You raise a good point about tampering. But, I own the wiring after Comcast taps and should be able to do whatever I want with it. If I want to add a signal to my cable, such as an IP security camera, this should be my right as cable owner, provided it doesn't interfere with neighbors upstream on the other side of Comcast taps. Do you agree?
Do you intend to use some kind of diplex / multiplex filtering or disconnect their end of the line ? If not, the noise police may just come along and do it for you if are injecting spurious frequencies into the neighborhood plant and it interferes with other subscribers. It doesn't matter if you own the lines at your premises, you are still connected to THEIR system at the demarc and they will disco you.
These days with systems going 100% encrypted digital, and a box is needed on every TV, it is extremely difficult to let alone inject a analog signal onto a full bandwidth coax, then at the TV end, you can no longer retrieve the analog signal, since the coax is going into the cable box.
Your a lot better off running a 2nd coax feeding composite video to the locations where you will be watching the camera, and run it into either the one of the video in inputs on the TV, or run a RF modulated signal on a separate coax to each location to the TV's tuner. There might even be wireless based systems to send your cameras around the house to the TV's.
Most Comcast systems have zero open channels unless you go above 860 or 750mhz depending on the system.
When our system was rebuilt from a 550mhz analog system to 860mhz digital, there was no space for my modulators, they had to be removed as the whole bandwidth was full with Comcast digital channels. I did have the luxury to run 2 or 4 coax runs to each TV outlet when I built the house.
I'm not sure why you were even asking about the sub channels, since no equipment goes that low.
The sub band question came from my equipment vendor who makes IP Coax switches. Their equipment places IP information such as camera or Internet service on the line. However, we don't want it to cause neighborhood interference. Actual application is a condominium building (MDU) wth 50 to 100 condos in a tower and we want everyone to receive the new signal. Running new coax is cost prohibitive. We're considering the distribution of security camera and/or Internet to every condo. Of course, this is not Comcast Internet, rather another vendor. It also helps that I am the condo board president and the board makes most decisions for the tower.
If this is commercial venue, then you should be discussing this with Comcast engineering or a close circuit camera installer to come up with a solution specific to your building. This is common for any MDU with camera's in common area's, or a neighborhood with with a camera out by the gate. Comcast or a local company would engineer a plan to insert the proper equipment in the building to insert your camera's. This is not a do it yourself project, but needs a professional that can insert the camera's properly especially on a digital channel.
This is much different than a homeowner wanting to see the camera pointing at his front door.
Very good points ! Agree, it's not a "do it yourselfer" project. Fortunately, I'm an advanced network engineer so RF and IP are well understood. Yes, it is a commercial building and association is incorporated identical to a business. Although an incorporated business, Comcast wants to treat it as residential to capture $50/mo. from every condo owner for Internet. Comcast would never assist us because the solution will cause them to lose revenue. Therefore, I have to do it. By the way, an alternative is in-building VDSL (100Mbps) using existing phone wires.
I have heard the upstream frequency is between 5-42 Mhz. Can anyone narrow it down to a smaller range as this seems awful broad? I want to place another frequency on line, but not interfere with Comcast. I think their downstream is 50 - 850 Mhz.