3D Projection Techniques
July 15, 2011 | 
Author Over the next several blogs I intend to get into more of the details of the 3D technologies now available for use by the consumer in a home theater. I specifically plan to cover the following 3D projection technologies and configurations:
- Use of a single 3D enabled projector that creates the 3D effect by sequentially alternating the right and left projected images and where the viewer must wear liquid crystal actuve shutter 3D eye-glasses that are synchronized with the projector. The synchronizing signal is usually send by the projector using an infrared (IR) emitter or a radio frequency (RF) transmitter. The IR emitter or RF transmitter can be either built into the projector or as an external accessory device.
- Use of a single 3D enabled projector that creates the 3D effect by projecting right and left images with a different light polarization orientation. These right a left images may be projected either concurrently or sequentially depending on the specific technology being used by the projection system. The viewer must wear passive polarized 3D glasses with this technology.
- Use of two standard 2D projectors each outfitted with a polarizing filter having a different orientation. This configuration requires use of an external processor to decode the 3D signal to send the right and left video streams to different projectors. The viewer must wear passive polarized 3D glasses with this technology. While a 3D system for a home theater can the assembled as a do-it-yourself project at moderate cost using this approach, the only currently available complete off-the-shelf commercial systems using this approach are from high-end manufacturers and their offerings are intended for very high-end home theaters.
- · A variation of the above two projector do-it-yourself 3D system is instead of using polarizing filters is to use filters both for the projectors and eye-glasses where the projected primary colors as slightly different for the right and left image streams.
For the remainder of this blog and probably for the next few blogs I will focus on the first of the above 3D techniques since that is what is being used with almost all 3D projectors currently being offered to the consumer.
As discussed in my previous blog, the 3D source device (using the industry standards applicable to that type of device) determines the resolution and frame rate of the 3D video that is provided into the 3D projector. Most Blu-ray 3DTMtitles are recorded at full 1080p resolution and at 24 frames per second (or 24 Hz) for each the right and left image streams. The lower resolution formats defined as mandatory in the HDMI 1.4a standard, may use frame rates as high at 60 Hz. My discussion here will focus on those “3D-Ready” projectors that have HDMI 1.4a input(s) and will accept all of the 3D signal formats required by HDMI 1.4a standard. Currently most of these projectors will process the incoming 3D signal and display the video at 60 Hz for each the right and left video streams regardless of the frame rate of the 3D video provided by the source device. This is essentially the same as with most standard 2D projectors, or other HD displays, that display all video at a fixed rate, which is most typically 60 Hz (or a multiple of 60) for HDTVs and Projectors sold in North America (50 Hz in some other parts of the world). With such 60 Hz displays, if the source device is providing the video at 30 Hz then each source frame may simply be repeated two times while if the source video is already providing video at 60 Hz then each source frame is displayed just one time.
However, for a 24 Hz source frame rate a technique called 2:3 pulldown (or sometimes called 3:2 pulldown) is used where one frame is repeated two times, the next frame three times, the next two times, the next three times, etc. This technique has been used for many years for displaying conventional movies (and all other programs shot on movie film at 24 frames per second) on TV systems that display at 60 Hz. This same technique directly carries over to 3D projectors that display each the right and left video streams at 60 Hz. The following illustration shows how a 24 Hz source video can be displayed at 60 Hz by applying 2:3 pulldown. This simple illustration is for a standard 2D display and assumes both the source and the display use progressive scan (i.e., as used with most Blu-ray discs and for most modern displays).
- Illustration of 2:3 Pulldown
However, not all projectors display 24 Hz 3D source material at 60 Hz. The current JVC 3D-Ready projectors (e.g., DLA-RS40) display 24 Hz 3D video, such as 1080p/24 from Blu-ray 3DTM, at 48 Hz per eye (96 Hz total) with each frame repeated two times (this is referred to as 2:2 pulldown). However, if the 3D video source provides the video at either 30 Hz or 60Hz then these JVC projectors will display the 3D video at 60 Hz per eye (120 Hz total) (i.e., by applying 2:2 pulldown to 30 Hz video sources). The disadvantage of using 2:3 pulldown for 24 Hz sources is this uneven 2-3-2-3 cadence of the displayed video produces a judder in the on-screen motion as compared to displaying each source frame the same number of times. Again, this is not a new issue brought about by the introduction of 3D displays. Rather it remains the same issue as we have long had with standard 2D displays. I have discussed it here just to help the reader better understand the relationship between the frame rates of the video source versus the frame rate at which the video is actually displayed.
Regardless of the rate at which the 3D video is being projected, the liquid crystal 3D active shutter eye-glasses worn by the viewer must be accurately synchronized to the projected image sequence with the respective right or left lens becoming transparent only when the image intended for that eye is being displayed and becoming opaque the remainder of the time. Accurate synchronization is necessary to prevent crosstalk, or ghosting, between the right and left images.
The following illustration shows the most basic case for such a 3D projection system used in combination with active shutter glasses. The result is only the projected images intended for the left eye are seen through the glasses by the left eye and only the projected images intended for the right eye are seen through the glasses by the right eye.
- Sequential 3D Display Illustration
Some consumer 3D projector manufacturers, such as Sony with their VPL-VW90ES 3D-Ready projector, advertise their projectors as operating at 240 Hz. However, the current generation of such projectors still display the individual right and left images 60 times per second because in 3D mode black frames are being inserted between each right and left video frame. This is done to reduce crosstalk (i.e. ghosting) by allowing addition time and helping to force one image (e.g., left image) to be fully replaced by the next subsequent image (i.e., right image) before that new image is presented to the viewer’s other eye.
