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Comcast (s CMCSK) customers, get ready for yet another TV transition: The cable provider has started to alert its customers in some markets that it is about to encrypt their basic cable signals, forcing them to order a digital adapter if they want to continue to receive basic programming through the service. Comcast is making adapters available for free in select markets, and the company even has a model that works with third-party set-top boxes — but some users could still be left in the dark.

Consumers who already use a Comcast-provided set-top box on all of their TV sets don’t have to worry, their service will continue to work as before. But if you have a TV in your den that’s hooked up to your cable outlet without a set-top box, then you’re going to have to get an adapter to keep it working.

Comcast is contacting consumers ahead of the transition, offering them up to two digital TV adapters for free for two years. These adapters are small boxes that come with their own remote control and are connected to a TV set with a coaxial (antenna) cable. Remember the converter boxes that consumers had to buy to receive over-the-air digital TV on old TV sets? It kind of works like that, except the sole purpose of this device is to descramble Comcast’s cable signals.

Comcast confirmed the move towards encrypted basic cable when contacted by GigaOM, and a spokesperson sent us the following statement via email:

“We are beginning to proactively notify customers in select markets that we will begin to encrypt limited basic channels as now permitted by last year’s FCC B1 Encryption Order. While the vast majority of our customers won’t be impacted because they already have digital equipment connected to their TVs, we understand this will be a change for a small number of customers and will be making it as convenient as possible for them to get the digital equipment they may need to continue watching limited basic channels.”

The company is also making a help page available online that goes into some of the details of the offering.

Cable companies have long lobbied for the right to encrypt basic cable channels, arguing that this will prevent cable theft and simplify remote management of their equipment. They succeeded last year when the FCC ruled that they could start to encrypt basic cable, as long as they provide consumers with some help during the transition.

The company also struck a separate agreement with Boxee to provide owners of the Boxee Cloud DVR with access to its encrypted basic feeds — and the new Boxee device also happens to be the first one that’s compatible with a new DLNA-based adapter that streams TV signals via an Ethernet connection.

However, Comcast’s adapters won’t work with Boxee’s old live TV dongle, which the company introduced a little over a year ago to bring live TV to the original Boxee Box. Also left in the dark are customers who use any other kind of digital TV adapter for their PC that are based on coaxial inputs, like the Elgato EyeTV. The last resort for many of these consumers may just be to invest in an antenna.