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LCoS 3D Projection Technology - Part 1

For this blog I will focus on 3D projectors based on Liquid Crystal on Silicon (LCoS) technology for which the viewer must wear 3D active shutter glasses.  As discussed in my previous blog, this type of 3D projector alternates the display of right and left images that are synchronized with the liquid crystal shutter lenses of the viewer’s 3D glasses such that the right eye only sees the stream of images intended for that eye (i.e., the right image stream) and the left eye only sees the left image stream.  LCoS is one projection technology that can be used to create such a 3D projector. For a high-level overview of LCoS projection technology see THIS WIKIPEDIA article.  Current consumer 3D projectors use LCoS micro-display chips manufactured by either Sony or JVC.  Sony calls their variation of LCoS technology “Silicon X-tal Reflective Display” (SXRDTM) where the term “X-tal” is an abbreviation for “crystal”.  JVC calls their variation of LCoS technology “Direct Drive Image Light Amplifier” (D-ILATM).  Nearly a year ago Epson announced plans for a line of projectors using “Reflective 3LCD” micro-display chips, which is essentially their variation of LCoS technology (see Art’s blog from September 2010).  LCoS technology differs from conventional liquid crystal display (LCD) technology in several important ways including:
  • LCoS displays are reflective while conventional LCDs, as used in projectors, are transmissive (i.e., require the light source behind the display)
  • LCoS display chips contain active electronics directly on the chip behind each pixel to control that pixel.  This allows LCoS displays to have a higher fill factor than LCD.  This means on LCoS displays greater than 90% of the display surface is occupied with active pixels, with less than 10% appearing a black space between pixels.  By comparison LCD’s are limited to about a 70% fill factor.
The result of these (and a few other) technical differences, the better implementations of LCoS produce projected images with a higher native dynamic contrast ratio (i.e, with deeper blacks) and with a smoother, more film-like appearance, as compared to what is possible with LCD based projectors. The latest implementations of LCoS are capable of superior display response times as compared to what is possible with current LCD-based projectors.  By “display response time” I am talking about how fast what is being displayed by a given pixel can be accurately changed from one value to the next value.  For example, from black-to-white or from dark grey-to-light grey.  When displaying conventional 2-dimensional video an excessively long response time can be seen on the display as some smearing of objects in motion or, if really bad, by as a trailing image behind objects in motion.  This was especially common with some early LCD flat panel displays.  When we try to apply these technologies to the projection of 3D video the display’s response time becomes more critical than for 2D projection.  This is because the alternating right and left images must each be clearly displayed without being impacted by the contents of the immediately prior image.  Otherwise 3D crosstalk or ghosting will result. Generically LCoS display chips are fabricated with a top layer of glass and under that a liquid crystal layer, a reflective layer, a light blocking layer, and finally a silicon layer (substrate) that contains the electronics that controls each pixel. All devices using liquid crystal technology rely on polarization of the light.  In the case of LCoS (and LCD) projectors, their light engines must incorporate polarizing filters and the light coming from these projectors will normally be polarized.  Likewise the 3D active shutter glasses, that viewers of this class of LCoS projectors must wear to view 3D, incorporate a polarizing filter element in order for the liquid crystal lenses of glasses to be able to alternative between dark (opaque) and clear states.  For a general discussion of how LCDs work see THIS WIKIPEDIA article.  The class of 3D projectors now being discussed do not utilize a different polarization of the right and left images streams as the means of creating the 3D effect.  However, consumers should be aware that the projected light is polarized, with a fixed orientation for its polarization, as this may influence the section of a projection screen and also compatibility with third party “universal” 3D active shutter glasses.  This will be discussed in more detail in a future blog. Now let’s consider more specifics for how LCoS is used with 3D projectors based first on SXRD and (in my next blog) D-ILA based light engines. SXRD based 3D Projectors Any projector using SXRD technology is using the Sony manufactured LCoS display chips.  Currently there are 3D projectors sold by Sony (of course), Mitsubishi, and LG that use SXRD display chips.  The following discussion will focus on Sony’s own 3D projectors, but most of the following observations should also apply to Mitsubishi’s SXRD based 3D projector (model HC9000D).  However, the current LG 3D projector (model CF3D) takes a substantially different approach to 3D projection, by essentially integrating two SXRD based light engines into a single projector supporting passive 3D using polarization (this will be discussed in a future blog). Sony introduced their first SXRD 3D projector in late 2010.  As can be seen by Art’s review of the Sony VPL-VW90ES, as a first generation 3D projector it does some things well (e.g., 2D projection) and some other things (3D image brightness) that will likely be improved with future generations of 3D projectors from Sony.  Also the first generation Sony 3D model does provide visible crosstalk (i.e., ghosting) with certain 3D video source material.  How objectionable this is becomes a very subjective judgment, but it is a performance area that is expected to improve with future SXRD based projectors. Sony is introducing their second 3D projector in August with an entry-level model VPL-HW30ES.  While this new lower-end model appears to be a 3D enabled successor to the currently 2D-only VPL-HW20 or VPL-VW Pro1 (i.e., it is not a replacement for the VPL-VW90ES), it does represent the first of the second generation of Sony 3D projectors.  