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Passive 3D Projection - Part 3

This blog is a continuation of my previous discussion on passive 3D projection (HERE).   Specifically, this blog is focused on do-it-yourself (DIY) dual projector passive 3D projection systems using polarization as the technique to separate the right from the left images.  Future blogs will discuss DIY 3D passive dual projector systems using other technologies. In my previous blog I included an illustration of a generic dual projector 3D passive setup, which is repeated below. I want to thank Rob Stewart for information on DIY passive 3D projection systems.  Rob has posted extensively on the AVS Forum concerning his efforts to put together a passive 3D projection system and has investigated a number of alternative technologies. The following paragraphs discuss the functional elements shown in the above diagram that make up this DIY setup.  Note that the following discussions of ‘3D source device’, ‘2-way HDMI splitter’, and ‘3D processor’ equally apply to all DIY passive 3D projection setups and not just those using polarization.

3D Source Device

While the 3D source is ‘outside’ of the 3D projection system (i.e., everything to the right of the ‘3D source device’ in the above diagram) the key thing here is the ability of the 3D passive projection system to accommodate the standard 3D sources the have a HDMI 1.4a digital signal output.  This means the projection system should, as a minimum, have the ability to support all of the 3D formats defined as mandatory by the HDMI 1.4a standard.  More information of the subject of HDMI 1.4a and mandatory 3D formats was presented in my previous blog of July 2, 2011 (HERE).   At the present time Blu-ray 3D is the highest quality 3D source, along with some 3D camcorders, that support full 1080p resolution in 3D using the ‘frame packing’ 3D signal format defined by the HDMI 1.4a standard.  Directv and some cable TV companies offer a few 3D channels, but because the HD set top boxes/DVRs/receivers used by these companies only have (at best) HDMI 1.3 outputs, they are only able to support 3D in the ‘compatible’ half-resolution 1080i side-by-side and the 720p top-and-bottom signal formats.  Directv also supports the half-resolution 720p side-by-side format, but this is only an optional format in the HDMI 1.4a standard.  The output of the 3D source device is assumed to be HDMI 1.4a for full 1080p resolution 3D source devices and either HDMI 1.4a or HDMI 1.3 (or 1.3a) for source devices providing the half resolution 3D signal formats.  While some home theater enthusiasts have used custom built/configured home theater PCs (HTPCs) as the 3D source, for this blog I have elected to address the more general case where off-the-shelf consumer 3D video sources are used.  

2-Way HDMI Splitter

Ideally there would be a 3D video processor that would have an HDMI 1.4a input, that would decode at least all of the mandatory 3D formats defined by the HDMI 1.4a standard, and provide separate HDMI outputs (HDMI 1.3 or 1.4a) that would deliver the individual right a left video streams with each being a standard 2D video stream with upto to 1080p resolution.  However, no such low cost consumer video processor is yet available (more information can be found in the next section below).  Therefore, it is necessary to use multiple boxes to accomplish these functions.  In the case of the 2-way HDMI splitter the essential characteristics are that it needs to be able to accept all of the mandatory HDMI 1.4a signal formats at its input and then provide two identical HDMI outputs while passing through the 3D signal formats (as well as the standard 2D signal formats).  Such HDMI 2-way splitters are available from a number of sources and can be purchased at relatively low cost.  The following link is for a 1 input and 2 output HDMI splitter that is the least expensive model (i.e., less than $50) that I have seen that has been verified to work for this purpose:

LINK:  Monoprice Pro Series Powered HDMI Splitter with 3D Support

Note if the only 3D source you wish to use is a Blu-ray 3D player, then you can eliminate the need for the HDMI splitter by using a Blu-ray 3D player that has dual HDMI outputs.  Such models are available from manufacturers such as Panasonic (i.e., models DMP-BDT310, DMP-BDT320), Oppo (models BDP-93 and BDP-95), and Samsung (models BD-D6700 and BD-D7900) as long the same video signal can be output via both HDMI connections.  Also if you are using an AV Receiver (AVR), to do your HDMI switching and audio decoding, and that AVR has dual HDMI 1.4a outputs (such as the Denon AVR-4311), then you may be able to eliminate the need for a separate HDMI splitter.

