Just as in analog video there is both a vertical and horizontal component to digital video resolution. However, the total image resolution displayed in DTV and HDTV is referred to in terms of number of pixels on the screen rather than lines. I will get back to that point shortly.
Digital TV Resolution Standards
In current digital TV standards, there are a total of 18 video scan rates that are approved by the FCC for use in the U.S. system. Fortunately, for the consumer, and for this Guide, there are only three that are commonly used. The three vertical scan line systems used in digital TV are 480p (480 lines vertically scanned in a progressive fashion), 720p (720 lines vertically scanned in a progressive fashion), and 1080i (1,080 lines scanned in an interlaced fashion).
Based on vertical scan rates, digital TV has much more capacity for a finer detailed image than that of analog TV. However, In order to display the raw scan lines, a video monitor has to be able to reproduce the full detail of the incoming DTV or HDTV program material. In addition, true HDTV is also dependent on a monitor that displays the image in a 16x9 screen shape. However, there are also HDTV monitors in the traditional 4x3 shape, in which case a 16x9 image is displayed in a letterbox format, with black lines on the top and bottom of the screen. Another factor on how a digital TV image is displayed is the actual size of the screen. Basically you can fit more dots on a 50-inch Plasma Screen than a 27-inch Direct View DTV.
HDTV vs EDTV
So, even though you may be inputting a 1080i image into your HDTV, your TV may not have the ability to reproduce all the dots within those lines. In this case the signal is often reprocessed (upconverted or downconverted) to conform to the number and size of dots (pixels) on the physical screen. At full resolution on a 16x9 screen, a 1080i image is comprised of 1920x1080 pixels (about a two-megapxiel field).
However, if your monitor is not capable of reproducing the total pixel field, then the image is scaled to fit the number of pixels in the display monitor's pixel field. So, an HDTV image of 1920x1080 can be scaled to fit 1366x768, 1280x720, 1024x768, 852x480, or other pixel field. The relative loss of detail actually experienced by the viewer will depend on factors such as screen size and viewing distance from the screen.
In essence, when purchasing an HDTV, it is not only important to make sure that you can input 480p, 720p, or 1080i signals, but you must also consider the pixel field of the monitor itself (and whether upconversion/downconversion is used).
To go on further detail on this, televisions that have to downconvert an HDTV signal (such as 720p, 1080i, or 1080p) to a pixel field of 852x480 (480p) for example, are referred to as EDTVs and not HDTVs. EDTV stands for Enhanced Definition Television.
EDTV Video Projection
In addition to EDTV TVs, some budget-priced video projectors are capable only of EDTV resolution and are not capable of projecting a true HDTV-resolution image. Just as with EDTVs, these video projectors will often have the capability of inputting an HDTV signal through HD-component, DVI, or HDMI input connections. However, if the projector has a native pixel resolution of 1024x768, the video projector will have to scale a 1080p, 1080i, or 720p HD image down to fit its lower native pixel resolution, in order to project it onto the screen.
Resolution Requirement For True HD Image Display
On the other hand, if a TV or video projector has a native display resolution of 720p, they are referred to as meeting HDTV specs. Most LCD and Plasma TVs, for example, currently have a native display resolution of 1080p. So, when faced with a 480i/p, 720p, or 1080i input signal, the TV will scale the signal to 1080p to display it on the screen. If a TV or video projector has a native display resolution of 720p or higher, it definitely has true HDTV resolution capability.
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