The following illustration shows the case for a 3D projection system that is using “black frame inserting” between each active video frame. In this case the active shutter glasses is synchronized to keep the appropriate lens in the clear state which an actual video frame is being projected then become opaque during the black frames and other video frames intended of the other eye.
- Sequential 3D with Black Frame Insertion Illustration
So let us now consider how this related to different 3D projector display technologies. Currently all consumer 3D ready projectors using sequentially alternating images in combination with active shutter glasses are base based on either DLP or LCoS micro-display technology.
LCoS display devices are currently manufactured by Sony (under the SXRD trade name) and by JVC (under the D-ILA trade name). Sony supplies their SXRD display devices to a few other 3D projector manufacturers including Mitsubishi. JVC is supplying their D-ILA display devices for the soon to be released 3D projector from specialty vendor Wolf Cinema. Current and soon-to-released 3D-Ready projectors using LCoS technology (all with full 1080p resolution) include:
- JVC: DLA-RS40, DLA-RS50, DLA-RS60, DLA-X3, DLA-X7, DLA-X9
- Sony: VPL-VW90ES and the soon-to-be-released VPL-HW30ES
- Mitsubishi: HC9000
- Wolf Cinema: SDC-15 (announced but not yet shipping)
DLP projectors use Digital Micromirror Device(s), exclusively manufactured by Texas Instruments (TI), within their light engines. TI makes several models of DMD chips with resolutions of 720p and 1080p that are used by a number of projector manufacturers. Current and soon-to-be-released 3D-Ready DLP projectors (equipped with HDMI 1.4a inputs) include:
- Sharp: XV-Z17000
- Optoma: GT750 (720p, unofficial, soon-to-be-release)
- Optoma: HD33, HD3300, HD8300 (1080p, some unofficial, soon-to-be-released)
- Mitsubishi: HC7800 (1080p, unofficial, soon-to-be-released)
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Next time I’ll get down into some more details about the pros and cons of the technologies/technical approaches used by some of these different projectors for supporting 3D.
Posted in 3D Sources, 3D Technology, Projectors | 
Tags: 3D Projector, 3D Technology, Active Shutter 3D Glasses, HDMI 1.4a, Projector Reviews blog
Your tech teaching is great. I would like to know more about an outboard processor I.e. the home grown system. The idea of buying one additional Epson and the processor sound great and might produce the best/brightest picture for much less then a new 3D 8k projector.
Can you provide some additional insight into the outboard processor mfg.
Thanks
Derrick
Optoma mades a 3D processor box, model 3D-XL. HERE is the link to the manufacturer’s web site. It is sold thru Optoma dealers, some of which have ads on this web site. The 3D-XL can be configurated so that the user can have it decode either the right of left video stream from the incoming 3D signal (via the 3D-XL’s HDMI 1.4a input) and then send that decoded video out via HDMI 1.3 to a connected projector. However, for a 2-projector 3D setup you need two of the these Optoma processors (one per projector with one set to decode the right and the other the left video stream). You also need to add a 3D capable HDMI 1-input to 2-output splitter ahead of the two Optoma 3D-XL boxes. These are inexpensive from sources such as monoprice and amazon.
Now the bad news is Epson projectors are not ideal for a 2 projector passive, polarized 3D setup. This is because LCD projectors inherently have internal polarizing elements as part of each of their 3 LCD imaging chips. For Epson projectors the orientation of the projector’s green polarizer, and thus the projected green light, is shifted 90 degrees from the red and blue polarizers which is not an ideal situation for using these projectors in a 2-projector 3D setup. Some hobbyists have added external linear polarizing filters with the external polarizing filter attached to one of the projectors having its orientation rotated 45 degrees colorwise from vertical and the external filter on the other projector rotated 45 deg. counter clockwise. However, this not only results in some light loss due to the offset of the orientation of the internal and external polarizing filters, but also the resulting projected images have some color shifts and non-uniform colors across the screen. The level of these color issues may be objectionable to some people while others may be able to live with it. But the bottom line is the Epson projectors are not the ideal starting point for putting together a 2-projector 3D setup, so unless you already own the two Epson projectors I suggest you investigate other projector alternatives.
Also note that with any passive 3D projection system using polarization as the means to seperate the right and left images, you must use a projection screen that maintains a very high level of polarization. These are typically called ‘silver screens’ and are designed specifically for polarized 3D projection systems.
I plan to have some future blogs on do-it-yourself two projector 3D setups. But that is probably more than a month away.
Thank you for taking the time for an in-depth explanation of a do-it-yourself 3D system. I now understand the complexities of 3D for the home. I will stand-by for your review/article describing a good or great setup using two projectors.
High end 3D is very expensive with pricing starting in the mid 20K range. The gap between current 3D entry projectors and the next step up is very wide both in performance and cost, unlike 2D. It will be interesting to see if any manufactures will soon offer a 2 projector setup that essentially reduces the great divide in performance yet only marginally more costly then what JVC, Sony etc.,currently offer as a single projector 3D system.
Thanks for the reply.
D
Hello Ron,
Very much looking forward to your blogs on do-it-yourself two projector 3D setups. I have been doing 2 projector passive setups for over 20 years with film projectors and am doing my research to move to digital now. A big issue is light loss, with typical polarization set up reducing light received by the viewer to about 16% of what would be received without polarization. So if you want 650 lumens, for example, you need to start with a little over 4000 lumens (not rated, but actual).