In recent years stereo transmission has become an accepted part of VHF FM transmissions. The system that is used maintains compatibility with mono only receivers without any noticeable degradation in performance. The system that is used is quite straightforward.
A stereo signal consists of two channels that can be labelled L and R, (Left and Right), providing one channel for each of the two speakers that are needed. An ordinary mono signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted in the normal way. If a signal containing the difference between the left and right channels, i.e. L - R is transmitted then it is possible to reconstitute the left only and right only signals. Adding the sum and difference signals, i.e. (L + R) + (L - R) gives 2L, i.e. the left signal, and subtracting the two signal, i.e. (L + R) - (L - R) gives 2R, i.e. the right signal. This can be achieved relatively simply by adding and subtracting the two signals electronically. It only remains to find a method of transmitting the stereo difference signal in a way that does not affect any mono receivers.
This is achieved by transmitting the difference signal above the audio range. It is amplitude modulated onto a 38 kHz subcarrier. Both the upper and lower sidebands are retained, but the 38 kHz subcarrier itself is suppressed to give a double sideband signal above the normal audio bandwidth as shown below. This whole of the baseband is used to frequency modulate the final radio frequency carrier. It is the baseband signal that is regenerated after the signal is demodulated in the receiver.
To regenerate the 38 kHz subcarrier, a 19 kHz pilot tone is transmitted. The frequency of this is doubled in the receiver to give the required 38 kHz signal to demodulate the double sideband stereo difference signal.
The presence of the pilot tone is also used to detect whether a stereo signal is being transmitted. If it is not present the stereo reconstituting circuitry is turned off. However when it is present the stereo signal can be reconstituted.
To generate the stereo signal, a system similar to that shown in Fig. 8.5 is used. The left and right signals enter the encoder where they are passed through a circuit to add the required pre-emphasis. After this they are passed into a matrix circuit. This adds and subtracts the two signals to provide the L + R and L - R signals. The L + R signal is passed straight into the final summation circuit to be transmitted as the ordinary mono audio. The difference L - R signal is passed into a balanced modulator to give the double sideband suppressed carrier signal centred on 38 kHz. This is passed into the final summation circuit as the stereo difference signal. The other signal entering the balanced modulator is a 38 kHz signal which has been obtained by doubling the frequency of the 19 kHz pilot tone. The pilot tone itself is also passed into the final summation circuit. The final modulating signal consisting of the L + R mono signal, 19 kHz pilot tone, and the L - R difference signal based around 38 kHz is then used to frequency modulate the radio frequency carrier before being transmitted.
Reception of a stereo signal is very much the reverse of the transmission. A mono radio receiving a stereo transmission will only respond to the L + R signal. The other components being above 15 kHz are above the audio range, and in any case they will be suppressed by the de-emphasis circuitry.
For stereo receivers the baseband signal consisting of the stereo sum signal (L+R) and the difference signal (L-R) centred around 38 kHz and the pilot 19kHz tone are obtained directly from the FM demodulator. The decoder then extracts the Left only and Right only signals.
The block diagram of one type of decoder is shown below. Although this is not the only method which can be used it shows the basic processes that are required. The signal is first separated into its three constituents. The L + R mono signal between 0 and 15 kHz, the pilot tone at 19 kHz, and the stereo difference signal situated between 23 and 53 kHz. First the pilot tone at 19 kHz is doubled in frequency to 38 kHz. It is then fed into a mixer with the stereo difference signal to give the L - R signal at audio frequencies. Once the L + R and L - R signals are available they enter a matrix where they are added and subtracted to regenerate the L and R signals. At this point both signals are amplified separately in the normal way in a stereo amplifier before being converted into sound by loudspeakers or headphones.
Today most stereo radios use an integrated circuit to perform the stereo decoding. Often the pilot tone is extracted and doubled using a phase locked loop. This provides a very easy and efficient method of performing this function without the need for sharp filters.