twolverton@mercurynews.com

Many Comcast customers may need to get a new box if they want to continue to watch local and public channels on their TVs.

The cable company is set to complete the final stages of its digital transition this fall. Starting next week in certain communities, Comcast will begin delivering digitally the more than 30 channels that it previously delivered as analog signals.

In the space needed to deliver one analog channel, Comcast can deliver 6 digital channels, said Bryan Byrd, a company spokesman. The company plans to use the space freed up by converting the channels to digital signals to add additional high-definition channels and to offer speedier Internet access and additional, unspecified future products, he said.

"When you take roughly 32 analog channels and switch that bandwidth over to digital, you can do a lot with the bandwidth," Byrd said.

To continue to receive the soon-to-be digital stations, which generally are found on channels 2 through 34 and include KGO, KQED and C-SPAN customers will generally need to have a Comcast digital tuner. Before this transition, customers have been able to tune in Comcast's so-called limited basic cable channel by just plugging a cable wire into the back of their televisions. Customers who do not get a box before the transition to digital who try to tune in the affected channels will just see a black screen with a message prompting them to get a box.

The move will affect those limited basic customers that don't yet have a set-top box from the company. But it will also affect customers who subscribe to Comcast's more extensive pay TV offerings who tune into limited cable channels box-free on one of their televisions.

Comcast has about 2.4 million customers in Northern California, Byrd said. Of those, about 10 percent are limited basic customers, he said.

The company will start this digital transition Sept. 12 in Daly City. It plans to complete the move by Dec. 5. San Jose customers will switch to all-digital signals on Sept. 26.

The cost of the boxes varies by subscription level. Comcast is providing up to three digital tuners for free to its limited basic customers. Those customers can get additional boxes for 50 cents a month. Customers of Comcast's more extensive and expensive pay TV services will be charged $2 a month for each additional digital tuner they need.

Customers can request a box by either calling the company or visiting its website.

In addition to continuing to receive the local and public channels that are currently analog, limited basic customers who get a digital tuner will also gain access to about 26 digital channels, including the so-called multi-cast stations offered by KQED and KTVU.

Comcast moved most of its channels to digital three years ago, but kept the limited basic channels on analog at the time.

Contact Troy Wolverton at 408-840-4285. Follow him at Twitter.com/troywolv.