Comcast is moving 47 basic analog channels to digital in Northern California, according to a story in The Mercury News.

The transition of the analog channels to digital is slated to start March 9 in Pleasanton and Santa Clara, and then a week later in San Mateo, San Carlos, San Rafael and other communities.

According to the newspaper story, Comcast has not said when it will move analog channels to digital in San Jose, but it intends to have the Bay Area transition finished by the end of this year.

Comcast is moving the analog signals to reclaim bandwidth for advanced services and technologies such as DOCSIS 3.0 data, as well as more high-definition (HD) channels and video-on-demand (VOD) offerings.

For no extra charge, Comcast will provide customers in the affected areas with one advanced digital set-top box that can access on-demand programming, and up to two low-end boxes that will need to be paired with digital terminal adapters (DTAs).

Comcast will be using the DTAs to reconvert the digital signals back to analog in customers’ homes with low-end set-top boxes. DTA adapters have emerged as a cost-effective way for cable operators to migrate toward a bandwidth-saving, all-digital environment.

The DTAs are a cheaper alternative to digital set-top boxes, but while they convert the digital signals back to analog at the TV set, the one-way devices do not provide other digital cable features, such as VOD. Thomson, Pace Technologies and Motorola are reportedly providing DTAs to Comcast.

Cable operators can reclaim between 250 MHz and 300 MHz in each system that goes all-digital. If a typical cable system has 79 analog channels and the operator decides to move 59 of those channels to digital, while perhaps leaving 20 or so as a life-line analog service for some select markets, it would reclaim 354 MHz.

Given 354 MHz of reclaimed spectrum in the example above – and the fact that, on average, 10 standard-definition (SD) MPEG-2 digital programs can be inserted into one 6 MHz slot – this yields enough bandwidth for nearly 590 channels.

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