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Is there a reason nobody uses two dmds to boost contrast?

Prodigy 80 points

Replies: 16

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If a single-chip dlp projector employed a second dmd that followed after the first in a single light-path, and the second chip simply slaved a mirrored image of the first..wouldn't it multiply its contrast upon the first? You would lose light efficiency, but wouldn't need to complicate video processing since the dmds are performing the exact same actions, and you would easily gain an enormous boost in on/off AND ANSI contrast. It seems like a fantastic and inexpensive building solution for creating a better picture, but I've never seen the design mentioned in with the other TI dlp designs. I have a hard time believing something both so simple and with such potential would be left in the dust without a good reason. My question is; what am I missing? Is there a major flaw that kills the design or some other glaring reason this isn't being done?
  • Hi.

    it's an interesting idea. We do have design for having two controlers interface to DMDs, but our controller cannot talk to 2 DMDs at the same time. It's for bandswidth, mainly for cost issue.

  • In reply to Frank Xu:

    So it would always require two controllers for bandwidth reasons, but would otherwise be very much possible? It just seems strange to me that manufactures would spend so much into small design tweaks, better optics, more specialized DMDs and other expensive parts/designs to squeeze out tiny contrast improvements when they could simply align a second inexpensive DMD+controller and receive a 100-TIMES contrast increase. A 1000X increase should be possible with the better DMDs. You could cheapen several aspects of any design and still come out quite a ways ahead in performance. Think of the improvement you could make for off-sets and zoom-range and lens-angles with high enough contrast that you can afford to throw some away for greater flexibility AND still have an impressive contrast ratio leftover! Is there anything TI can do to help this along? A single(double)chip dlp with the placement flexibility of an LCD, on/off contrast exceeding LCoS, with all the motion and 3D benefits of DLP..and it could be made with cheaper components and higher profit margins. A lower price moves more units and each unit moved sells TWICE as many TI DMDs. The consumer, PJ manufacturer and TI all profit..everybody wins! Is there anything holding this back aside from the comparatively small price of a second DMD/controller and a fairly straightforward design change?
  • In reply to dus java:

    The concept here is not new. Somebody has done it before. DLP Cinema has taken this approach before and indeed showed very impressive contrast. However, For mainstream applications, it is too expensive (2 dlp chipsets + optics) and too inefficient (~68% efficiency per dmd) to be practical.

  • In reply to Frank Xu:

    Would a consumer be willing to trade a 32% brightness loss (minimally more than the loss from eco mode) in exchange for going from 1000:1-2000:1 up to 100,000:1-200,000:1? The price of high-end parts currently used to achieve 5000:1-8000:1 contrast at a solid profit margin dwarfs the comparatively small $800or less increase to make a 100,000:1 single-lightpath device. Hometheater screens up to 135" only require 800 lumens to reach 14ftL, and a 110" screen only requires 500lm. People seem more than willing to pay $3000 more for a 50% brightness loss and 10X contrast gain over a $1000-1500 DLP; it happens every time somebody chooses a JVC or Sony LCoS. Tell a consumer they can instead pay $800 more, lose 32% brightness and gain 100X contrast by staying with a DLP device and anyone would be pleased.
  • In reply to Frank Xu:

    Thanks for the information so far. It's understandable that years ago such a design wouldn't make sense when brightness was hard to come by, DLPs led the pack for contrast, and a watchable unit already cost $8000-10,000. But now there's brightness to spare, $2500-4500 LCD and LCoS units are slaying DLP contrast levels, and parts prices are allowing for $500-800 DLPs that rival the high-end of old. If all it takes is an old design, $500-800 more parts on top of an $800machine, and the will the try it again to create a 800lumen, 100,000:1CR, $1,600 device.. It looks like the old design has become relevant. What other hurdles are there? I have to guess there are more reasons this hasn't popped back up.
  • In reply to dus java:

    The two DMD approach you mentioned is actually built into product by Zeiss planetarium http://www.zeiss.com/planetariums/en_de/products/products/velvet.html.

    Any customer would like to have this design, we certainly will support it.

  • In reply to Frank Xu:

    Are there any particular licenses or rights that need to be bought/rented for others to use that design or a very similar one?
  • In reply to dus java:

    By all means, I want a consumer two-chip DLP to become a reality. As Dus Java pointed out it's a much more efficient upgrade to a DLP than basically everything else including a change of a light engine (LED, laser). Like was already pointed out it's a win-win situation for everybody. Frankly speaking, without this upgrade I can't see how DLP can compete with LCoS projectors in the middle price market (2500$-8000$) in the nearest future.

  • In reply to Elix Berd:

    I first thought two DMD's in series would be too complex/expensive because of the off-axis light path, but might it be as simple as 45 deg bounces off of two DMD's that are 90 deg from each other, with the rest of the optical path the same as before?

    I'd love to have a DMD projector for the simplicity and robustness, but the contrast just doen't cut it; I'm on my third JVC for 

    The new JVC's with dynamic iris give real-world contrast >100k:1 which really puts the nail in the coffin for DLP for me.

    I'm surprised to hear DMD's have only 68% reflectivity; is that typical or does something about using two lower it?

  • In reply to Noah Katz:

    Another thing: I wonder how close to squaring the CR (contrast ratio) two DMD's would actually give.

    I've long wondered how much of the CR is determined by diffraction off of the mirrors' edges, i.e. what would the CR be if the rest of the light were perfect.

    Frank, are you able to share that info?

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