Abstract: A system is disclosed for encoding and decoding the audio signal portion of a television signal to prevent unauthorized reception of the audio signal. At the encoding stage, the audio signal is applied to a multiplier which combines the audio signal with a programmable carrier having a frequency controlled by a frequency selection protocol to produce an output signal which is spectrally inverted from the original audio signal. The spectrally inverted signal from the multiplier is applied to a sideband filter which filters out the upper sideband of the spectrally inverted signal to produce an encoded signal having the same bandwidth as that of the original audio signal. At the decoding stage, the carrier signal is phase-shifted to compensate for phase delay imparted to the encoded signal in the encoding process and combined with the encoded signal to decode the encoded signal.