What is the definition of HDTV?

Television (image by ark - CC-BY)

Television (image by ark - CC-BY)

HDTV comes in many flavors, but there is a resolution threshold below which television doesn’t count as high definition, and there are two standard HD TV resolutions.

Prior to digital television, the resolution of the picture was specified as a number of scanlines. The most common formats were NTSC (480 visible scan lines out of 525) and PAL (576 visible lines out of 625). Neither of these counts as high definition.

The first HDTV resolution is 1280 x 720 pixels. In other words, the image comprises 1280 dots across, and 720 dots downwards. That’s a total of 921,600 pixels, giving an aspect ratio of 16:9. In other words, the screen size is in a ratio of 16 units across and 9 units down.

The second HDTV resolution is 1920 x 1080 pixels. That’s a total of 2,073,600 pixels, again giving an aspect ratio of 16:9. This resolution (1920 x 1080) is about as “future-proof” as any home electronics standard can be. Although higher resolutions are possible, none are currently slated for use in television broadcasting. The Quad HDTV format is 3840 x 2160 pixels, but it’s only used in cinemas.

Quarter HDTV, with a resolution of 960 x 540 pixels, is used in some portable entertainment devices.

The display of a television or computer monitor may have a resolution other than these, in which case the HD image can be stretched to fill the screen (resulting in non-square pixels), or a black band can be displayed at the top and bottom (or sides) of the image to “use up” the extra pixels. For example, a computer monitor having a resolution of 1920 x 1200 pixels can display a full HD 1920 x 1080 image with 120 horizontal rows of pixels “left over”.

The standard HDTV resolutions are often informally refered to as 720 and 1080, after the number of pixels in the vertical direction. A suffix can be added – either “i” for interlaced, or “p” for progressive. A progressive display includes every pixel in every frame of the image. An interlaced display includes the odd-numbered pixel rows in every second frame, and the even-numbered pixel rows in the intervening frames.

Regardless of the resolution of the display, the program material needs to have a matching level of detail. If a broadcast program has been heavily compressed, it may not retain enough detail to gain the benefit of a high definition display. Blu-ray discs are able to hold enough detail to “max out” a 1920 x 1080 HDTV display.

The claim is sometimes heard that the output of a DVD player (normally 720 x 576 for PAL or 720 x 480 for NTSC) can be “upscaled” to high definition. It’s true that the format of the signal can be changed to high definition, and with careful interpolation a very high quality picture can be obtained, but it will never have the degree of detail that can be supplied by a true HD program source.

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