SIFM is the Sennheiser Software for rapid calculation of intermodulation-free radio frequencies for wireless microphone systems.
Interference due to intermodulation generally occurs when at least two transmitters close to the receiving antenna produce very strong signals in the receiver. The two signals form intermodulation products at non-linearities, such as transistors or other semiconductors in the receiver (e.g. in the mixer). Unwanted signals are produced which may interfere with the wanted frequencies of the system. Intermodulation signals are also produced when two or more transmitters operate in close proximity to one another. In this case, the transmitter not only transmits its own signal but also receives the signals from the other transmitters. From both signals, the transmitter generates and re-transmits mixture products which can interfere with the wanted frequencies. For reasons of operational reliability, a wireless UHF transmission system has a limited switching bandwidth (e.g. 36 MHz). This switching bandwidth is determined by input filters in the receiver. Intermodulation products within this range can interfere with the selected receiving frequency or with the whole system and can make the system inoperable. Chopping noise or hissing in the background is an acoustic indication of frequencies interfered with by IM products.