HMI lamps, flicker can be avoided by the use of electronic ballasts that cycle at frequencies thousands of times faster than the mains frequency. Ballast operation To power an HMI bulb, special ballasts act as an ignitor to start the arc, and then regulate it by acting as a choke. Two types of ballasts exist: magnetic and electronic (square-wave or flicker-free). Magnetic ballasts are generally much heavier and bulkier than electronic ballasts, as they consist primarily of a network of large inductors. They are usually cheaper than electronic ballasts. Since the magnetic type of ballast does not maintain the discharge continuously, the lamp actually extinguishes at zero-crossings of the mains waveform; unless the camera is locked to the mains waveform, the difference in frequency between the lamp and the shutter will produce a beat frequency that is visible in the resulting recording. This is why TV standards typically use the power grid frequency as their basic frame rate. Magnetic ballasts are simple devices compared to electronic ballasts. Essentially, a magnetic ballast is a large, heavy transformer coil that uses a simple principle to generate the high startup voltages needed to create an arc in a cold lamp. Input power is routed to a choke coil connected between the main input and the lamp. The coil may be tapped in several places to provide for various input voltages (120 V or 240 V) and a high start-up voltage. Capacitors are also included to compensate for the inductance of the coil and improve the power factor. Because of the high amount of current through the ballast, a low humming sound is often heard due to magnetostriction of the ballast iron laminations. Some magnetic ballasts have insulation around the coil for silent operation. Since the early 1990s, electronic flicker-free (or Square-Wave) ballasts have become increasingly popular and affordable as an alternative to magnetic ballasts by eliminating most of the problems associated with HMI flicker. Unfortunately, their operation is not as simple as a magnetic ballast. Electronic ballasts can be thought of as operating in three stages—a DC intermediate converter, a power module, and an AC inverter. Power initially flows through the main breakers into an RF mains filter that prevents the flow of noise back onto the incoming power line. Then, rectifiers and capacitors charge and discharge to invert the negative half of the AC cycle and convert the line to positive DC