I spent a large part of my life as a technologist with a major broadcaster. But long before that I was an unruly kid who built electronic gadgets like radios and things and often caused little explosions and minor household disasters. My parents were very long suffering.
During my early years I installed quite a few car radios. In those pre-transistor days they were valve sets, and those valves needed a high DC voltage on their anodes to make them work. To achieve that we had to install a vibrator pack in the engine compartment. It made a loud clattering sound as the vibrating contact blade switched the voltage from the car battery fast between positive and negative to produce an alternating current. The voltage could then be stepped up by a transformer to 90 volts or more for the valve anodes. The high voltage then had to be reconverted to DC (continuous current) because that’s what the valves needed. The vibrator pack was bulky and noisy and interfered electrically with the radio, so all sorts of screening and silencing systems had to be used. In the end, all we could receive was a pretty ropy AM (medium-wave) radio signal. Some radios had shortwave as well but interference from the vibrator & the engine usually made that unlistenable. To buy a set of car spark-plug leads today is a pretty expensive exercise because instead of using normal wire they are made of special carbon-based composites. This allows the high voltage for the spark plugs to pass whilst at the same time preventing the leads from causing radio interference.
When FM (Frequency Modulation) came on the scene in the 60s, it was by its nature far less sensitive to interference from electrical pulses given off by the various parts of a car engine. That’s because spark interference has the same nature as an AM (Amplitude Modulated) radio signal. The interference would mix with the radio transmission and the AM radios of the time couldn’t discriminate between the interference and the program. FM on the other hand ignores the amplitude variations of the radio carrier and detects instead variations in the incoming radio wave frequency i.e the rate at which the signal changes polarity. The frequency is varied by the transmitter in a manner that represents the music or whatever. Electrical sparks in cars don’t affect the frequency of the FM wave so, if strong enough, the program is heard without noise.
Today we are moving a step further with technology by introducing digital radio. The signal at the studio is turned into a pattern of pulses which digitally define the program signal, and the receiver knows what the code should look like. It therefore totally rejects any interference that is not digital code and the result is a perfectly clear signal. It also works much better than FM in weak reception areas. Digital transmissions are also not confused when ghost signals reach the receiver slightly delayed after reflecting off buildings or mountains. This was a problem with earlier technologies.