Standard Definition Video
Before addressing what qualifies video as hi-def, we should start with some information about standard definition technology to compare it against. Standard definition video is designed around either the PAL or NTSC television standard. Digital sampling (capturing) is performed according to some variation on the ITU-R BT.601 (Rec.601) standard. This results in resolutions referred to as Full D1 with rows of 720 pixels for PAL and NTSC. PAL signals consist of 576 rows, while NTSC has 480. The visible picture (active area) actually corresponds to resolutions of 702x576 for PAL or 711x480 for NTSC with extra pixels added on the sides both to hide sampling or encoding artifacts on the edges of the frame and ensure a multiple of 16.
Framerates
Like SD resolution, framerates are intended for either NTSC or PAL playback. That means 29.97fps (or 23.976fps with pulldown) for NTSC or 25fps for PAL.High Definition Video
Generally speaking, anything with a resolution of at least 1280x720 is considered high definition. Since most hi-def video is encoded with HDTV playback in mind only a few standard framerate/frametype combinations exist. Most are used by both ATSC and DVB video systems - the replacements for NTSC and PAL, although formats like HDV allow other resolutions.
720p
Generally considered the best format for smaller HDTVs, 720p gives you 1280x720 progressive frames. This is more than twice the number of pixels encoded in a DVD frame. Depending on the constraints of the video Codec being used, you could see framerates as high as 60fps. Besides being useful for newer technology for recording and broadcasting at higher refresh rates, by doubling the framerate of interlaced video it can be bobbed, creating deinterlaced video with twice the framerate of the original and no interlacing artifacts.1080i
There are actually multiple formats that use 1080 horizontal lines in two interlaced fields. While ATSC, DVB, Blu-ray, and HD DVD all use the full 1920 vertical lines on a 1080i HDTV, camcorder formats like HDV and HDAVC may be limited to 1,440, for a resolution of 1440x1080. 1080i is almost always delivered at 25fps (50 fields per second), 29.97fps (59.94 fields per second), or 30fps (60 fields per second). This is the least efficient format because interlaced frames always require a higher bitrate to obtain the same quality as progressive. One big advantage, however, is that older (analog) video doesn't have to be deinterlaced for playback.1080p
1080p, or 1920x1080, is the highest quality of all the standard consumer video HD formats. Since it's also supported in the fewest displays and requires a much higher bitrate than 720p, it's also the rarest in terms of content. Like 1080i, framerates commonly peak at 30fps.Framerates
Although framerates for HDTV haven't changed drastically, but support for all standard framerates means that no matter whether video runs at 23.976fps (film slowed down for NTSC), 24fps (film), 25fps (PAL), 29.97fps (NTSC), or 30fps, any HDTV should play it. Depending on the resolution used, higher framerates like 50fps, 59.94fps, and 60fps are possible. Hopefully that means no reason for crimes against video quality perpetrated in the name of standards conversion.Video Codecs
The most basic video encoding used for hi-def video is MPEG-2. However, more advanced Codecs have displaced it as the primary tools for HD video encoding. As of the writing of this guide MPEG-4 AVC (H.264) and VC-1 have become the standards by virtue of efficiency and quality at high resolutions. Although VC-1 is currently gaining market share in HD DVD and Blu-ray encoding, AVC is used for both formats, as well as camcorders (AVCHD), broadcasting (DVB), and Nero Digital AVC. Although not as good as either next-gen Codec (AVC or VC-1), MPEG-2 is used for ATSC broadcasting, as well as HDV camcorders and some HD DVD and Blu-ray movies.
