Broadcast Radio Stereo Generator, or Stereo Encoder: In broadcast radio, a stereo generator is a device used to generate the stereo audio signal that is broadcast over the airwaves. The signal generated is a Composite Stereo Signal otherwise known as an MPX (multiplex) signal. The composite stereo signal is then modulated onto the FM carrier by the broadcast exciter.
A composite stereo signal refers to a combined audio signal that contains both left and right audio channels multiplexed together. In FM broadcasting, for example, the composite stereo signal includes the left and right audio channels along with additional signals such as the pilot tone and subcarrier signals for transmitting stereo and auxiliary data. This composite stereo signal is usually generated by the stereo generator, which takes the left and right audio channels and combines them into a single signal suitable for FM transmission.
The pilot tone is often used as a reference signal and is a specific frequency (usually 19 kHz) that helps stereo receivers decode the stereo signal accurately. The exciter generates this pilot tone and sends it along with the audio signal. Receivers use it to demodulate the stereo signal correctly.
MPX (Multiplex) audio output refers to a type of signal commonly used in FM broadcasting. It carries a combination of the baseband audio (left and right stereo signals) along with other ancillary signals like the pilot tone and the RDS (Radio Data System) signal. MPX is essentially the composite signal that includes everything needed for stereo FM broadcasting.
MPX output / Composite Stereo Signal may include:
- Baseband Audio: The left and right stereo audio channels. (required to be composite stereo)
- Pilot Tone: A constant 19 kHz tone that serves as a reference for stereo receivers to properly decode the stereo signal (required for FM exciter)
- RDS Signal: Data encoded according to the Radio Data System standard, providing additional information such as station identification, program information, traffic alerts, etc. (optional)
- Subsidiary Communications Authorization (SCA): If applicable, SCA signals may also be included in the MPX output. SCA allows broadcasters to transmit additional content such as background music, foreign-language programming, or data services. (optional)
The MPX output is typically transmitted from the FM transmitter to the broadcast antenna, where it is radiated over the airwaves. Receivers equipped with stereo capabilities and RDS decoders can then demodulate and decode the MPX signal to reproduce the stereo audio and access additional information provided by the RDS.