How 3D content works: Blu-ray vs. broadcast

Not all 3D is the same. Knowing how to get the best 3D content for your TV will ensure your TV looks its 3D best.

How 3D Content Works
Geoffrey Morrison: Image, Screenshot: ESPN 3D

As 3D content becomes ever more available, the differences in how it works become important. Just like all HD isn't the same, all 3D isn't the same either.

More than a simple "how it works," this guide will help you understand why some 3D looks better than others.

How 3D works

As I'm sure you've already read, 3D TVs work by sending each eye a slightly different image. The latest generation of TVs and 3D Blu-ray players can create a fake a 3D image from a 2D source, but the most "realistic" 3D comes from original 3D content. This means shows/movies that were shot in 3D originally, and transmitted in a 3D format.

3D Blu-ray

The main source of high-quality 3D is via 3D Blu-ray discs. There are a growing number of titles, though these still number in the dozens, not hundreds. Worse, many of the most desirable titles are brand exclusives. So in order to get "Avatar" in 3D, you need to buy a Panasonic product. To get "How to Train Your Dragon" you need to buy a Samsung product. This is staggeringly stupid, but that's a rant for a different article. Keep in mind, though, that these disc still work on any 3D BD player, its just they're only available with gear (or at outrageous prices on eBay).

So how does 3D on BD work? The logical assumption is that each eye's content is encoded on the disc sequentially (the frame for the left eye, then frame for the right eye), just as how its shown on the TV. This is not the case.

All Blu-ray 3D content is encoded at 1080p/24. However, the two frames are stacked on top of each other. In a rare example of simple nomenclature, this is called frame packing. It looks like this:

Full HD 3D
With 3D on Blu-ray, each eye gets its own 1,920x1,080-pixel image. These are combined into one extra-tall frame, of which there are 24 per second. 45 pixels of blanking are included (the black space in the middle), resulting in a 1,920x2,205/24 signal. Geoffrey Morrison: Image, Screenshot: Dreamworks

Because of extra data included with the two frames, the total amount of bits is slightly more than if it were a true 1080p/48 (1080p/24 x 2) signal. It is, as you see above, a 1,920x2,205p/24 signal. Because of this, a theoretical full resolution 1080p/30 3D signal would require greater bandwidth than 1080p/60. True, the HDMI spec can handle this extra bandwidth, but that doesn't mean the current generation of HDMI transmitters/receivers can. In order to get 3D to the market in a reasonable amount of time, the choice was made to limit the frame rate to 24p (nearly all movies), rather than incur the extra development time and money on chips that could handle an increased bandwidth to handle higher 3D frame rates. So it goes.

This isn't a major issue, though, as there is very little native 1080p/60 content, and no 1080p/60 3D content that you're missing out on. At least, none that would be on Blu-ray.

The most important thing to understand is that because 3D from Blu-ray is actually less data than 1080p/60, you probably don't need some "3D-capable" HDMI cable to get 3D to work. It's very likely your current high-speed HDMI cable will work. All the ones I have do. Check out my article on why all HDMI cables look and sound the same for more on this and why cheap HDMI cables work great.

Thanks to the efficiency of the video codecs on Blu-ray, even though double the amount of data is shown from the TV, it doesn't take up double the amount of storage on the disc. More than 2D, but not double.

3D Cable and Satellite

Several cable and satellite providers are offering dedicated 3D channels, and/or 3D pay-per-view. The bandwidth available to these providers is significantly less than what's possible with Blu-ray. Most providers max out at 1080i, though 1080p/24 is available from some (as it requires less bandwidth than 1080i). Because of this, the frame stacking method is not possible.

Instead, the resolution is reduced so that the different content for each eye can be squeezed into a single frame. There are two main methods for this, Top and Bottom, and Side by Side. They look like this:

Side by Side
This is an example of what a frame of Side by Side 3D looks like. Each eye gets a 960x1080 or 640x720 image. This is stretched horizontally to fill the screen. Geoffrey Morrison: Image, Screenshot: ESPN 3D
Top and Bottom
This is what Top and Bottom 3D looks like. Each eye gets a 1280x360 image. This method is not used with 1080i, though a 1080p/24 version would result in a 1920x540 image per eye. Geoffrey Morrison: Image, Screenshot: ESPN 3D

The Side by Side method halves the horizontal resolution, so each eye gets a 960x1,080-pixel resolution frame in the case of 1080i, and a 640x720-pixel frame with 720p.

