VHF Receiver, transmitter, lapel & headset microphones with two switch selectable radio channels. All wireless devices of any type are subject to interference caused whenever a receiver can 'hear' more than one transmitter broadcasting on the same frequency. This 'jamming' effect can sound like a howl or squeal distortion of the mic signal or even no sound at all coming from the receiver. Should this occur on a single-frequency system you're simply 'out of luck' until the interfering transmitter stops broadcasting or moves out of range. But when using the S1600 you simply set the A/B switch on both the transmitter and receiver to their opposite position. The chance of both frequencies being simultaneously interfered with by two outside transmitters is extremely small. This is great insurance that your presentation will go smoothly. 2 frequency groups available (171.105 / 171.845 or 169.505 / 170.305); requires two 9-volt batteries (not included). Includes cables to connect to amplifier.