Older Samsung Home Theater systems all use the NECx2 IR protocol, device 67.83. As it happens, no Comcast remote has this IR protocol built in.

Of course, I don't know if the HTC-6900 uses the same IR protocol. If it does, it may make sense to get the RCA RCRP05B for $9 at Walgreens (assuming it is still on sale).

You would use Audio 1295 with the Samsung HTC-6900. If your cable box is Motorola, the setup code is CBL 1376 (DCX3400 is 1982), or if the box is a Scientific Atlanta, the setup code is 0877.