) sweeping back and forth, creating a blank raster on a phosphor screen. With no signal, you can see these scan lines near the screen.

A video signal creates an image by modulating these scan lines. Vertical resolution differs from scan lines. This resolution is the number of alternating black and white test pattern lines visible from the top to bottom of a screen. When TV images are scanned at the source, a scan line can miss a detail entirely or two scan lines may straddle details. With vertical motion, this occurs more often. As a result, vertical resolution with most TV is typically only about 0.7 the number of scan lines for stationary images and about 0.5 the number of scan lines for moving images.

Horizontal resolution is how much detail is visible along each scan line. Usually this is the number of alternating black and white lines visible along a scan line length equal to the height of the picture. To obtain the full-width horizontal resolution for 4:3 screens , multiply by 4/3 = 1.33. For 16:9 screens, multiply by 16/9 = 1.78. For CRT displays this resolution depends on how finely the electron beam spot(s) can be focused, how much contrast can be obtained, whether the CRT has a fine-pitch shadow-mask screen (if direct-view), and whether the video circuits can pass the higher frequencies carrying fine details (good bandwidth). Other factors such as the optical resolution of lenses and the optical pitch of elements in rear-projection screens are involved with projection TVs. -- John
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