What Does "3D Ready" Mean?
Dispelling the Myths about 3D Projection
August 19, 2010
If you are like most consumers, you think "3D Ready" means a projector is ready to display 3D in all its various forms. Well, that's not quite true. Many 3D Ready projectors are available right now, as you read this article, but 3D is still a confusing subject. What 3D signal sources will work with your 3D projector? What do you need to know to make sense of all of this stuff? After finishing this article, you will know exactly what 3D is and how it works.
Editor's note: This article has been updated to reflect the current state of 3D projection as of February 2012. -bl
What is "3D Ready?"
To differentiate 3D projectors from their 2D brethren, there is usually a logo on the case declaring their 3D capability. But what does this logo actually mean? In short: a 3D Ready projector will accept and display at least one stereoscopic 3D transmission format. By transmission format, we mean the 3D signal format used to transmit from your source--a computer, set-top box, game console, or Blu-ray player--to your projector. We are not talking about the difference between passive polarized 3D glasses and active shutter 3D glasses (if you need more information about these display technologies, see our article "The 3D Renaissance").
3D Transmission Formats
At last count, there are at least four stereoscopic 3D transmission formats currently in wide use, called frame sequential, frame packing, side-by-side, and checkerboard. There are other transmission formats as well, but we will focus on the four main formats for now.
Frame sequential. Frame sequential, also occasionally called page-flip, is in some ways the simplest of the 3D formats. A frame sequential signal is a full resolution picture sent at 120 frames per second to the display. The frames alternate in sequence, so the display receives a left eye frame, then a right eye frame, then a left eye frame, and so on. This is simple because the projector itself does not need to do any decoding of the source; it just needs to be capable of accepting a 120Hz signal. Correspondingly, this format requires a lot of bandwidth, since it is essentially sending a full resolution signal at 60 frames per second for each eye. This is double the bandwidth of a comparable 2D signal.
In the world of projectors, frame sequential is an important format. Today's inexpensive DLP projectors that are touted as "3D Ready" accept only frame sequential 3D. And at this writing, their 3D capability is limited to a maximum of 1280x720 resolution. Currently, the only way to send them such a signal is to use a computer, such as one equipped with NVIDIA's 3D Vision system. Consumer electronics like Blu-ray 3D players and set-top boxes do not output frame sequential 3D. In short, all those inexpensive DLP 3D Ready projectors you've been seeing do not work with Blu-ray 3D or broadcast 3D content--it's PC or bust.
Frame packing. Frame packing is closely related to frame sequential, but they are not the same thing. Frame packing sends the left and right eye images to the projector simultaneously, stacked on top of one another with a small space between them. Essentially, the source sends one giant double-height frame instead of two smaller frames. The signal is transmitted at either 24Hz or 60Hz. The projector must then separate the two images and display them sequentially.
Frame packing is the default format used in the HDMI 1.4 specification, and any product labeled as HDMI 1.4 compatible must support this format. It is the standard output format of Blu-ray 3D players, though some have additional options. Frame packing requires more processing power on the part of the projector, since it must separate the two frames and then display them in sequence.
Side-by-side. In the side-by-side transmission format popularized by DirecTV, two frames are compressed to half of their original horizontal resolution and sent simultaneously. For a 1080p signal, which is 1920x1080 pixels per frame, this would be two 960x1080 frames side by side. The projector then separates these compressed frames, expands them back to their original 1920x1080 format, and displays them sequentially. Side-by-side comes in both interlaced and progressive variants, with interlaced taking up less bandwidth and progressive being higher in image quality.
As you might imagine, this format loses some resolution in the process of compression and subsequent expansion. Essentially, it leaves you with half resolution to each eye. At this writing, DirecTV is the only game in town using the side-by-side format, but it should be compatible with newer (2010 model) 3D televisions and current DirecTV HD boxes. Older 3D televisions probably will not be able to display this format, and the inexpensive DLP "3D Ready" projectors that have been brought to market thus far cannot display it either.
Checkerboard. Many DLP 3D Ready televisions (not projectors) accept what is called the checkerboard format. In this format, the two images for left and right eye are interleaved, with every other pixel going to the opposite eye. Look at an actual checkerboard and pretend the squares are pixels. The black squares would go to the left eye, while the red squares would go to the right eye. The television separates the two interleaved images and displays them sequentially. The resulting images are half-resolution.
Why do you need to know this, since projectors do not support this format? Well, checkerboard is important for its legacy status. Older DLP 3D Ready televisions would accept checkerboard and nothing else, and many of these televisions were sold in the past few years. When consumers discovered that their televisions were not compatible with broadcast and Blu-ray 3D formats, they were understandably incensed. The solution came in the form of converter boxes that are able to convert frame-packed or side-by-side 3D to checkerboard TV for display on DLP televisions. This is important because DLP 3D Ready projectors cannot display checkerboard 3D. If they could, it would be a simple matter to buy a conversion box and live happily ever after. However, converter boxes that change frame-packed or side-by-side 3D into frame-sequential 3D are not available, and the converter boxes for televisions output checkerboard 3D and nothing else.
A note about HDMI 1.4 One of the important things included in the HDMI 1.4 standard is a list of 3D transmission formats that must be supported by any device claiming 1.4 compliance. The catch is that a non-HDMI 1.4 device can still support these transmission formats. An excellent example is Sony's Playstation 3 game console, which can play 3D games and Blu-ray 3D movies even though it is an HDMI 1.3 device. Some projectors may have HDMI 1.3 yet be able to decode frame-packed 1080p 3D. To determine a projector's compatibility with modern 3D transmission formats, you need to look beyond the bullet points on the spec sheet and find out what transmission formats it is actually compatible with.
The currently available, inexpensive DLP "3D Ready" projectors are good for a lot of applications. For gaming, nothing beats the big-screen experience of 3D through a projector. In education, they can be used to display diagrams of three-dimensional shapes and objects, from electronic dissections to statues in Art History courses. But as far as home video is concerned, they have some serious limitations. They are incapable of displaying frame-packing and side-by-side, the two most popular and important 3D transmission formats for video. While most of these products are designed as data presentation projectors, people have been buying them in the hopes of using them for home theater anyway. Without support for the right formats, you will find yourself purchasing another 3D projector in the future once support for these formats is incorporated. As in all things, caveat emptor--let the buyer beware.
If you want full 1080p 3D projection, look for one of the many HDMI 1.4 compatible full HD 3D projectors available. Prices start at $1499, with a notable cluster around the $2500-$3500 mark. These projectors will indicate somewhere in their specifications that they are HDMI 1.4 or Blu-ray 3D compatible, and that's your cue that they are safe to purchase.
Looking forward: the future of 3D projection
3D has changed a lot since 2010 when we first published this article. These days, it is more common to find a projector that is full HD 3D compatible than it is to find one that is only 3D Ready. Blu-ray 3D is already well established, and a wide selection of movies is making its way to market slowly but surely. Several satellite and cable providers will occasionally show 3D programming, as enough of their subscribers own 3D displays that it is worth their time to do so. As long as you keep your head about you and make sure you know exactly what you're buying, in-home 3D can be a rewarding experience.