Comcast (NASDAQ: CMCSA) customers with older MPEG-2 based set-tops in Chicago have been told they need to upgrade to newer boxes that support MPEG-4 in order to keep receiving HD channels.

Comcast spokeswoman Amanda Vallejo told the Chicago Tribune that customers in the region were notified about the change with letters, emails, TV alerts and automated phone calls starting in the early fall. Most of these customers have already completed their conversions, she added.

Reached by FierceCable, a Comcast rep had no comment. A source familiar with the company, however, said the move is being made to improve the efficiency of Comcast's network in the Chicago region. Conversion to the less data-intensive MPEG-4 format will free up bandwidth for Comcast's broadband network.

The source did not say if customers in other regions are being similarly asked to convert their set-tops.

As for the Chicago customers, the conversion is free, so long as they conduct a self-install; those that require a truck roll must pay $50.

The obsolete set-top models include the DCH3416, DCT6200, DCH3200, DCT6412, DCT3416, DCT6416, DCH6416, DCT5100, DCH6200, DCT3412 and DCT6208.

For more:

- read this Chicago Tribune story

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