The operation mode ISB (Independent Sideband) is a special operation mode in which two audio contents can be transmitted simultaneously modulated on one signal. It is also referred to as A3B or 6B8E.

In normal amplitude modulation, a carrier signal and two sidebands with identical information are transmitted. In the operation mode Independent Side Band, the two sidebands are modulated with different audio content.

In terms of reception, the signal is processed as two SSB signals. The receiver is tuned to the carrier frequency, both sidebands are usually separated with appropriate filters for upper and lower sideband and both sidebands are demodulated simultaneously. The content of the lower or upper sideband is selected by means of a selector switch and sent to the headphones or audio output.

The ISB method was used, among other things, to feed relay stations of international short-wave broadcasters and for supplying military bases and ships with radio programmes; for example, the French and English language service could be transmitted simultaneously with one signal.

An ISB capable receiver must be able to demodulate ISB signals with two independent demodulators. Usually, both signals are fed to a line-out jack; for monitoring purposes, the upper or lower sideband with programme A or programme B can be selected with a switch.