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Home Theatre
Projector, Screen, HTPC, High Definition


Here I will tell you about my experience of projectors and Home Theatre. I have also written down something about the physic and math that can be a help for you. It's a little bit old (2000) information now but maybe you still can find something interesting.

Please note that you use the information at your own risk, I do not take any responsibility for it !

Contents

  1. Projectors
  2. Thoughts about projector type
  3. HTPC
  4. HDTV
  5. Screen
  6. What about our eye, what is its resolution and optimal Luminance
  7. Some easy physics and math about projectors light output
  8. Some physics and math about gamma

1: Projectors:

My experience of projectors come from data CRTs, LCD XGA (1024x768) and nowadays 720p (1280x720).

CRT projectors is an old construction, goes back to the 1950:s. It was great for films and superior to all other projectors that come in market in the middle of 90:s with its sharpness and low black levels.

For today's demand the CRT-technology is too soft to display computer graphics (resolution is OK at the center but not at the edges and contrast is low for small details). CRTs also has problems with the temperature drift and magnetic influence, always needs more or less convergence adjustment.

Do I here anyone say that it's bulky too? And yes there are of course a lot of problems with LCD and other digital projectors too!

With the beginning of LCD-projectors (or other digital technology) of XGA (1024x768) resolution this type of projectors begin to be very interesting.

One important rule, never ever do a downscaling! Here in Europe with the PAL-system and 576 effective lines in height you need 1024 pixels (square pixels) per line for anamorphic movies (16/9 * 576 = 1024). A lot of projectors sold here in Sweden are told to be of true 16x9 format (~850 x 480), but that low resolution is for NTSC movies! Less than this 1024 pixel per line (PAL anamorphic) will result in downscaling (in height or maybe both).

There are projectors that have resolution of 1024x576 pixels today, but better to use a standard XGA (1024x768) projectors. One problem with scaling is that the projector pixels are square and the DVD pixels are not. In the example above you only get pixel to pixel matching in height. If you have to do oversampling it's preferable to do it in a multiple of 2 (or more) example 2048x1152 (2.84x resp. 2x oversampling). But you can't find a digital projector with that resolution today.

CRT works in the analog way and doesn't have pixel raster and therefore doesn't have this problem. Some of the new HDTV format have square pixels and is much easier to match to a digital projector if there only was projectors to find with this high resolution (up to 1920x1080) at a price that you can afford.

Today there are a lot of 1280x720 projectors on the market. Look at the new Sanyo Z4, Hitachi TX200 or Panasonic AE900, cheap and very good! I have a Hitachi TX100, little bit surprised of the bad optical mechanical quality, but in other ways it's ok.

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2: Thoughts about projector type:

If you only want to display movies I think a data CRT of high quality is still one of the best projector construction, if you can find one not too expensive and if you have space for it at your home. You must also be a very skill technician to handle a CRT, a not properly adjusted CRT looks like shit.

Be prepared to that the projector can be very expensive to repair nowadays (they are old now). To take advantage of the CRTs low black level the room must be very dark, for a LCD the dark demands are much lower because of it's lack of low level output (modern LCDs also have a much higher light output).

No projector today (2003) in the price range you can afford can give the HDTV format full resolution. Maybe if you split the image to two CRT-projectors side by side. But if you have the money, by a digital 1920x1080 resolution projector.

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3: HTPC:

The real fun starts when you use the projector together with a computer (that's what all people today call HTPC) over the VGA or the DVI (digital) output.

  • Advantages:

  • You have much more control over the image.
  • Progressive scan from DVD and digital satellite TV and maybe from analog TV.
  • You get the signal from DVD (internal) in its digital domain and can do scaling without any analog step.
  • You get no over scan (a normal TV clip the edges so you don't see the whole image!).
  • No flickering (CRT).
  • If the computer is fast enough you can get a digital video recorder too.
  • You can play games, but be very careful if you use a CRT to not burn the tubes.
  • Computers are not expensive and the player is in software.
  • With a CRT connected you can set the graphics card to 720x576 or 1440x1152 and adjust the projector (if these controls have that large range) to work with not square pixels (however it will give other problems).
  • Newer graphics card have digital output (DVI) and then you can skip one analog step.
  • But there are also some disadvantages:

  • It's hard to get the Graphics card to synchronize with the internal DVD player or Satellite/TV-tuner card. Reclock maybe solve that problem.
  • The graphics card resolution must match the resolution of the digital projector, exact! If not the projectors internal scaler take over and an extra scaling will take place. For standard graphics resolution there is normally not a problem. 16x9 projectors could cause problem (but perfect for HDTV 1280x720).
  • Cheaper LCD-projectors doesn't synchronize with the graphics card refresh rate, some of the DLP and most data CRTs (those with no internal scaler) will do it.
  • Lot of work to get it to work perfect. It's a challenge to get the movie too run smooth.
  • The handle of the computer isn't as easy compare to maneuver a stationary DVD player.