With Top and Bottom, you get 1,280x360-pixel (with 720p) resolution images. There is no Top and Bottom with 1080i, as the interlaced fields are already 1,920x540 pixels and you wouldn't want to halve that again. A 1080p/24 signal is possible in the current broadcast spec, and with this you'd be getting the aforementioned 1,920x540.

In both cases, the TV scales (stretches) the signal to fill the screen, ideally without distortion and minimal artifacts, the same way it would if the signal was from a DVD

Side by Side v. Top and Bottom

With broadcast 3D you're getting less resolution than with 2D. In theory, the Top and Bottom method will look better with sports, as you have more horizontal resolution. But it's actually more complicated than that. As you can see with the numbers above, if you halve 720p in either direction, it's not much higher resolution than DVD. ABC and Fox, and all their related sports programming (including ESPN), made the decision in the early days of HD to go with 720p, as they felt it looked better with fast motion and therefore sports. That means that now they're looking at either having to shoot in 1080i, or supply their viewers with 3D content that's little more than 15 percent better than DVD.

Remember, your cable/satellite box likely defaults to output everything at 1080i, but that doesn't mean that the original source is 1080i.

It's unlikely any cable/satellite provider will increase their bandwidth in the near future to allow for Full HD 3D. So we're left with low resolution images with broadcast 3D.

The passive 3D wrinkle

Vizio and LG are pushing passive 3DTVs. These use inexpensive polarized glasses and an added component on the TV's screen to ensure each eye gets the correct data. The pros and cons of passive vs. active are largely left to another article, but one aspect is relevant to this one: All current passive 3D displays supply each eye with half the vertical resolution. In other words, each eye is seeing 1,920x540 pixels. All the odd numbered lines go to one eye, all the even lines to the other. Check out Active 3D vs. passive 3D: What's better? for more info.

But what if you're watching Side by Side from cable/satellite? Now you're getting 960x540 per eye, and that's not really HD at all. And that's with a 1080i feed. If you're watching 720p 3D you could potentially be getting 640x360 per eye, barely better than VHS.

The problem is, LG claims their TVs are full HD resolution with 3D, though it's debatable if this holds up in testing. Other companies (notably, Samsung) claim LG's 3D TVs are half resolution. Both are pushing their own agenda, and truth is not likely the top factor in either case. At the moment we on the press side are trying to sort through it all and come up with testing procedures to test the validity of LG's claims. Stay tuned.

Conclusion

3D on Blu-ray is the only way to get full-resolution images for each eye. Cable and satellite 3D content is handicapped by their limited bandwidth, and will therefore have softer images, likely with artifacts like jaggies (depending on many factors, not least the TV).

It is true that we are still in the early days of the 3D TV evolution, but the cable/satellite hurdles are exceedingly expensive to remedy, and are unlikely to happen with such a small installed base of 3D TVs.

You don't have to take my word for it:



Got a question for Geoff? Click "Geoffrey Morrison" below, then click the E-mail link in the upper right to e-mail, wait for it...Geoffrey Morrison! If it's witty, amusing, and/or a good question, you may just see it in a post just like this one. No, I won't tell you what TV to buy. Yes, I'll probably truncate and/or clean up your e-mail. You can also send me a message on Twitter: @TechWriterGeoff.

About the author

Geoffrey Morrison is a freelance writer/photographer for CNET, Forbes, and TheWirecutter. He also writes for Sound&Vision magazine, HDGuru.com, and several others. He was Editor in Chief of Home Entertainment magazine and before that, Technical Editor of Home Theater magazine. He is NIST and ISF trained, and has a degree in Television/Radio from Ithaca College. His bestselling first novel, Undersea, is available in paperback and as an ebook on Amazon, B&N, and elsewhere.

 

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