After three years of painstaking work, Comcast, (CMCSA) the nation's largest cable provider, Tuesday will begin marketing TiVo's (TIVO) interface as a premium software upgrade for DVR customers in the Boston area.

The significance of the offering reaches far beyond Beantown.

For Comcast, it's a test of a technology — called Tru2way — that enables TVs, DVRs and other consumer electronics to handle its interactive services, including video on demand. Those services currently can be accessed only with Comcast-provided gear and software.

TiVo is "the first application running on our network that comports with the Tru2way platform," says Kevin Casey, who's president of Comcast's NorthCentral Division.

Meanwhile, TiVo wants the partnership with Comcast, which runs at least until 2014, to be a model for other cable deals that offer access to millions of new customers.

TiVo serves 4.1 million DVRs and has lost hundreds of thousands of subscribers since 2005, when DirecTV (DTV) stopped marketing TiVo in favor of a home-grown DVR.

"Being able to port (TiVo) software into somebody else's box opens up a way to begin total integration of cable services into TiVo," CEO Tom Rogers says. "A lot of elements make it quite important."

The companies say they initially didn't appreciate how complicated it would be to develop software to offer TiVo's features on Comcast's DVRs and operating system.

"It ended up to be rocket science," Rogers says.

TiVo's most popular features include its ability to track and capture all episodes of favorite shows with a Season Pass and sophisticated search capabilities that find and record programs using a Wish List.

Their press release in March 2005 said that they expected the TiVo service to be available "in a majority of Comcast markets in mid-to-late 2006."

Having blown that deadline, neither will say when Comcast will offer TiVo to most of its customers.

"We expect to begin to make it available in additional markets later this year," Comcast spokeswoman Jenni Moyer says.

Comcast can provide TiVo to its systems using Motorola (MOT) DVRs. TiVo recently began work on software for other areas that use boxes from Scientific Atlanta (CSCO).

Also dropped is the original promise for Comcast to "showcase TiVo's home networking, multimedia and broadband capabilities."

Unlike stand-alone TiVos, Comcast's version doesn't allow TV watchers to access music and photos on the home PC, watch Web videos or transfer recorded shows to other rooms, PCs or mobile players.

But Comcast's service has some unique features.

Users can continue watching a show when they check the programming schedule or plan other recordings. The sound stays and the video appears in a window in a corner of the screen.

In addition, program searches in Comcast's TiVo include the cable company's growing array of video-on-demand movies and shows.

The companies say they have a road map for enhancements, which they decline to discuss.

Meanwhile, they're still squashing bugs, including one that, in some cases, causes a noticeable delay when flipping between channels or TiVo screens.

"Some of these are bugs that were known and were deemed to be reasonable to get the product out there," TiVo general manager Jeff Klugman says. "We have several additional releases of software that are planned over the next several weeks and months to address those issues."

Comcast will try to minimize potential problems, at least at first, by requiring those who want its TiVo service to schedule an hour-long service call.

"What we want to avoid is that the customer doesn't understand how to use this," Casey says. "This is a brand-new product for us, and we want to make sure we roll this thing out in a high-quality, high-touch way."

Customers who have a problem will call Comcast for help — with TiVo personnel available, if needed.

It may be difficult for outsiders to assess public reception to the new service. Rogers says he probably won't disclose the number of Comcast TiVo customers. He likely will report a single number that includes his remaining subscribers from DirecTV.

TELL US: Does Comcast's TiVo sound like a service you'd buy? Why or why not?

Enlarge A screen shows how Comcast users can access video on demand. Users of regular HD TiVo don't have access to Comcast VOD.