Some software media player allow you to connect software filters. The filter ffdshow will do lovely things to the image quality. See the AVS-forum under the page of links to get information how to get it to work.

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4: HDTV:

At last, January 1st 2004 we will get HDTV (1920 x 1080i x 50) here in Europe by satellite, and it has already started with test transmitting in September of 2003. My first test was success full but need a faster computer. Maybe a 3Ghz is enough. The other equipment you need is a 60cm parabola (Stockholm, Sweden) and a PC based satellite receiver, cost only 100 to 300 Euro, the parabola 50 Euro. The cheaper ones as I have only allow free to air (FTA) channels.

Another exciting news is that HDTV can be delivered on DVD from the Windows Media9 player format. And it's even better than the HDTV format! It's stored in the format 1920 x 1080p, which means 24 frames progressive, exactly as the original movie.

The sound for both media is the Dolby Digital 5.1 format or better.

You can read more about this, see under home theatre at the menu page Links.

Today the best solution to receive and play HDTV movies is from a HTPC (if you like that concept).

  • For the wm9 format you need something like this:
  • PC P4 3GHz (never get fast enough)
  • 128Mb graphic card 1920 x1080p resolution and 48, 50, 60, 72, 75 fps, or even 24 fps
  • Soundcard with digital SPDIF output (or internal decoding to 5.1, 7.1), the latest standard 96khz sampling and up to 24 bit resolution
  • Windows XP
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5: Screen:

Screen, it's important that you choose a screen of the right construction and quality.

One important thing about screens are the screen gain, that's the ratio of the light reflected from screen compared to the light reflected from a standard white board with a surface of magnesium oxide.

A normal screen has a gain between 0.9 and 1.3, a high gain screen has a gain between 1.8 and 2.3. High gain screen doesn't really amplify light, what they do is that they direct the reflected light in a more narrow beam (concentrate it), and that's also the problem with this kind of screens.

For a CRT never use a screen of gain higher than 1.5. Higher gain will result in color shift for different viewing angles (a CRT normally has three independent lenses one for each color) and maybe amplify the spot in center. High gain screens are expensive too and more sensitive to dirt. The only use for this type is in conference room where you need the light on to take notes.

Long time ago when projectors had very low light output they where used with screens of gain 8 to 12, only one person at a time could see the image properly. A few centimeters out of the image center spot the image was terribly. So don't spend money on expensive high gain screens. Spend them on a better darkening (compare to a cinema) of your room and maybe buy a projector with higher light output.

As a rule 1000 ANSI lumens (a figure of the projectors light output) is enough for a 3 square meter screen of gain 1. Remember a screen work with reflected light, if light belongs from other sources than the projector it make the image too look more gray (low contrast).

My own experience of how close you can sit in front of the screen is about 2.3 times the screen high for a XGA-LCD projector. Closer will bring out the pixels in a disturbing way. DLPs and CRTs are better in this area.

  • Here is a simple formula for viewing distance (it vary from person to person):

  • distance=factor*768/(number of vertical pixel)*(height of screen)
  • factor=1.9 for CRT (together with a HTPC or a lined doubler)
  • factor=2.1 for DLP
  • factor=2.3 for LCD

But of course you can't take a 2000 pixel wide projector and a movie on VHS-tape and use this formula.

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6: What about our eye, what is its resolution and optimal Luminance:

Did you know that an eye have 125 million rod cells (very sensitive but do not distinguish colors). There are also 6 million cone cells (not so sensitive) for color vision. The later are concentrated in the center of the visual field. Compare that to how many pixels a digital camera have!

How dense (close) lines can an eye see? This is often expressed in line-pair per degree or cycles per radian. One line-pair is one bright line and one dark line together, you need 2 lines of pixels to build this line-pair. For a bright black/white (500 cd/m2) display the peak sensitivity is at 3 to 4 line-pair/degree with a contrast ratio of 500. The cut off is at 50 line-pair/degree. A darker one (0.05 cd/m2) will give us only 1 line-pair/degree and the cutoff at 5 line-pair/degree.

That was for one eye, binocular (two eye) looking will raise the cut off resolution a bit, about 10%.

In our home theatre the image will have a practical brightness in the range 70 to 170 cd/m2 so the resolution will be a bit lower than the first example. If the Luminance is lower than 70 cd/m2 it will be problem with the color vision and contrast. In the higher end there is a technical limit and over 170 cd/m2 will stress the eye.

What's the maximum no of pixels per line that I have need of?

Example, you sit 5 meter away from the screen and have a 3 meter wide screen. That's a viewing angle of 33 degrees, now with the maximum resolution of 50 line-pair/degree this correspond to 3300 pixels! (33 * 50 * 2 = 3300) in practice much lower number is needed.

Our home theatre image is darker than 500 cd/m2, the image is not still, it's moving and it's in color and our color resolution is much lower. The new HDTV format with it's 1920 pixel per line will in practice look very sharp. Compare to the DVD-format that has a black/white resolution of 720 pixels or 360 line-pair per line and the color resolution is half of this (4:2:2 format). It doesn't sound much but still it looks very impressive.

