One Seg allocates one of the 13 segments that make up a digital terrestrial television channel in Japan to broadcasting aimed at handheld devices. It launched in April 2006 and let feature phones and car navigation units show live television. The name comes from that single segment; broadcasting for home sets, which uses the remaining segments, is called Full Seg.
Picture quality is QVGA (320x240 pixels), coarser than Full Seg, but the narrow band carries heavy error correction so reception holds up while the viewer is moving. What actually trips people up is the antenna. Many handsets need their built-in antenna extended, or an earphone or antenna cable plugged in to act as one, so starting the app with nothing attached can leave a blank screen. Deep inside buildings or underground the signal simply does not arrive, and moving outdoors or next to a window is often the only fix.
Watching costs nothing in data or subscription fees, but owning equipment capable of receiving broadcasts brings the question of an NHK receiving contract. Whether someone who keeps no television and owns only a One Seg phone still owes that contract was litigated, and in 2019 the Supreme Court rejected the appeal, leaving in place the ruling that the obligation applies. In short, having no television set at home is not on its own a reason to fall outside the contract.
One Seg was one of the features that symbolised Japan's feature phone culture, and peering into a small screen on a commuter train was an ordinary sight in the late 2000s. Once smartphones spread and watching video over a network connection became the norm, a tuner for broadcast waves mattered less, and from the late 2010s more models shipped without one. As of 2026, very few newly released smartphones support One Seg or Full Seg.
The disaster-preparedness argument still holds, though. Broadcast waves are not affected by congestion on the mobile network, so where the signal reaches, video, audio and data broadcasting arrive intact. The trade-off is battery: a lit screen drains a handset quickly, and during a long outage a radio lasts far longer. A workable division of roles is to learn of the danger through emergency alerts such as Area Mail, check the details on One Seg or radio, and confirm that family members are safe using the steps in how to make phone calls during disasters. The history of telephone hardware looks back on how many functions feature phones came to carry.