FM stereo - FM estéreo

The broadcasts in the VHF-II transmission band (more popularly known as commercial FM or simply FM) have special or specific characteristics (at the frequency, modulation and bandwidth level) that make them ideal for being able to emit stereophonic signals. FM Stereo is thus modulating stereophonic audio signals into FM .

However, stereophony in analog radio is not unique to FM. Contrary to what happens in Europe, in North America it is very common to find AM radio stations that broadcast in stereo using the C-QUAM method .


At the end of the 1950s, up to fourteen different systems were patented in the United States for transmitting stereo radio signals over frequency modulation, which were evaluated by the United States agency in charge of the control of telecommunications FCC (acronym for Federal Communications Commission ).

Each system was evaluated and tested through the KDKA-FM station (located in Pittsburgh , Pennsylvania ), making it the first station in the world to broadcast in stereo. The tests consisted of evaluating the stereo separation of the two channels, the SNR (Signal-to-Noise Ratio), the effects of multichannel propagation or the possibility of incorporating other services through sub-carriers for authorized subsidiary communications (SCA).

After multiple tests, the FCC concluded that the two most suitable systems for stereo broadcasting were those presented by General Electric and Zenith Electronics Corporation respectively. These two systems were so similar that they were considered as one, and finally in April 1961, the FCC approved this system as the standard for FM stereo broadcasting in the United States. Later, most countries in the world would also adopt the same standard.

In Venezuela , the Cultural Station of Caracas became active on January 1, 1975 and, nine days later, in Spain , the first station to broadcast regularly in stereo system was RJ2, the second channel of the defunct Radio Juventud de Barcelona (EFJ-15). [ 1 ]


Compatibility with mono receivers

One of the important features that stereo broadcasts had to have was to ensure compatibility with Mono receivers. In other words, with the change from mono to stereo, users who did not have a stereo radio receiver did not have to notice any type of change or lose any type of information.

Until the appearance of stereo in radio broadcasting, radio signals were Mono and if in the radio studio it was produced in stereo, the two channels (L + R) were previously mixed in order to broadcast them. The result of this is that monophonic FM broadcasts carry a single baseband audio signal (30Hz to 15kHz) that is the sum of the two channels (L + R). This L + R signal is called "Sum Signal".

If you want to broadcast in stereo (that is, go from broadcasting 1 channel to 2 channels) without harming monophonic receivers, the Sum Signal (L + R) must be conserved by force.

In order to broadcast in stereo, what is done is to send another audio signal in parallel with the Sum signal, which is called "Subtraction Signal". The subtracted signal is nothing more than the signal resulting from subtracting the 2 channels L and R (LR).

That is, to broadcast radio in stereo we do not directly broadcast the two channels L and R, but rather we broadcast two signals resulting from combining the L and R signals: the Add and Subtract signals.

Sum Signal:

Subtraction Signal:

To recover the original L and R signals, the radio receiver operates as follows;

To get the signal from the L (left) channel, add the SS to the SR: .

To get the signal for the R (right) channel, subtract the SS with the SR: .

As an observation, we can see that when the receiver recovers the signals from the L and R channels, it recovers them with twice the original amplitude.

In this way, Mono receivers will continue to receive the same signal as before (the Sum signal), and Stereo receivers will combine the Add and Subtract signals in the manner described above to reproduce the two original channels L and R.

In order to emit the Add and Subtract signals at the same time (in addition to other services), these signals are added into a general signal that we call the MPX signal, and that is the signal that we finally inject into the transmitter.

Instantaneous signal deviation

International conventions indicate that the maximum deviation of a carrier signal for FM stereo has to be 75kHz. This deviation varies depending on the amount of modulation (level of the L and R audio channels) the modulator receives. To calculate the instantaneous deviation the following formula is used, where A and B are the audio signals L and R with pre-emphasis and= 19 kHz (pilot tone):



Pre-emphasis is a noise reduction technique applied to the L and R channel audio before generating the Add and Subtract signals. It consists of increasing the amplitude of the high frequencies, since these are more sensitive to noise. In Europe, the pre-emphasis time constant is 50µs, while in America it is 75µs.

The multiplex signal (MPX)

The MPX signal or multiplex stereo signal is the signal from which the transmitter is fed and, therefore, is the signal that is emitted and the one that is received at the receiver. The MPX signal is a signal that contains the Add and Subtract audio signals, as well as other services such as SCA or Radio Data System (RDS). With this composite signal the only carrier frequency is modulated. [ 3 ] The MPX signal has a bandwidth of 100kHz. It is also important that the audio is cut to 15kHz in both the Sum and Subtraction Signal, so as not to interfere with neighboring services.

