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A high-definition (HD) downconverter is an electronic device that converts HD video signals into standard-definition video, which can then be viewed on a conventional television or older computer monitor, or can be used for recording. The HD downconverter is able to do the downscaling of video signals with minimal loss in quality as well as minimal blurring, aliasing and any conversion noise.
High-definition downconverters are also known as HD Serial Digital Interface (HD-SDI).
A high-definition downconverter is able to convert HDTV signals, often 720p and 1080p, into standard-definition signals (480p) and more or less maintain quality. These devices, depending on the model, can automatically sense the aspect ratio of the HDTV input signal and then convert appropriately to the suitable SD aspect ratio by adding letterbox or pillarbox, or cropping and zooming depending on the desired broadcast output. These devices can have multiple outputs such as simple SD, composite and can even separate audio and video signals. Some might use these devices for recording HDTV signals into the smaller DVD format.
The main users of HD downconverters are TV stations or broadcasters who might still support subscribers that do not own HDTVs, so they need to also be able to provide standard definition broadcasts.