What is multicasting?

This is archived content from Digital TV Facts. For up-to-date information on the digital TV transition, see the federal government’s site, www.DTV2009.gov.

Multicasting is a digital television technology that gives viewers access to additional local broadcast TV channels.

That’s right: More local channels. A single station can now provide multiple channels of separate programming simultaneously, free and over the air. Each separate program stream is called a multicast. New multicast channels are already on the air in many metropolitan areas throughout the U.S., and in some smaller markets, too.

Thanks to digital TV, we can now have five or six channels in one. This magical feat is accomplished by using the broadcast spectrum more efficiently.

Compared with analog TV, local broadcasters can now send pictures, sound and other information over the public airwaves in smaller packages. The packages are so small, in fact, that your local digital TV station can broadcast not just the single channel you’ve always had, but up to five more.

HDTV broadcasts—which must be sent over the air in bigger packages—limit broadcasters’ ability to multicast. See Are HDTV and multicasts competing technologies?

(Also, multicast technology allows some stations to rent out unused portions of their FCC-licensed spectrum allotment to pay-TV providers, or to use them for other information services.)

Many broadcasters already offer multicast channels today, which are available over the air to viewers who own newer TVs equipped with digital tuners. For example, channel 4 in your area may offer separate digital programming streams on channels 4.1, 4.2, and 4.3 in the digital band. A converter box will allow you to watch multicast channels on your old analog TV.

Muticasting can also be called “multiplexing.”

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