[opendtv] Re: State of the Art LCD 45" HDTV

At 10:34 PM +0200 10/27/04, Olivier Houot wrote:
>Recently, I saw a demo of a 1280x720 LCD front projector.
>
>The picture was approximately 2 meters wide (with a cinema aspect
>ratio), and it was not an impressive size at all, i found it wanting
>from this point of view.
>I would certainly feel frustrated if after putting money in a costly
>HDTV setup, i could not get more than that size out of it.

What kind of room do you have for a larger screen?
Based on your description I calculate the following about the display 
you were watching (I am converting to inches to take advantage of a 
cheat sheet that I use).

This assumes that the projector is 1280 x 720 and that the image is 2 
meters wide (~78 inches).

Thus for a full 16:9 raster, the screen height would be 44 inches, 
and the screen diagonal `90 inches. Based on this info alone, I can 
understand why you perceived what you did.

I have often used the following rule of thumb to determine the raster 
resolution needed for various screen sizes at preferred viewing 
distances.

Screens up to 40 inch diagonal - 480P is sufficient - Preferred 
viewing distance ~7 picture heights

Screens between 40 to 100 inch diagonal - 720P is sufficient - 
Preferred viewing distance ~5 picture heights

Screens larger than 100 inch diagonal - 1080 lines are needed - 
Preferred viewing distance ~ 3-4 picture heights

  The preferred viewing distance is the location that the average 
observer would choose to sit from the display  when given the option 
to choose a comfortable viewing distance.  Several studies have shown 
that the distance (as measured in multiples of the screen height) 
decreases as the screen size increases. The average viewer does not 
choose to sit 3.3 picture heights from the screen until it is about 
117 inch diagonal - a distance of about 17 feet, which is not 
feasible in most family rooms.

The preferred viewing distance for typical HDTV displays (40-60 
inches is about five picture heights).

>The corner of the store where the display was did not allow me to get as
>far as the screen as i would have liked, yet a rough measurement with my
>steps suggested a distance on the order of 3.5 to 4 meters.

So your viewing distance was about 12-13 feet, or right at 3.3 picture heights.

It is not surprising that you saw the good old LCD screen door effect 
at this viewing distance. You should have.

Unfortunatley, LCD light valves suffer from the manufacturing reality 
that the signal traces must be run between the LCD sample sites. As 
the image is magnified by increasing its size, eventually you will 
see these fine lines between the samples. The solution is simple, 
increase the viewing distance or reduce the projected image size. 
Your example places the display conditions near the transition point 
where 1080 lines is beneficial.

I strongly suspect that if the projector had used DLP technology 
rather than LCD, you would not have detected the raster. The reason 
is that seams between the samples are much smaller, as the 
micromirrors nearly touch one another.


>In those conditions, the pixels of the LCD matrix were just at the limit
>of visibility, on the bad side of the limit, meaning i could see them.
>So to me, 1280x720 is not enough.

Exactly as they should be, although without the LCD screendoor, it is 
unlikely that you would have seen the actual pixels. That is, you 
would be just on the good side of the limit.

>
>Some recent european reports consider that 720 is enough for 130 cm
>screens (diagonal), but that beyond that, 1080 should be considered.
>After that (far from scientific) experience, i am tempted to concur .

I would like to see these studies. I think there may be some serious 
errors in their observations, based on the limits of the human visual 
system.

a 130 (51") cm screen is near the low end of the range of screen 
sizes for 720P. I have seen 72 inch DLP displays with 720P resolution 
that can easily be viewed at 3 picture heights without the perception 
of the raster ( not that i would choose to sit that close).

Regards
Craig
 
 
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