While setting up my new Home Theater PC (HTPC) the past week or so, I've been doing some reading online and some comparison of different resolutions. There's quite a bit of confusion floating around out there about the differences between 720p, 1080i and 1080p. I decided for my own reference to jot down some numbers and clarifications. As usual with this blog, this is 90% for my future reference, but maybe somebody else will find it useful too.
Let me go through the basics of the math I'm using here with an explicit example, then I'll report various numbers of a number of different resolutions. Starting with the standard HD resolution of 720p, in counting pixels we have a resolution of 1280 x 720 pixels = 921,600 pixels on the screen. This is a progressive scan resolution, so each pixel is updated every refresh. In speaking of standard resolutions, this means 60 Hz for the NTSC world. (The PAL world is 50 Hz, but I don't want to introduce that confusion here -- just know the US is NTSC.) So refreshing at 60 Hz (60 cycles per second) means we're looking at 1280 x 720 x 60 = 55,296,000 pixels/second. This is a number we could use to compare all different resolutions, but let's take it a step further to get into more familiar units, even though the values just change linearly. To get more information, consider 24-bit color - this is where each of Red, Blue and Green values are defined by an 8-bit number. Since 8 bits are 1 byte, we have 3 bytes of color per pixel, so we can multiply the pixel/sec number by 3 bytes/pixel and get a number in more familiar bytes/second unit. Since the numbers are large, we divide by 1024 to get KiloBytes/sec or again by 1024 to get MegaBytes/sec.
For 720p, the whole calculation is 1280 x 720 x 60 x 3 /1024 / 1024 = 158.2 MB/sec.
With all that verboseness out of the way, a 720p display pushes through 158.2 MB/sec. One more word on progressive scan versus interlaced to have the following numbers make more sense. In an interlaced world (standard TV resolution is this), every other row of pixels are updated with each scan, which requires exactly half the bandwidth of a progressive scan at the same resolution. Standard TV is 480i and you'll see "progressive scan" DVD players that have been available for a long time that output 480p pictures. On even a low-end HDTV, you will see a nice difference here - basically twice the resolution from the same source. This translates to 30 Hz in the numbers above - 30 frames per second is standard interlaced display.
On to the numbers. To standardize more, I'm using 16:9 wide screen resolutions here.
480i = 853x480@30Hz = 35.1 MB/s
480p = 853x480@60Hz = 70.2 MB/s
720p = 1280x720@60Hz = 158.2 MB/s
1080i = 1920x1080@30Hz = 178.0 MB/s
1080p = 1920x1080@60Hz = 356 MB/s
Some conclusions:
480i sucks. At least play all digital content (DVDs, etc.) on an HDTV in progressive scan mode if possible. The difference is clear to just about anyone on any TV - we are talking a full factor of 2 here.
1080i versus 720p is a judgment call. Lots of people claim one is better than the other, but these numbers show 1080i is only slightly better than 720p. So don't bash your neighbors 720p HDTV compared to your 1080i -- hardly anyone will be able to see much difference. Exactly, there's a 12.5% difference. When talking about what's noticeable and what's not, 40% is usually the key minimum value.
1080p rules. It's literally twice as good as 1080i. You can argue weather upscaling a standard resolution DVD to 1080p versus 1080i is much different, and that's really kind of a judgment call there. HOWEVER, 1080p sources (Blu-Ray, HD-DVD, .mkv files, etc.) carry twice the amount of information as a 1080i source and 10 times that of standard TV. Incredible.
Finally, for the rest of the internet out there -- YES both 1080i and 1080p can be characterized as 1920 by 1080 resolution. However, the difference between an interlaced and progressive scan display is a factor of two in true resolution, giving a much smoother picture at 1080p.
Personally, I've been keen on 1080i because that's what my TV and cable box both support. Hooking up the HTPC, I've got both 1080i and 720p running well, but have to say that I'm probably going to stick with 720p almost all the time. First, as shown above, upscaling standard DVDs to either is going to result in almost the same quality. Since I can do both, 1080i makes sense -- but there's the pragmatic side too. At 1920x1080 my icons and text are TINY. Even on a 52" screen, it's really hard to see. Plus, with so many pixels and a trackball/keyboard setup it takes a long time to scroll across the screen. at 1280x720, the icons and text are much more visible, and navigation is a lot easier too. Finally, most of the true high-def content I've been downloading is 720p already. It does look good at 1080i, but not worth dealing with tiny icons. I will of course set things up to switch between, but unless I see artifacts or am working with 1080i sources, I think I'll be sitting at 720p most of the time.
