The technique, which the companies are collectively seeking to patent, is a variation on the "broadcast flag" rules mandated by the Federal Communications Commission and going into effect later this year.
Under ordinary broadcast rules, devices will be barred from redistributing digital TV content over the Internet. However, companies have come up with a variety of ways of figuring out how to keep content inside a home network.
The IBM-Fox technique will let devices that read or receive TV content use the digital broadcasts to identify themselves as part of a home broadcast market. They will then refuse to hand TV content off to devices from other local markets, using that measure as a way to keep content in its authorized area, the companies said.
"The primary purpose of the broadcast flag is to not allow for broad redistribution of content over the Internet," said Steve Canepa, IBM's vice president of global media and entertainment industry issues. "This way links it to the way broadcasting actually happens, which is tied to your metro area."
The broadcast flag issue has drawnas it has passed through the FCC and into the courts, where a panel of judges is consumer groups' appeal of the ruling.
The ruling stems from Hollywood and TV companies' concerns that the move to digital television will make it much simpler to record high-quality versions of TV broadcasts and post them online. The broadcast flag is essentially a string of data that will be included in most digital broadcasts which, when read by a compliant device, will block that piece of content from being redistributed.
One trick has been to come up with ways to tell whether something is being sent over the Net, or just over a poorly constructed Internet Protocol-based home network, however.
Companies have used a few different tests, such as seeing how long a "ping"--a computer's version of a "hello" to another computer--takes to get to its destination and back. If it takes a long time, the computer assumes that the content is going online and blocks content redistribution.
Alternately, a computer might look at how many different networks are being passed in the course of the transmission. The problem with either of these techniques is that a home network set up by someone without much technical experience could fail both these tests, keeping people from getting to their content legally, Canepa said.
By letting devices assign themselves to home TV markets and transmit only to other devices in the home market, that difficulty will be avoided, the companies believe.
The companies are still working on the technology, and are likely to file a patent in the next few months, said Andrew Seto, Fox president of engineering. The collaborative process has been productive, and should serve as a model to other joint projects between technology and copyright companies, he added.
"Historically, the relationship between copyright and (technology) implementation has been one of antagonism and tension," Seto said. "Rather than arguing over too much flexibility or not enough protection, here we worked together."