Sony's latest 4K TVs continue the rapid price erosion of the next-generation TV resolution.

The 55-inch XBR-55X850A will cost $3,499, and the 65-inch XBR-55X850A will go for $4,999 when they're available in October. They represent the least-expensive 4K TVs from a major manufacturer.

Sony also recently reduced the price of its step-up XBR-X900A series, which now cost $500 more at $3,999 for the 55-incher and $5,498 for the 65.

Aside from their different 3D technologies, the major appreciable difference between the two has nothing to do with the picture: the X900A has bulky built-in speakers on either side of the screen, where the X850A is a more traditional design.

Unlike the X900A's passive 3D system, the X850A utilizes active 3D. If you actually care about 3D picture quality that's potentially a bit of a disappointment, especially if the less-than-perfect active 3D of the W900A is any indication. Passive 3D combined with 4K delivered the best 3D I'd ever seen in my brief look at Sony's 84-incher.

Otherwise, the X850A and X900A seem pretty much identical judging from their specifications. UHD/4K in these TVs' (and most others') case means a pixel count of 3,840x2,160 -- four times as many pixels as today's 1080p TVs. The advantage, according to its proponents, is an even sharper picture. One problem, according to us, is that you'll have to sit very close, especially to a screen this small, to appreciate the difference (see our in-depth look at the 84-inch Sony for more). There are many other issues, too, to the extent that we currently consider 4K TVs pretty stupid.

To its credit, Sony is starting to address one big problem, lack of content, by selling a 4K video player. Today the company announced that the video download service for that player is now live, offering more than 70 feature films and TV shows. I also applaud Sony's decision to buck the "UHD" moniker in favor of the more established, and frankly catchier, "4K."

Aside from 4K, the XBR-X850A is packed with picture-quality enhancements, although unlike the 2012 XBR-HX950, it doesn't have full-array local dimming. On the other hand, its edge-lit display does include edge-lit local dimming (more info), which in Sony's implementation works better than just about any other example of its kind, judging from models like the W900A we liked so much.

The X850A also has what Sony calls "Triluminos Display LED backlighting." No, it's not the same as the three-color LED backlight of the vaunted XBR8 from 2008. Instead it delivers somewhat improved color response, judging again from our W900A review. Check out our in-depth look at the technology for more nuts and bolts.

The XBR-X850A also offers all of the features of the step-down sets like the R550A, including NFC and RVU (click through for an explanation). Beyond those fancy extras is a full helping of Smart TV, including the Sony Entertainment Network.