Projector Reviews

3D Projection Techniques

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.