While the 2D performance of the VW90ES will be clearly superior to the VPL-HW30ES in areas such a black levels and contrast ratio, Sony’s press release for the VPL-HW30ES indicates there are certain 3D related areas where this lower priced model will have better performance than its more expensive sibling.  Finally, it appears that from several unofficial sources that Sony is prepared to release a new model VPL-VW95ES later in 2011, but no official announcement is expected until the IFA and CEDIA trade shows in early September.  By some unofficial reports posted on the web, the VPL-VW95ES will be 3D enabled successor to the VPL-VW85 (rather than the successor to the current flagship model VPL-VW90ES) and a new flagship model (i.e,, replacing the VPL-VW90ES) will be coming out later. In my July 15th blog I presented diagrams illustrating sequential 3D display timing between the projected image and the operation of the 3D shutter glasses.  In one case I depicted the case where black frames were inserted between each projected right and left image.  Sony uses a somewhat modified version of this technique with their 3D SXRD projectors.  Below is a 3D timing diagram taken directly from the Sony (Japan) press release for the new model VPL-HW30ES projector. The above figure may tend to be little confusing to the reader as to what is really happening because it shows snap shots of what being displayed at 1/720 sec. intervals - ie., not at the 240Hz refresh rate.  I have noted where this is the case on the right side of the above illustration.  Sony’s LCoS implementation does not update the full image at one instant.  Rather it requires time ‘draw’ the new image.  In 3D mode the Sony SXRD projector displays one image for 1/240 second then uses the next 1/240 second interval (or 3 of the 1/720 second snapshots in the above illustration) to clear the previous image and to draw the next image.  During this second 1/240 second interval both lenses of the 3D shutter glasses become opaque thus blocking the screen image from both eyes. Thus with the 240Hz refresh rate a given eye sees one frame for approx. 1/240 sec. then sees a black screen (i.e., blanking) for the following three 1/240 sec. intervals.  This means that each eye sees 60 actual video frames per second in 3D mode.  Thus the sequence for both eyes is (each 1/240 sec. long): ....Right View - blanking - Left View - blanking - Right View - blanking - Left View -...... Or for just the Right Eye the sequence is: …Right View - blanking - blanking -blanking - Right View - blanking - blanking - ..... Or for just the Left Eye the sequence is: …blanking - blanking - Left View - blanking -blanking - blanking - Left View -..... With such a scheme each the right and left image streams are only being fully displayed to the viewers 25% of the time.  Thus, there is a light loss of 75% due to just this scheme for the way in which 3D is being displayed.   There are also additional sources of light loss within the 3D active shutter glasses, as they never become totally clear.  As a result in 3D mode the image brightness (at the viewer’s eyes) will be less than 20% of what is possible with the same projector when used for displaying standard 2D video (i.e., and without the viewers wearing the 3D glasses).   According to the press release for the new VPL-HW30ES Sony has attempted to make a number of improvements to increase the image brightness in 3D mode.  One of these techniques is to modulate the brightness of the projector’s lamp to provide increased brightness when actual video frames are being projected and decreasing the brightness during the blanking intervals.  This modulation of the lamp appears to be what is depicted across the bottom of the above Sony diagram.  Sony also claims to have reduced the light loss with a new generation of 3D active shutter glasses and perhaps by a small reduction in the blanking time between the actual video frames.  Preliminary information offered for the new HW30ES in Sony press releases and in other documents claim as much as a 2.7-to-3 times increase in 3D image brightness as compared to the VPL-VW90ES.  However, tests results published on-line by one European reviewer of a pre-production VPL-HW30ES showed similar brightness in 3D mode as for the VPL-VW90ES they had previously tested.  Sony also claims the level of 3D crosstalk (ghosting) has been reduced in the new VPL-HW30ES.  Art will have an upcoming full review of a production VPL-HW30ES here at Projector Reviews.  So until the results of that review are available it is best to hold off any judgment on how successful Sony has really been with improving the 3D performance with this second generation 3D model. The current Sony VW90ES as well as the upcoming VPL-HW30ES offer the option for video processing to do motion interpolation in 3D mode, as well as in 2D mode.  For an overview of video motion interpolation see THIS WIKIPEDIA article.  Sony calls this feature “MotionFlowTM”.   Many film lovers do not like the “soap opera” or “live video” effect created by motion interpolation, especially when viewing 2D movies.  However, since the whole idea behind 3D is to pull the viewer into a virtual reality 3D world, the use of either higher frame rates for the original source material or to use motion interpolation to simulate higher frame rates can be an effective enhancement.  Some filmmakers clearly recognize the benefits of using higher than the standard 24 frames per second rate for 3D movies.  James Cameron has announced plans to shoot the two sequels to Avatar with a 48 fps or higher frame rate.  The rapidly increasing use of digital cinema projectors in commercial cinemas has opened the door to having movies displayed at higher frame rates and we will probably be seeing a movement to higher frame rates for future big screen 3D releases. These Sony 3D projectors are equipped with HDMI 1.4a inputs and support all of the 3D signal formats required by that standard.  Some VPL-VW90ES owners have reported compatibility issues in displaying certain of the "optional" 3D formats now being used by Directv for delivery of certain 3D channels and programming (e.g, ESPN 3D).  Most of these reports were from late 2010 and early 2011, and I don't know if the compatibility with the optional 3D formats being used by Directv has been addressed with more recently firmware updates, or not. My next blog will focus on the JVC D-ILA based 3D projectors.

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