3D Video Processor (3D-to-2D Demultiplexer)

As mentioned in the above discussion, ideally there would be a single 3D video processor/demultiplexer box that would have an HDMI 1.4a input, that would decode all of the mandatory 3D formats defined by the HDMI 1.4a standard, and provide separate HDMI outputs (HDMI 1.3 or 1.4a) that would deliver the right a left video streams with each being a standard 2D format with upto to 1080p resolution.  However, currently the lowest cost solution requires two video processors (see the info below on a potential single processor alternative).  Therefore, the most effective solution is currently to use two identical 3D video processors with each decoding the incoming 3D video signal and one processor decoding/outputting just the right video stream and the other 3D video processor configured to extract and output just the left video stream.  Currently, what appears to be the most popular (and least expensive) 3D video processor (i.e., 3D-to-2D demultiplexer) that can be used for this purpose is the Optoma 3D-XL. While Optoma offers this product mainly for use with their 720p 3D ready DLP projectors, the 3D-XL also can be configured to decode the incoming 3D video and then output either the right or left video stream in 1080p signal format.  The 3D-XL is available from a number of on-line e’tailers for under $300 each. There are other more expensive alternatives for the 3D video processor.   The Lumagen Radiance Mini-3D video processor offers state-of-the-art video processing with a built-in test pattern generator and a comprehensive color management system (CMS) as well as 3D video processing.  Although these processors retail for approximately $2000 each, they are an viable alternative for those users that demand the maximum capability for calibrating their projection system for accurate colors and grey scale, in both 2D and 3D modes.  As with the Optoma processor described above, two of the Lumagen Radiance Mini-3D processors would be needed with a dual projector passive 3D setup. A 3D video processor/demultiplexer box priced similar to the Optoma 3D-XL is being offered by an on-line dealer in Australia.  The 3D Demultiplexer Box – CH-322 is priced at $299 (in US Dollars).  I have not seen any reports from owners of this box as to how well it works.  The dealer describes this box as a:

3D to 2D Demultiplexer Box system is designed to convert a 3D HDMI v1.4 video signal in ‘Side-by-Side’ (Left-eye/Right-eye) or ’Frame Packing’ (Top/Bottom) format, to an HDMI 2D v1.3 video signal compatible for use in applications using two HDMI 2D projectors or one HDMI 2D HDTV. A quality product perfect for converting a 3D video signal input from your 3D DVD or BluRay player to an HDMI 1080p@60Hz 2D video signal output. Ideal for quality movie viewing using your existing 2D HD equipment.  To use with twin 2D projector applications, 2 x 3D to 2D Demultiplexer Box’s are needed, one each for ‘Left-eye/Right-eye or ‘Top/Bottom’ 3D formats. An HDMI 1in:2out Splitter is also required to interface the 3D HDMI source to each 3D to 2D Demultiplexer Box. To use with a 2D HDTV application, simply connect 1 x 3D to 2D Demultiplexer Box between your 3D source and 2D HDTV using two HDMI cables.”

More recently there has been a single 3D processor solution announced by a company in Taiwan.  Although I have seen so reports from users with experience with this product, the VNS model G-501 3D video processor is described by the manufacturer as:

.... a standalone system that can automatic decode 3D signal from standard 3D signal sources, such as Blue Ray DVD or STB, and output RH/LH 3D signal to two projectors for passive 3D display. It can also support any other 3D signals from media player, website and PC with frame packing, side by side or top/down formats. Low cost polarized glasses can be used.”

I have not yet seen any dealers in the USA offering this product for sale.  The estimated price, when/if it becomes available in the USA, is approximately $1,000.  The manufacturer has indicated they are also working on a less expensive version, with fewer features, for release in the 2nd half of 2012.  


