HDTV Resolutions Explained

According to the high-defintion television standard (BS EN 60107-7:1997) two resolutions are available: 1080 & 720 (just to confuse you only one figure is often quoted!) This figure, however, relates to the height of the image (height is often quoted as lines due to the television's origins in cathode ray tube technology).

The HD TV standard also states that a HD television must support a widescreen signal. That is, the image must have the ratio of 16:9 (width: height) and not be reduced in any way to 'fit' the television screen.

Confused? Well, putting all of this information together gives us the following:
If the minimum specification for image height is 720 pixels, then in order to support HD the image width must be a minimum of 1280 pixels:

width ? height = 16 / 9 = w / 720 pixels

therefore:

w = 720 x 16 / 9 = 1280 pixels

So going back to our example resolution of 1024 x 768; on first pass, as the height is 768 pixels, one might assume that the resolution is HD TV compliant but it is not as the width is only 1024 pixels instead the required minimum of 1280 pixels!

To summarize: for a line resolution of 720 a television needs to have a screen size of 1280 x 720 pixels or if the line resolution is 1080 a screen size of 1920 x 1080 is required for the television to be HD TV compliant.

Interlaced or Progressive Scan?

So we have both 720 and 1080 resolutions but these can come in two formats progressive (p) and interlaced (i) giving the following options:
720p
1080i
1080p

Interlaced (i) scanning is 'old' technology as it was developed for the analogue television signal. Basically the way interlacing works is to divide the total number of lines into two fields: Field A being lines 1, 3, 5, 7, etc and field B being lines 2, 4, 6, 8, etc.
Filed A & B are then shown alternately on the screen for a duration of 1 / 48th sec for PAL (1 / 60th sec for NTSC). The display of both field A & B equates to 1 frame of broadcast. It therefore follows that there are 24 frames displayed per second in a PAL broadcast (and 30 frames in a NTSC broadcast). The result of this is that at any one time the viewer is only seeing 'half' the broadcast image! The image 'looks' complete because the viewers brain 'fills in' the missing information as its all happening so fast. Interlaced scanning was a great way to minimize the data broadcast in an analogue signal while maximizing screen resolutions.

With the advent of digital technology and the ability to transmit great quantities of data, progressive (p) scanning can be used. Here the complete frame (i.e. the complete image) is displayed for 1 / 24th sec (1 / 30th for NTSC). This produces a much higher quality image on the television screen particularly when viewing high speed images (such as televised sports events)

The High Definition TV standard requires that televisions are capable of both interlaced & progressive scanning. While a resolution of 720 lines will show both formats 1080i images will be best viewed on a 1080 line screen i.e. in un-scaled format. 1080p is highly unlikely to be used by broadcasters (due to the increased transmission data requirements) but may well be available via either HD DVD or Blu-ray technology.

720p Vs 1080i HDTV sheds more light on the issue.