In order for any Television to be classified as an HDTV, the television must be able to display a vertical resolution of at least 720 lines, represented as a 1280x720 pixel field (1280 pixels across the screen and 720 pixels down the screen). Some smaller LCD TVs sets are referred to as EDTVs (Extended or Enhanced Definition televisions) as they only display a resolution lower than that of 720 lines.
EDTVs typically have a native pixel resolution of 852x480 or less. 852x480 represents 852 pixels across (left to right) and 480 pixels down (top to bottom) on the screen surface. The 480 pixels down also represent the number of lines from the top to the bottom of the screen. This is higher than standard television, but not HDTV resolution.
The images on these sets can look good, especially for DVDs and standard digital cable, but it is not HDTV. LCD TVs that are capable of displaying HDTV signals directly have a native pixel resolution of 1280x720 (where 720 also represents the number of lines from the top to bottom of the screen) or higher.
Since LCD televisions have a finite number of pixels (referred to as a fixed-pixel display), signal inputs that have higher resolutions must be scaled to fit the pixel field count of the particular LCD display. For example, a typical HDTV input format of 1080i needs a native display of 1920x1080 pixels for a one-to-one point display of the HDTV image. Also, since LCD Television only display progressively scanned images, 1080i source signals are always either deinterlaced to 1080p or scaled down to 768p, 720p, or 480p depending on the native pixel resolution of the specific LCD television.
In other words, there is no such thing as a 1080i LCD TV. LCD TVs can only display video in a progressive scan format, so if your LCD TV accepts a 1080i input resolution signal, the LCD TV has to deinterlace and rescale the 1080i input signal to either 720p/768p on TVs with a 1366x768 native pixel resolution or 1080p on LCD TVs with a 1920x1080 native pixel resolution.
Also, if your LCD television only has a pixel field of 852x480 or 1024x768, the original HDTV signal must be scaled to fit the 852x480 or 1024x768 pixel count on LCD screen surface. HDTV signal inputs have to be scaled down to fit the LCD Television's native pixel field.
In conclusion, when shopping for a LCD Television, make sure you check to see if it is an EDTV or an HDTV. Currently, just about all LCD Televisions, 23-inches and larger, are indeed HDTVs, with most having a 1280x720 (720p), 1366x768 (768p), or 1920x1080 (1080p) native pixel resolution, but some of the smaller (20-inch and less) units may not be.