automatic mono stereo switch

Since the end of February, owners of stereo FM receivers will have noticed that the stereo indicator lamp stays on continuously. This is due to a decision by the BBC to transmit the 19 kHz pilot tone with all programmes, ostensibly to eliminate annoying clicks that occur when switching the pilot tone on and off. The commercial radio stations have, of course, followed this practice for some time. As the mono-stereo switch in a stereo receiver operates by detecting the pilot tone, this means that such receivers are now incapable of distinguishing between mono and stereo trans-missions and will be permanently switched to stereo unless the `mono` switch is pressed. Apart from the minor annoyance of not knowing if a programme is in stereo (unless one buys the Radio Times) there is the greater inconvenience of listening to mono transmissions with the poorer signal-to-noise ratio inherent in stereo transmission. This is particularly a problem in areas of fringe reception. The circuit described in this article distinguishes between mono and stereo transmissions by detecting whether or not stereo information is present in the received signal, independent of the pilot tone.