Car radio interference generally originates in the ignition or charging systems or other electrical accessories. Interference is either picked by the aerial or by the wiring. Interference is fairly easy to identify. The most common source of interference, a rapid crackling or ticking that intensifies with engine speed is from the ignition system. Two types of suppressors depending on the type of set (AM or FM) connected in different ways are used to eliminate interference. An alternator or dynamo produces a whining sound that increases in pitch as the engine speed increases. Never connect a capacitor to the alternator field terminal. To eliminate this interference a capacitor is fitted between the warning lamp and earth. Interference from electrical components such as horns, windscreen washer motors, fan heaters, indicators disappears when these components are switched off. A capacitor fitted between the feed terminal and earth eliminates all interference.