Sonm10’s comment “There will still be interference, though.” relates to the use of the same frequency. Although differing NAC’s may be used, a signal can be present but ignored by your scanner because it’s not using the same NAC as your scanner. This signal, although intelligible with the proper NAC, will have the same effect as a high noise floor on the frequency. What this means to us is this... there may be times when we don’t receive a transmission as clearly as usual, and start thinking there is something wrong with either the transmission or our scanner, when all it amounts to are two signals using the same frequency at the same time.
Thanks to Somn10 for mentioning this. It’s another instance (there are a few) that can create confusion, especially with a newbie who just plunked down hundreds of dollars on a state-of-the-art scanning receiver only to find it’s not performing as advertised, at least in their minds... when all it was is just another instance of a lack of knowledge on how things work. There is nothing wrong with the scanner in situations such as this.