Digital Home Thoughts: The HD TV Evolution: Part 6 – Resolution Wars

The Interlacing Question
Now, there is one technical aspect of 1080 displays that does need addressing - interlacing (and de-interlacing). Two aspects of it:

First, most folks don’t realize that all fixed-pixel HDTVs are, internally, 1080 displays. Every last one. LCDs, rear-projectors, even ED Plasmas. They have to be in order to properly decode all the 1080 content they receive.

The way the process works is that the incoming video signal (whether via cable or over the air) gets decoded into a 1080i data stream that then gets de-interlaced into a 60 frame per second frame buffer. On native 1080 displays, this frame buffer simply gets presented to the viewer with no further processing or scaling, unless the user asks for it (zoom factors, overscan settings, that kind of stuff). On other displays, the frame buffer gets downscaled and mapped to the display’s native resolution, whether WXGA, 720, or even ED.

Here's where things get touchy: 1080i data streams provide interlaced video fields in pairs (even scan lines in one, odd scan lines in the other) but the field pairs are not each one half of one 1080p/30 frame. Instead, each one is half of a different 1080p/60 frame so that the data from one field to the other is displaced in time by one 60th of a second. Proper de-interlacing of this content requires motion-adaptive processing of the data so that the true matching field for each field received can be properly calculated to generate a full 1080p/60 data stream for display. To put it another way, each displayed frame should contain one transmitted field and one calculated field. And a lot—repeat, a lot—of pre-2005 HDTVs do not properly calculate the “missing” field data in building the frame buffer data stream. Reports are that something like half of the HD display models tested do not build the internal 1080 frame buffer properly. Instead of calculating the proper complementary field, they just replicate the received field line by line, resulting in an actual frame buffer resolution of 1920 by 540 instead of 1920 by 1080, and then they scale down that mangled data stream to the native resolution.

Of course, 720 content—what there is of it—doesn’t get shredded this way, which is why a lot of people properly swear that 720 content looks way better on their sets. It does. Not because 720 content is better but because 1080 content is being improperly processed.

Which brings us to the second technical aspect of the 1080 "war" - interlaced data streams versus progressive streams. Lots of sound and fury about the lack of 1080p content, or a given display’s lack of 1080p inputs. Well, yes, it would be great if all 1080 displays accepted 1080p signals. But it’s not all that critical. If you think about the reality of what the data transmission process described above does, it's clear that interlacing in a modern HDTV is just another form of data compression and one that is very, very effective. You get 50% compression for an almost un-measurable loss in image quality. Frankly, anybody who can see the difference between the exact same content on the exact same display transmitted from the exact same source, via the exact same port, at 1080i and 1080p had better be hitting .300 with power in the major leagues.

Anybody out there looking to buy a 1080 display today and who isn’t going to run PC productivity applications via that display should not hold back on that account. (There are plenty of good reasons to wait to buy an HDTV until you absolutely positively have to—things like 25% yearly price drops, for one, rapidly improving technology for another. But not that one - 1080 content is 1080 content and it all ends up in a 60fps frame buffer anyway).

Simply put: the best way to watch 1080 HD content is a 1080-native display. Just make sure it has good quality electronics and you’ll be covered because if the electronics aren’t up to snuff the rest of it won’t matter one whit. And since the vast majority of all available HD content is and will be 1080-resolution…

Enough said.

Next up: wrapping it all up with a bow so I can skip town.

Felix Torres is a dabbler in home entertainment electronics and a survivor of both the home computing wars of the 80's and the multimedia wars of the 90's who is currently most interested in home media networks and the North American transition away from broadcast media.