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7: Some easy physics and math about projectors light output:

How many lumens do I need from my projector ?

Some useful units:

concept Symbol SI unit comment
Luminous flux F lumen (lm) ex. projectors output
Illuminance E lux (lx) (1 lux =1 lm/m2) light per m2 screen
Luminance L cd/m2 reflected light from screen

Calibration reduction = the projectors figures about light output normally doesn't represent a calibrated projector. It could be reduced as much as 40%, (60% left). And another thing, the lamp age, at the end of its life you have lost 20 to 50% of its output.

Screen gain = ratio of the light reflected from screen compared to the light reflected from a standard white board.

Screen area = the width x high of screen, meter is the SI unit.

The formula to get the Luminance (the reflected light from screen):

Luminance (L) = flux * calibration reduction/screen area*screen gain / PI (PI = 3.14).

Example:
A 4x3 projector that have 1000 lumens output and a screen with a gain of 1.5 and width and high of 2.6m x 2m.

1000 * 0.8 / (2.6x2) * 1.5 / 3.14 = 73 cd/m2

Normal values are 70 cd/m2 for a dark room and 170 cd/m2 for a not perfectly dark room. I prefer 80 to 120 cd/m2. Divide the Luminance by 3.426 to convert it to U.S. foot lambert (fL).

Or in the other way, say that we want to have a Luminance of 100 cd/m2 from screen:

Lumens = Luminance / calibration reduction * (screen area) / screen gain * PI

Lumens = 100 / 0.8 * (2.6 x 2) / 1.5 * 3.14 = 1360 lm

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8: Some physics and math about gamma: (this is complicated!)

Our eyes are not linear, that means if we increase the luminance by a factor 10 you will not se it as 10 times brighter.

This is a common but rough formula that describe what happens.

B = a * L^g , B = Brightness, a = constant, L = Luminance, g = gamma factor

For a well-lit original scene our eyes and brain compare to a gamma of 1/2

B = a * L^1/2

This is how our eyes see it in a reproduction in dark surrounding (home theatre), now gamma appears to be closer to 1/3.

B = a * L^1/3

There must be a correction here, 1/3 divided by 2/3 = 1/2, this gamma correction 2/3 I hope the producer of the video take care of. Some people say that this gamma correction should be about 0.8.

Now we (the producer) have corrected an outdoor scene to viewing it in a dark room. That was easy, but we have to correct for other devices in the chain to.

In old days the camera had a tube (vidicon) as an image sensor. A vidicon has a gamma about 0.5 a Television has CRT as a display device and gamma for that is about 2 to 3. Normally g = 2.8 Now we see that the total gamma value is built up from a lot of devices in the signal chain.

gtot = g1 * g2 * g3 * g4.... gn

There is a standard for gamma of video signals, the video is adjusted to deliver a signal that is connected to a system that display the image with a gamma of 2.2 If we have a system of devices that has this total gamma = 2.2 the gamma correction will look like this (that gcorr is built in in the video signal).

Example, 0.5*2.2/gcorr = 1.5 gives gcorr = 0.73

But nowadays the cameras has a CCD or CMOS sensor, and that device could be linear! Linear means the gamma = 1. There is normally a correction in the camera itself to give an output gamma as the old vidicon cameras.

We don't have to worry of that part of the chain, that is already corrected in our video signal (but if we have produced video by ourselves then maybe we must gamma correct it).

The display device could be a LCD monitor or projector and it's linear. For the video input there is a gamma correction built in. And if it is a fine device maybe it's also adjustable to. But if you connect to the data input i.e. VGA it could be linear (not common). If you have a HTPC or a progressive DVD player that's the input you want to use because it will give you a better image quality.

If you have a linear display and don't do a gamma correction the dark areas in the image look grey. What to do? Now we talking about corrections in your system that handles the video signal.

gtot = 1 / gcorr = 2.2, math give that a gamma correction of g = 0.46 will fix it.

Or for a CRT with gamma 2.8.

gtot = 2.8 / gcorr = 2.2, math give that a gamma correction of gcorr = 1.3 will fix that.

But watch up, maybe the gamma can be adjusted at many places, camera, dvd-player, graphic-card, display etc. Remember that gtot = g1 * g2 * g3...gn / (gc1 * gc2 * gc3... gcn), gcx = gcorr.

A gamma correction of 0.5 is a very big correction, and that can be a problem. Your graphic-card normally only work with a resolution of 8-bits (256 steps) per color. The dynamic resolution could be to low and you get a blocky result. 10-bit resolution is better, the projector can have this resolution internally. Try to adjust it at different places and se where you get the best result.

There are test pattern to control gamma direct from signal to display output, but I have only seen this for still images not video. Se under my calibrate-map, it's my self made still image, it's a bit tricky but it works for my HTPC. With an instrument you can measure it in a more advanced way.

Sorry to say, it's much more complicated than this! There are three colors red, green and blue and they can have different gamma correction. There can also be advanced system that dynamically adjust the gamma.

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