Yes. You can purchase your own device or DVR available from a legitimate retailer and-or authorized reseller in the United States.
Below are a list of a few of the models of "purchasable equipment" that Comcast supports:
Any Tivo device.
Any customer owned device (above) would need to be activated via a Comcast-supplied cable card.
Will I be charged to use a CableCARD? The first CableCARD in a retail CableCARD device is free to Comcast customers. If a second CableCARD is needed for the same device (as is the case with certain older model TiVo devices), a charge will be applied based on your area.
Will I receive an equipment credit if I use my own CableCARD-compatible retail device? If you own a CableCARD-compatible device with an activated CableCARD and subscribe to an XFINITY TV service package that includes equipment as part of the service, you will receive a monthly equipment credit of $2.50 for each CableCARD-compatible retail device on your account. To learn more, see our Customer Owned Equipment Policy. http://www.xfinity.com/equipmentpolicy
Some additional detailed information can be found at the links below:
What about Motorola or Scientific-Atlanta or Cisco or Pace cable boxes on online auction sites?
The monthly cost for Comcast Digital Cable service includes rental charge for the digital cable box. There is currently no savings for purchasing your own digital converter on your primary cable outlet.
Comcast is compliant with the FCC ruling that we offer customers the option to purchase a digital descrambler through authorized retail outlets. Comcast would then supply the customer with a POD (point of deployment) authorization card (cable card).
However, we are not aware of any Motorola/Scientific-Atlanta/Cisco/Pace digital converters being offered through authorized retail outlets in the United Sates. Since we know of no authorized retailers or cable companies yet selling these digital converters in the United States, any such device could be the property of Comcast or another cable company.
If we determine by the serial number that the box is lost or stolen and/or the property of another cable company, we would not support it. Information received would then be turned over to the proper authorities.
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