A digital TV service intended for mobile phones and other devices officially kicked off in Japan over the weekend.

The service, known as 'one-seg', offers simultaneous broadcasting of terrestrial TV channels and is available at no cost. Broadcasts began in many regions across the country and in Tokyo viewers should be able to watch at least seven TV channels through the service.

The broadcast is transmitted as part of Japan's digital TV service. The service divides each roadcasting channel into 13 segments, 12 of which are typically used for an Mpeg2 high-definition stream for television sets. The final one is used for an Mpeg4 stream for mobile devices.

This Mpeg4 stream is QVGA resolution (320x240), which means it's better suited for processing by small, portable devices such as mobile phones.

Carried alongside the TV stream is a data service that can provide general information to viewers and data related to the TV programme being broadcast. Japan Broadcasting Corporation, the national broadcaster, is sending news and weather information alongside its programmes, and interactive services are planned. If the receiving device is a mobile phone, a communications channel back to the broadcast station is easy to establish.

Knowledge of the new service is already high among potential users, according to a survey conducted among 6,000 people in early March by NTT Advertising. Just over half of the people surveyed said they had heard of one-seg broadcasting. When asked how they might use the service, the main reason given by the respondents was to kill time while waiting. News, weather and music programming were ranked as the most attractive content, the survey found.

Service-ready phones are already on the market, with more planned for release. NTT DoCoMo, Japan's largest mobile carrier, offers one handset from Panasonic, while KDDI's Au service has two available handsets, from Hitachi and Sanyo. Vodafone plans to offer a one-seg TV handset later this year.

With the new service Japan becomes the second country to begin dedicated terrestrial digital broadcasts for mobile phones and other portable receivers. South Korea started a similar service in December 2005.