How a Radio Program Gets From The Studio DJ To Your Radio
Listening to music on the radio is as normal as eating breakfast or combing your hair. It's an integral part of most people's day. But if you stop a moment and think about it, it is a bit of a miracle how music can come through the air wirelessly and play in your kitchen. This is how it works...
The DJ in the studio talks into a microphone, and plays CDs, and these stereo sounds are sent to a mixer desk, which combines all these signals and gets their volume and tone right. These sound signals then go to an audio processor, which adds other signals like traffic info, a pilot tone, RDS signals and possibly NICAM signals, among other things. The stereo tracks are melded into an encoded stereo, and then all these signals are cleverly combined into a single channel called baseband.
This signal is sent to a digital link, which digitises the baseband, adds it to a very high frequency carrier, and then transmits this over the air to a nearby transmitter station, which is normally situated close by on a high building or on a mountain. In the transmitting station, this signal is received, the digital info is decoded and transformed back into baseband.
THE TRANSMITTING STATION
The baseband is then routed to the transmitter, where it is modulated (ie added) onto an FM high frequency carrier. FM stands for frequency modulation, which means that the FM frequency of say, 95.6MHz, is changing slightly all the time, so one second it will be 95.5998, then it will be 95.6005, and so on. What 95.6MHz means is that the signal is being switched on and off and on again 95.6 million times every second. This high frequency carrier signal is amplified by the transmitter equipment, and then the signal is sent up a cable to the top of the transmitter mast, at the top of which are antennas that convert this electrical signal to an electromagnetic signal - one that is capable of travelling through air.
TRAVELLING THROUGH THIN AIR
The transmitted signal is able to do this because it consists of two fields: an electric one and a magnetic one. When a electric field collapses, it produces a magnetic field, and when a magnetic field collapses, it produces an electric field. So the two fields work like an inchworm, pushing against each other to move forward through thin air.
Often this signal travelling through the air is received by a more distant transmitting station, where the signal is taken back down to baseband, and then remodulated onto a different frequency and once again transmitted. This is to achieve a greater area of coverage for a particular radio station.
YOUR DEMODULATOR IS YOUR RADIO
Once this electromagnetic signal reaches your radio aerial, the electric part of the signal is extracted, and this is demodulated by your radio. What this means is that the high frequency signal is separated from the baseband signal, and then this baseband signal is again separated into it's various parts, which are stereo sound, RDS (which gives the name of the station on your radio's display) traffic report info, and NICAM, which has the capability of giving you CD quality sound or dual language tracks.
No wonder it's been said "Any sufficiently advanced technology is indistinguishable from magic!"