On DMDs, screen-door e‰ffects and cre­at­ing 4K...

Sound+Image - - Test - SD

In the body of this re­view I in­di­cated that the prob­lem with much pixel shift­ing tech­nol­ogy is that the pix­els are just too big. If a pixel is shifted a half-pixel width to its right, most of its left side will over­lap the po­si­tion where it pre­vi­ously was, and most of its right side will over­lap the pixel which was pre­vi­ously to its right. (The pre­vi­ous pix­els are no longer be­ing dis­played, but the per­sis­tence of vi­sion in your eyes will make it look like they are still there. DLP pro­jec­tors do ev­ery­thing over time, rather than si­mul­ta­ne­ously, and can only work due to this prop­erty of hu­man vi­sion.)

Yet with this pro­jec­tor, as our test photo (left) shows, there is clearly res­o­lu­tion down to the UHD level, even if it’s not all that clean. Let’s try to ex­plain this at the tech level.

The Dig­i­tal Mi­cromir­ror De­vice used in this pro­jec­tor ap­pears to be the Texas In­stru­ments DLP470TE, a 0.47-inch model with 1920 x 1080 res­o­lu­tion. It has a pixel pitch (the dis­tance from an edge of one pixel to the equiv­a­lent edge on the next) of 5.4 mi­crome­tres, or thou­sandths of a me­tre. As it hap­pens, this ap­pears to be a ‘cut down’ ver­sion of the DLP660TE DMD, the 0.66-inch ‘4K’ model, which also has a pixel pitch of 5.4•m. (It doesn’t re­ally have 4K’s worth of pix­els, but 2716 by 1528 pix­els, which it dis­plays twice.)

The DLP470TE chip is newly de­vel­oped; in­deed it isn’t yet (as we write) fully listed on the site of the man­u­fac­turer Texas In­stru­ments. But it shares its size with the 1080p DLP470LE, while TI’s other 1080p DMD model is the DLP650NE, a 0.65-inch model with a pixel pitch of 7.6 mi­crome­tres, nearly 41% larger.

Which brings us to the ‘screen door’ ef­fect. Since 1080p pro­jec­tors ap­peared, this hasn’t been much of an is­sue in home theatre. But with 720p and ear­lier, some­times the grid pat­tern of the pix­els would be vis­i­ble, with thin black lines around each pixel. (The name came from the sim­i­lar­ity with look­ing through fly wire.) These days the pixel den­sity is so high one re­ally can’t see the in­ter-pixel bound­aries un­less un­re­al­is­ti­cally close to the screen.

Now con­sider what hap­pens when you use a 0.47-inch DMD at 1080p in­stead of a 0.65-inch one. The pix­els are smaller of course, but are the spa­ces be­tween them smaller as well? Re­mem­ber, a 1080p Dig­i­tal Mi­cromir­ror De­vice has on its sur­face more than two mil­lion tiny mir­rors. They are slightly un­der 7.6•m wide for a 0.65-inch DMD, and slightly un­der 5.4•m wide for a 0.47 inch DMD. Are the gaps less wide? My guess is no, or al­most no. Some clear­ance tol­er­ance must be pro­vided, and there’s lit­tle rea­son to think the tol­er­ances could be re­duced. If a pow­er­ful enough lens were used to dis­play the im­age at 1080p to the same size as with the larger DMD, it would likely have a more not­i­ca­ble screen-door ef­fect.

That prob­lem at 1080p lends it to ef­fec­tive use with pixel shift­ing, since it re­duces over­lap be­tween stan­dard and shifted pix­els. The pix­els are smaller, which is just what you want.

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