Generally speaking, premium channels are definitely encrypted. Other than that, I can not say for sure. I have Comcast service and I am able to watch non-premium channels using Ceton InfiniTV or HDHomerun Prime. Example: channels of your local OTA stations, CNN, Discovery Channel, Disney etc. I have never done an testing of all channels, so I can not say for sure. Furthermore, I do not know if Comcast has different policy for different locations.
Maybe you can ask Comcast?