Analog signal processing is the conversion of analog signals into another form using analog (continuous) circuits or systems. Analog signal processing includes filters, shaping circuits, combiners, and amplifiers to change their shape and modify the content of analog signals.

Audio signals contain a wide range of frequencies and it is usually not necessary to transmit all of these frequencies. These high and low frequencies can be removed by filtering. To remove these frequencies, audio signals may be processed by shaping circuits that add or remove emphasis to frequency (tone) or intensity (volume). When analog audio signals are used to modulate a radio wave, the high frequency components of the audio signal are emphasized (called pre-emphasis) to allow the modulator to be more effective. Because the intensity of an audio signal can vary dramatically (some people talk loudly and others talk softly), an amplification of the audio signal may be reduced as the user talks louder (called companding). When pre-emphasis and companding circuits are used to process an audio signal in a transmitter, the opposite functions (de-emphasis and expanding) are used in the receiver to restore the audio signal back to its original form.

In some cases, additional signals may be combined with audio signals prior to transmission. These signals are used for control purposes. If control signals are added to an audio signal in a transmitter, they must be removed from the audio signal in the receiver by filtering. Audio amplifiers are used to adjust the volume of an audio...