n 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 and right only signals. By adding the sum Broadcasting 209 and difference signals, i.e. (L + R) + (L - R), gives 2L, i.e. the left signal, and subtracting the two signals, 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 8kHz subcarrier itself is suppressed to give a double sideband signal above the normal audio bandwidth as shown in Figure 7.2. This whole baseband is used to Amplitude Pilot Mono signal Stereo difference signal Frequency (kHz) Figure 7.2 The modulating (baseband) signal for a stereo VHF FM transmission frequency modulate the final radio frequency carrier. This signal is also what is regenerated after the signal is demodulated in the receiver. To regenerate the 38 kHz subcarrier, a 19 kHz pilot tone is transmitted. This provides a reference in the receiver when the signal is reconstituted. The frequency of the 19 kHz pilot tone is doubled in the receiver to give the required 38 kHz signal. This is used to demodulate the signal by reinserting the subcarrier into 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. As higher noise levels are present when stereo is being received it enables the stereo circuitry to be on only when it is required. Also when the signal is weak and there is a high background noise level, the pilot tone will not be detected and the ster