Although some versions employ to prevent unlawful reception by scanner radio receivers, which operate in the 900 MHz, 2.4 GHz, or 6 GHz ISM bands, the receiver is frequently used for multiple microphones. Some types employ (two antennas) to prevent transmission interruptions due to actor movement. Some inexpensive (or specialized) devices employ light, but they need a straight line of sight between the microphone and the receiver.