1seg (ワンセグ, wansegu) is a mobile terrestrial digital audio/video and data broadcasting service in Japan, Argentina, Brazil, Chile, Uruguay, Paraguay, Peru and the Philippines.

1seg, like ISDB-T uses QPSK for modulation, with 2/3 forward error correction and 1/4 guard ratio.

Broadcast Markup Language (BML), is a data-transmission service allowing text to be displayed on a 1seg TV screen.

Further information can be found through links to content on websites, frequently those belonging to the television station itself.

Until the end of March 2008, Japanese regulation required that the programs on 1seg were fundamentally the same as those broadcast on the equivalent HDTV channel.

On April 1 the regulation was revised, and experimental programs by 1seg or third parties have begun airing on several stations.

[1] In the fiscal year of 2007, on average 45% of mobile phones had 1seg reception capability out of the 22.284 million units sold.