The typical spectrum of the multiplex signal is composed of the following signals:

  • 30Hz to 15kHz: Sum signal of the L + R audio channels.
  • 19 kHz: Stereo pilot
  • 23kHz to 53kHz: Subtraction signal.
  • 57kHz: RDS data service
  • 67kHz to 94kHz: Subsidiary Communications Authorization Services (SCA)
Frequency spectrum of the MPX signal

Sum Signal

The Sum Signal is sent in baseband , within the 30Hz to 15kHz range. This part of the MPX (up to 15kHz) is the only part that monaural receivers decode.

Subtraction Signal

The Resta signal is sent within the 23kHz to 53kHz band, centered at 38kHz and modulated in DSBSC (Double Side Band Suppressed Carrier Signal). The reason for modulating in DSBSC is that, with this modulation, it is not necessary to emit the carrier signal, so that emitted energy is saved and the effective power of the emitter is used more. If the Resta signal were emitted separately, the emission of the carrier of the AM modulation would be necessary to be able to transport the signal through the waves, but since only the AM is necessary to modulate the carrier and generate the sidebands of the signal, it can the carrier be suppressed.

One of the characteristics of DSBSC modulation is that it generates side bands to the original signal (Lower Side Band and Upper Side Band); This means that if we have a signal that occupies 15kHz (as is the case of the Subtraction Signal), modulated in DSBSC it will occupy twice as much, in our case 15 · 2 = 30kHz. Therefore, the Subtraction Signal is positioned frequently far from the Sum Signal. If the Subtraction Signal started just after the Sum (from 15kHz) there would be interference because the Lower Side Band would overlap with the Sum signal. Besides, a guard frequency interval must be left to avoid possible overlaps and facilitate filtering for the teams.

The suppression of the carrier signal of the Subtraction Signal entails the design of a mechanism that facilitates the synchronization and is emitting a pilot tone at the frequency of 19kHz; thus the receiver can synchronize with the Subtraction signal (centered at 38kHz) by multiplying the pitch by two: 19 · 2 = 38. With this we manage to regenerate the carrier that we avoid emitting.

Stereo Pilot

The stereo pilot is a 19kHz tone that has the same phase as the Subtraction Signal carrier (which we have previously removed), and an amplitude of (typically) 10% of the total signal amplitude.

The stereo pilot has three main functions:

  • Informs the receiver that the broadcast is stereo
  • It allows to regenerate the subcarrier of the Subtraction Signal at 38kHz that we have not emitted thanks to modulating in DSBSC.
  • It allows to regenerate the RDS subcarrier at 57kHz that we have not emitted thanks to modulating in DSBSC.

RDS data service

The RDS is also modulated with DSBSC at a frequency of 57kHz. The phase of the RDS signal is the same as that of the Subtraction Signal. Thus, the receiver can regenerate the original signal carrier and self-synchronize, multiplying the 19kHz tone by 3 (19 * 3 = 57kHz).

SCA services

Services Authorization Communications Subsidiaries (SCA) are modulated from 67kHz. These services are aimed at the professional sector, and cannot be tuned to conventional radios. There are several types of SCA services, such as the broadcast of paid music threads, radio feeds or telemetry.

Steps to generate and recover the MPX signal

In the issuer

  • 1.- Generate the Sum Signal.
  • 2.- Generate the Subtraction Signal.
  • 3.- Generates a 19kHz pilot tone
  • 4.- Modulate in AM the Subtract signal at 38kHz with the phase of the pilot tone.
  • 5.- Eliminates the carrier of the modulated Subtraction signal.
  • 6.- Add the Sum signal (1), the Subtract signal without the carrier (5) and the pilot tone (3).
  • 7.- Modulate in FM (from 87.5Mhz to 108Mhz) the signal resulting from section (6).

If there were RDS, it would be injected directly into the transmitter through the multiplex signal input. Previously, the sender must have provided the 19kHz pilot tone phase to the RDS encoder.

Generation and emission diagram of a basic MPX signal

On the receiver

The steps to decode the MPX signal vary depending on whether it is a stereo receiver or not:

Receptor Mono

1.- Decode only the sum signal.

Stereo Receiver

If the broadcast is not stereo, do the same steps as the Mono Receiver.

If it is stereo, then proceed as follows:

  • 1.- It receives the 19kHz pilot tone and activates the internal stereo decoder circuit.
  • 2.- The decoder extracts the Suma signal.
  • 3.- The decoder generates a 38kHz signal in phase with the 19kHz pilot.
  • 4.- The decoder demodulates the Subtraction signal.
  • 5.- The decoder combines the Add and Subtract signals to obtain the original L and R channels.

If the receiver has the possibility to display the information provided by the RDS, it will generate a carrier at 57kHz in the same way as it does with the 38kHz, and it will be able to correctly receive the data from the RDS system and display, for example, the name of the station that we are tuning in on the screen.


  1. History of Radio Juventud de Barcelona
  2. «Stereophonic Broadcasting: Technical Details of Pilot-tone System», Information Sheet 1604(4) (BBC Engineering Information Service), junio de 1970
  3. Stereo Multiplexing for Dummies

external links

See also

Stereophonic sound

Modulated frequency