"Future Sony Ericsson phones launched under its new Xperia brand may not be based on Microsoft  Windows, the handset maker said at the launch of its digital marketing campaign for the Xperia X1 on Monday.The X1, which will be the premium phone in Sony Ericsson's portfolio when it starts shipping later this month, is Sony Ericsson's first smartphone powered by Windows Mobile and was seen as boosting Microsoft's presence in the smartphone market.But the loss-making joint venture of Japan's Sony and Sweden's Ericsson  was reluctant on Monday to reveal any details of future models that may be launched under the Xperia brand, of which the X1 is the first."The brand is not tied into any specific technical platform," said Magnus Andersson, product manager for the X1.Asked how much pressure Xperia was under, given Sony Ericsson's shrinking range of must-have products, Andersson said: "I think there is a lot of hope riding on everything Sony Ericsson does."The X1 is Sony Ericsson's most advanced smartphone, offering fast uploads to the Internet as well as fast downloads -- where suitable networks are available -- and combining a touch screen with a slide-out keyboard.It also allows up to nine applications -- such as email, video or Internet browsing -- to run simultaneously.Sony Ericsson, best known for its Cybershot camera and Walkman music phones, has slid to fifth place in global rankings of mobile phone makers, most recently falling behind LG Electronics  in the first quarter. Sony Ericsson, which denied earlier this month the X1 would miss the key Christmas sales period, reiterated on Monday that the phone would start shipping to Britain, Germany and Sweden on September 30. It declined to give prices.An announcement about North America -- a market Sony Ericsson has said it would make a top priority this year -- will be made on November 3, a spokeswoman said.The phone is due to start shipping to other markets in Europe, Asia and Latin America in the fourth quarter. No announcements have yet been made about China and Russia.Dean Bubley, founder of wireless research firm Disruptive Analysis, said the later rollout in North America could be due to the greater complexity of wireless networks there.U.S. operator AT&T  had teething problems with the recent introduction of the third-generation (3G) version of Apple's iPhone, which uses advanced networks, and the X1 is more complex in its requirements."I imagine testing will be an issue in the U.S., given what's happened with the iPhone and AT&T," he said. Bubley also noted: "There seems to be scant reference going forward about committing to the Microsoft brand." He added that the frequencies supported by the X1 -- which contains a Qualcomm chipset -- suggested it would likely be better suited to AT&T's networks than those of other U.S. carriers. " via reuters.com