Try this :
On Demand: Device Code: 0 Function: 26
0000 006C 0012 0002 0153 00A9 0013 0054 0013 00A9 0013 0054 0013 00A9 0013 00A9 0013 0054 0013 0054 0013 0054 0013 0054 0013 0054 0013 0054 0013 0054 0013 00A9 0013 0054 0013 00A9 0013 0054 0013 0468 0153 0054 0013 0D13

The DCX3400 actually responds to two different IR protocols-- General Instruments device 0, and XMP device 62.16.

The Comcast supplied remote sends the XMP protocol (setup code 01982) which can be quite difficult to learn. Jason provided the GI IR protocol, and the Pronto Hex above is also GI.

If you want to exactly duplicate the IR protocol being sent by the Comcast remote, I can provide those Pronto Hex codes. I'm not aware of any particular reason to use XMP rather than GI.