What is blocking Vista users from recording NBC shows?

Update at 7:10 a.m. Wed., May 14: A correction was made to this story. See details below.

Some users of Windows Vista Media Center say they were blocked from recording the NBC Universal TV shows American Gladiator and Medium on Monday night.

"Restrictions set by the broadcaster and/or originator prohibit recording of this program," the error message read.

At Thegreenbutton.com a message board popular with DVR enthusiasts, those affected tried to figure out what was causing the block. When this kind of thing happens, people immediately question whether TV and cable networks are trying to thwart time-shifting technologies and force people to watch commercials again. Even though it's rarely done, content providers do have the ability to communicate with some time-shifting hardware to block the recording of shows or broadcasts.

"This is indicative of why the current DRM schemes are flawed," said Marcel Good, an IT director in Northern California and one of the people prevented from recording the NBC shows. "It affects people who intend to legally consume content. They have no intention of stealing the content. The people who want to steal already have ways around DRM."

On Tuesday, the cause for the block of the NBC Universal shows was still unclear. Representatives from both the network and Microsoft said they would need time to look into the matter.

Until they do, here are the few available clues. Most of those who posted at Thegreenbutton were Vista users recording digital cable with ATI cablecard tuners.

A TiVo spokeswoman said the DVR company received no customer complaints about being blocked from recording.

This isn't the first time a Vista Media Center user has complained about being barred from recording. A person identified as Alex from Canada wrote 14 months ago at Thegreenbutton: "Often a small number of my shows fail to record, and in the Windows event log I see errors about 'copy protection set by the broadcaster.'"

The shows Alex tried to record came from the Toronto-based W Network. Alex was running Windows Vista Ultimate 32bit and when attempting to record the show Friends: Trial by Jury received this message in Media Center's event logs: "Friends was not recorded. Recording of this content is prohibited by the content provider."

Jessica Zahn, a program manager in Microsoft's Media Center TV group, who according to Thegreenbutton has volunteered to answer questions at the site, apparently responded to Alex (a Microsoft spokeswoman said she could not confirm the message came from Zahn). The person who responded under the username JessZahn said the problem had either one of two causes.

"The broadcaster is setting the 'Copy Never' flag and Media Center's behavior is expected or Media Center is misinterpreting data it receives," and someone at Microsoft would need to study the stream of the show to know for sure.

What remains unclear, however, is whether those who posted complaints at Thegreenbutton shared the same cable provider.

I'll update this story as soon as I get more information.

Correction: This story erred in stating when Alex in Canada posted to Thegreenbutton.com. The post was from March 2007.