An SDTV tuner is a digital tuner, included with a television or with a set-top box for a cable or satellite television service, that accesses channels in standard definition. Standard definition is digital signal broadcast for televisions that have no enhancements to the visual presentation such as high definition television (HDTV). An SDTV tuner receives a picture, in 480i resolution, while an HDTV tuner receives a picture in 1,080p resolution. A slightly enhanced definition resolution is also offered, EDTV, providing between 680p and 720p.
Every television produced since 1997 has an SDTV tuner, even HD and ED televisions. The tuner is the most basic model of television tuner on the market, and it is compatible with higher resolutions in that an SDTV tuner can view an HDTV signal. The HDTV signal, however, will appear choppy, blurred, and poor. SDTV provides the equivalent picture quality of an analog television with the absence of white noise and static due to signal interference being the only notable improvement.
More TVs are appearing in the marketplace that do not have integrated tuners. To save space, several makes and models of flat-panel televisions do not include a built-in tuner. In order to watch live television broadcasts on such a TV, the user must purchase and install a set-top peripheral tuner. Most peripheral tuners feature both SD and HD capability. Some SDTV tuners, however, require either an antenna or cable or satellite service.
The standard in North America for an SDTV tuner is a resolution of 480i. This means that a television renders a picture composed of 480 interlaced lines within a vertical frame. Depending upon the aspect ratio, an SDTV tuner presents a picture that is between 704 and 720 pixels in width. The European standard for SDTV features a resolution of 525 interlaced lines, with only 480 of those lines used to present signal information.
An SDTV tuner typically presents an image with a 4:3 display aspect ratio. This means that the width of the picture relayed is slightly larger than the height of the picture. Some such tuners, along with all HDTV tuners, present an image that features a 16:9 display aspect ratio. Some television models, along with several set-top boxes, allow a user to stretch an SD signal received in the 4:3 ratio to cover the 16:9 aspect ratio of full screen HDTV. This stretching process degrades the quality of the resolution and is not a substitute for an HDTV tuner.