DVD upscaling is a technique used by some DVD players to make the picture from a DVD appear as better quality on a higher resolution screen. This will most commonly be done on a high definition television. The technique is only a workaround, however, and does not produce an HD quality picture.
The main use of DVD upscaling is for viewers who have an HD television but do not have a Blu-ray or HD-DVD player. The upscaling resolves the fact that a standard DVD player can output pictures with a maximum resolution of 480 vertical lines of pictures, but an HD television usually has either 720 or 1080 vertical lines. Though the DVD picture will be stretched to fill the screen, this does not make the most of the screen's capabilities.
Some DVD players have a built-in upscaling feature. This will convert the signal so that it sends out information to "fill" all 720 or 1080 lines. To do this, it artificially creates the information for the extra lines using a series of mathematical calculations. While the actual process is more complex than this, the general principle is that each added pixel is given the color which is most appropriate given those surrounding it.
It is worth remembering that an LCD or plasma television will provide its own upscaling if required. The theory behind DVD upscaling is that better results are provided by performing the upscaling on the playback device itself. This is because there is the possibility that some of the original signal will be lost or subject to interference while being carried to the television. Although this isn't usually notable, it will leave the TV with less information to work from when upscaling.
The quality of pictures produced by DVD upscaling is disputed. Some sources believe it provides a sharper and more detailed picture than a standard DVD playback on the same high definition screen. Other sources maintain that the upscaling process makes little genuine difference and may even introduce picture errors.
Although it produces pictures with at least 720 vertical lines, DVD upscaling does not constitute a high definition picture. This is reserved for pictures which have 720 lines in the original source. For DVDs, this is only possible with discs using Blu-ray or the now defunct HD-DVD format.
It's also debatable whether standard DVD players with an upscaling feature represent value for money. By 2010 most Blu-ray players had reached a low enough price that the upscaling feature in a DVD player was arguably no longer needed as a stopgap measure. Virtually all Blu-ray players have DVD upscaling built-in.