quote:
The DTV standard used in the U.S., Canada, and Mexico is named ATSC, after the Advanced Television Systems Committee, the industry-led group that originated it. ATSC allows resolution as high as 1080 by 1920 pixels, but only in an interlaced format. That means the picture is scanned in two passes, or “fields,” each lasting 1/60th of a second. The first pass leaves blank spaces between lines, which are filled in by the second pass. Because each field takes 1/60th of a second, and there are two of them, 1080i actually needs 1/30th of a second to convey a full frame—it is a 30-frames-per-second medium. Fixed-pixel displays don’t need to use scan lines but approximate the same thing in dots.

The upside of interlaced scanning is that it conserves bandwidth while still providing more than two million pixels onscreen (1080 times 1920 equals 2,073,600). The ATSC adopted interlacing for the 1080-line format specifically to fit over-the-air HDTV into the same 6MHz bandwidth as an analog TV channel (though 1080i also travels via cable and satellite). The downside of interlacing is that it induces motion artifacts and other forms of distortion. The best example would be a horse race. With interlacing, the horses’ legs become a blur.

quote:
Remember, 1080i delivers only a half-frame every 1/60th of a second, while 1080p can deliver a full frame in that amount of time. That gives the video processor some headroom that it can use to manipulate the signal. This is exactly what a progressive-scan DVD player does, except that it converts a 480i signal to 480p. If you can see the difference between an interlaced-scan DVD player vs. a progressive-scan model, you may see the difference between 1080i and 1080p.

quote:
Another reason why 720 looks better than 1080 in HDTV is because the 1080-line formats that can offer better temporal resolution than 720 are not part of the standard. Although the ATSC includes 1080p formats, they are capped at 24 and 30 frames per second; so there are neither 1080p50 nor 1080p60 variants, which are the only 1080-line formats that will (in practice) surpass 720p60.

quote:
The reason for that is that 1080p50 and 1080p60 would exceed the 19Mbit/s bandwidth allotted in a 6Mhz channel as required by the ATSC standard. The highest HDTV formats that don’t exceed that limit (and in fact are very close to it) are 720p60, 1080p30 and 1080i60.

quote:
Just because the ATSC standard doesn’t include 1080p formats with higher frame rates than 30 frames per second doesn’t mean there are no higher display rate formats in existence, or that TV manufacturers can’t go beyond the ATSC 1080 line formats. TV manufacturers have already produced displays that can display a 1080 progressive signal up to 60 frames per second; those are the HDTV sets advertised as 1080p.