ACSSB (amplitude-companded single-sideband) is a narrowband modulation method using a single-sideband with a pilot tone, allowing an expander in the receiver to restore the amplitude that was severely compressed by the transmitter. It offers improved effective range over standard SSB modulation while simultaneously retaining backwards compatibility with standard SSB radios. ACSSB also offers reduced bandwidth and improved range for a given power level compared with narrow band FM modulation.
The companding used in ACSSB is a type of dynamic range reduction where in the difference in amplitude between the louder and softer sounds is reduced prior to transmission. A corresponding expander circuit in the receiver inverts this transformation in order to restore the dynamic range. If a conventional SSB receiver is used to receive ACSSB signals, some distortion may be noticed but generally the signals are quite intelligible. Similar techniques are used in audio noise reduction circuits such as those developed for Dolby.
ACSSB is being used by amateur radio operators, air-to-ground phones, as well as mobile-satellite services.