Background information on passive 3D projection, including polarized systems, can be found in Part 1 and Part 2 of this series of blogs on passive projection.  Selection of the ideal projector (from a performance and cost point of view) for use with a passive, polarized 3D projection setup is a complex topic that will require future blogs in this series to fully discuss.  For now I will just give an overview and point out some of the factors that make this a fairly complex subject. The ultimate goal is to have one projector/filter (i.e. projector equipped with an external optical filter) whose projected image is being polarized with one orientation and having the second projector/filter whose projected image is being polarized with an ‘opposite’ orientation.  If linear polarization is being used this means the orientation of the polarization is shifted 90 degrees between the two projected images.  This most commonly use arrangement will be with one orientation set to vertical and the other horizontal or with one orientation +45 degrees from vertical and the other –45 degrees from vertical.  The other form of polarization that can be used for 3D passive projection is circular polarization and in this case one projector/filter will project a right-hand circular polarized image and the other projector/filter will project a left-hand circular polarized image.  Regardless of the type of polarization used, when the lens of the 3D glasses worn by the viewer has the same type and orientation of polarization as the projected image the light will be passed thru the lens while if the glasses' lens has the opposite orientation the light will be blocked.  HERE is an informative Wikipedia article on polarization in general providing details on both linear and circular polarization. While at first glance it might appear that all one has to do is to add an appropriate polarizing filter to the front of the lens or each projector.  However, many projectors already produce polarized light and for these projectors it can be complex (or even impossible) to obtain the desired type of polarization that is needed for a dual project 3D setup.  Future blogs in this series will get into the details of this topic.  Therefore, for this current blog update, I will limit my discussion to other, more general factors associated with the selection of suitable projectors for a passive 3D system. For this current discussion I am assuming that two projectors will be purchased and will therefore the following discussion is intended to steer the reader to selecting projectors that are well suited for this application.

1. Select two identical projectors.  There are several reasons for this advice.  First you will want to have similar image brightness and image characteristics such as contrast, gray scale, black level, etc.  Second, different projector models will have different delays between when the signal is input into the projector and when the resulting image is displayed on the screen.  This delay comes from the video processing being done within the projector and it is important to have the displayed right and left images that make up the 3D pair closed synchronized in time.

2. When installing two projectors for a passive 3D setup the projected images should have near perfect alignment.  While pixel perfect alignment is not absolutely essential for an acceptable 3D presentation, the closer to pixel perfect alignment that can be achieved the better.  If the two projectors are to be located one above the other (i.e., stacked vertically) and are located directly back from the center of the screen (along the screen’s right-to-left center-line), then having a vertical lens shift feature on the projector will be necessary to achieve optimum alignment of the two projected images.  If the projectors are to be placed beside each other, or if a vertically stacked pair are located off of the screen’s center-line, then having a horizontal lens shift feature will be necessary to achieve optimum alignment of the two images.  Also note that projectors with lens shift may have a limited range of adjustment and this can constrain where the projectors can be mounted in order to achieve correct image geometry on the screen (the same as with a single projector 2D installation).  So my bottom line advice is when selecting projectors for a dual projector 3D passive setup, choose models with lens shift.  Some DIY hobbyists have used projector’s without lens shift, but the results will be less than ideal.

3. Select projectors that you will be satisfied with for 2D projection.  Let’s face it you will probably be watching a lot more 2D video in your home theater than 3D.  So select projectors that will provide the level of 2D performance you will be satisfied with (i.e., for such factors as contrast ratio, black levels, color accuracy, gray scale accuracy, etc.) and which have the mounting flexibility needed for your specific home theater installation.  This latter point involves selecting a projector with adequate zoom ratio, throw distance, lens shift adjustment range, and lumens of light output appropriate for your specific home theater setup (e.g., screen size and gain).  However, when you throw the additional requirements and constraints for use within a dual projector 3D passive setup into the set of requirements, this will likely rule out some projector models that you might otherwise consider for a single projector 2D installation.


For my next blog (Part 4 of this series) I will get into specifics on the use of different types of projectors for a dual projector 3D passive setup using polarization.

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