 and the pilot tone. These have to be extracted and converted into the two audio signals L and R. Broadcasting 211 Mono filter Matrix (cut off 15 kHz) Right Signal from Stereo difference FM demodulator signal between Stereo filter 23 and 53 kHz (accepts Mixer 23-53 kHz) 38kHz Pilot tone 19 kHz filter (allows pilot tone Frequency through doubler 19 kHz tone) Figure 7.4 Block diagram of a stereo decoder The block diagram of one type of decoder is shown in Figure 7.4. 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 its frequency is doubled using a phase locked loop. This provides a very easy and efficient method of performing this function without the need for sharp filters. The initials RDS stand for radio data system. It is a system that uses inaudible data signals added to a VHF FM transmission to bring a variety of information and automatic tuning facilities to the listener. Initially aimed towards the car radio market RDS is now a standard feature on many hi-fi tuners and other sets. The system is well established in Europe and is used in a number of other countries as well. The concept of using a subcarrier included in the baseband modulation to carry data for information and other serv