Processing is, indeed, mostly irrelevant. There are three main causes of interference to aircraft radios:
1. Harmonics or mixing products from the transmitter. An FM transmitter can generate signals at twice or three times the operating frequency. This is normally filter out by harmonic filters built into the transmitter. Some older exciters do not have such filtering, relying on a filter later on in the transmitter. So if they are used for low power operation they can generate interference.
Mixing products can be generated at sights with more than one transmitter. Signal from one antenna is picked up by the antenna of another station, and is mixed in the final amplifier to produce a third signal. Also can be cured by external filters.
Some exciters, when they age, will also generate spurs up and down --and beyond, the FM band.
Usual culprit is dried out capacitors, especially in frequency synthesis circuits. A fail fan and too much heat can also be the culprit, especially in the various re-branded RVR exciters (Energy-Onix, Bext, etc.)
2. IF beat interference in the aircraft radio. The receiver has a broadly tuned amplifier that amplifies the incoming signal, then mixes it with a local oscillator to produce a third signal, the intermediate frequency. Tuned stages are then used to amplify the IF signal to a useable level. Most receivers use an intermediate frequency of 10.7 mHZ. When the plane (usually "general aviation"--think Piper Cub) flies near a radio station that is 10.7 MHZ below the aircraft band signal, the radio may pick up the radio station instead of the signal in the aircraft band.
3. De-sense. Many aircraft band signals are AM. The "front end" of these radios may be very broad. Again flying over the radio station--even, sometimes, an AM station, the radio's front end is overloaded and detects the radio station. Even if the station is not generating any harmonics, or mixing products that actually fall into the aircraft band.
