Media for Modulation and Digital Cable with Comcast

Modulation and Digital Cable with Comcast

I have Comcast Digital Cable for my home. The box is a Motorola DCT-2000. I also have a Channel Plus 5445 (4-channel modulator)

I am able to inject 4 channels into my home signal but I have a few questions / problems.

Depending on what channel I inject to ( 77, 79, 81, 83), I lose some of my digital channels. The channel displays something similar to , "signal unavailable should be available soon". If I disconnect my modulator the channels appear fine. If I choose different channels to inject to, I lose different digital channels.

How can I find which channels are available for creating modulated channels? Also, I'm unclear on Low Pass Filters The ChannelPlus Low Pass Filter LPF-750 passes CATV channels 2 to 116 Would this work? Does Comcast use any channels higher than 116?

thanks, tom