There are other concerns in determining how much return bandwidth is needed. Some of the return bandwidth may not be usable at all times during the day. There have been numerous reports of interference, particularly below 15 or 20 MHz, due to ingress into home wiring. It is either ingress from off-air short wave radio signals, which can be quite strong at times, or it is impulse noise from things such as electric motors and light dimmers. Whatever the source, the noise can sometimes prevent use of certain frequencies.
Some operators use high pass filters installed in drops to block noise from entering the network.
These filters have been shown to yield dramatic improvement in some cases. The filters are installed on the drops of homes that don't use the two-way services. If a home just has a set top terminal with a return path transmitter, then a special filter may be used that enables that required limited passband. Many systems that use the return path are designed to be frequency agile. If the return spectrum they are using develops problems such as ingress, the system has the ability to jump to a new frequency, hopefully without the user being aware of the jump.