A PSK31 operator typically uses a single-sideband (SSB) transceiver connected to the sound card of a computer running PSK31 software. When the operator enters a message for transmission, the software produces an audio tone that sounds, to the human ear, like a continuous whistle with a slight warble. This sound is then fed through either a microphone jack (using an intermediate resistive attenuator to reduce the sound card's output power to microphone levels) or an auxiliary connection into the transceiver, from which it is transmitted.