The transmission of stereophonic audio signals over radio using either amplitude-modulation (AM) or frequency-modulation (FM) techniques. See also Stereophonic sound.
To be commercially acceptable, the signal transmitted by a stereo broadcast station should be decipherable by either a stereo receiver or a monophonic receiver. For the monophonic receiver to correctly receive a stereo broadcast, some component of that broadcast must be a single signal that includes information from both the left and the right channel. This is called the L + R signal. A monophonic receiver simply demodulates the L + R signal and delivers it to the listener.
The stereo receiver must use the L + R signal and other transmitted information to demodulate separate L and R signals. For this, stereo broadcasts include an L − R signal. In the stereo receiver, after detection of the L + R and L − R signals, the L and R signals are separated as shown in Eqs. (1) and (2).
1. 
2. 
The subtraction used in Eq. (2) is realized through phase shifting and addition. Shifting the phase of a signal by 180° inverts the signal. Thus, (L − R) put through a 180° phase shift is −(L − R).
In FM stereo transmissions, the L + R signal is directly frequency-modulated onto the radio-frequency carrier, assuring compatibility (see illustration). The L − R signal is used to amplitude-modulate a 38-kHz hypersonic subcarrier. The 38-kHz carrier is suppressed, creating a double-sideband (DSB), suppressed-carrier AM signal at 38 kHz. This hypersonic signal is then frequency-modulated onto the radio-frequency carrier. A 19-kHz pilot signal is also transmitted to indicate that the broadcast is stereo and to provide an accurate frequency reference for demodulation of the 38-kHz L − R signal. See also Amplitude modulation; Frequency modulation.
Baseband spectrum of an FM stereo signal. This audio and supersonic spectrum is frequency-modulated onto a radio-frequency carrier.
In the FM stereo receiver, the entire baseband signal that has been frequency-modulated onto a radio-frequency carrier (L + R, pilot, and double-sideband L − R) is first detected with an FM discriminator. The L + R signal is isolated by a low-pass filter. The pilot tone is doubled and mixed with the double-sideband suppressed-carrier L − R signal, so that the L − R signal can be demodulated by an AM envelope detector. The L + R and L − R audio signals are then passed through a network that implements the functions expressed in Eqs. (1) and (2). The resulting left- and right-channel audio signals are amplified and presented to the listener. See also Amplitude-modulation detector; Electric filter; Frequency-modulation detector; Frequency-modulation radio.
There are five AM stereo modulation systems permitted by the Federal Communications Commission. Several of these mutually incompatible systems are in use. The most widely used system, installed at around one-third of the AM stereo broadcast stations in the United States, is the Compatible Quadrature Amplitude Modulation (C-QUAM) system, which uses two radio-frequency carriers 90° out of phase. One of these carriers is modulated with the L + R signal, and the other is modulated with the L − R signal. The carriers are then added to create a single signal and limited to remove amplitude variations. The resulting radio-frequency signal is amplitude-modulated with the L + R signal. This last step provides compatibility with monophonic AM receivers. A 25-Hz pilot tone is provided for identification of stereo transmissions. A C-QUAM receiver uses a standard AM detector to demodulate the L + R signal. The L − R signal is demodulated by a sophisticated synchronous FM detector. See also Phase modulation; Radio receiver.
