The national television networks ABC, CBS, NBC, Fox, PBS and The CW all provide free digital over-the-air signals, often in high definition. Independent, Spanish-language, religious, home shopping, sports or nostalgia networks fill in the remaining channels.
Channel availability varies by area depending on geographic features and distance from the broadcast tower. Sites such as Antennas Direct or TV Fool allow ZIP code entry to view available networks, distances and channels. Additionally, antenna type and location greatly influences reception. Antennas Direct has articles and instructional videos to help users select suitable antennas. Indoor, low-cost devices work in many situations, avoiding the expense and effort of installing an outdoor, fixed antenna.
Aside from a large variety of free programming, digital television networks often have several channels associated with them. Digital transmission allows multicast signals, so more than one channel is broadcast on the same spectrum, a feature not available with analog transmission. Networks thus have main channels and additional channels to broadcast alternative lineups or time slots. Multicast channels are normally standard definition, which is well-suited for older content that was not originally recorded in high definition.
Broadcast television transitioned entirely to digital signals in 2009. Low-powered analog signals shut down entirely as of September 1, 2015. All televisions sold since 2009, plus older high definition televisions, have the ability to receive digital signals. Converter boxes allow analog-only televisions to use digital signals. Some antennas have built-in converters.