operating bias and gain of the final RF amplifier can be controlled by varying the voltage of the control grid. This method requires little audio power, but care must be taken to reduce distortion. Clamp tube (screen grid) modulation The screen-grid bias may be controlled through a clamp tube, which reduces voltage according to the modulation signal. It is difficult to approach 100-percent modulation while maintaining low distortion with this system. Doherty modulation One tube provides the power under carrier conditions and another operates only for positive modulation peaks. Overall efficiency is good, and distortion is low. Outphasing modulation Two tubes are operated in parallel, but partially out of phase with each other. As they are differentially phase modulated their combined amplitude is greater or smaller. Efficiency is good and distortion low when properly adjusted. Pulse-width modulation (PWM) or pulse-duration modulation (PDM) A highly efficient high voltage power supply is applied to the tube plate. The output voltage of this supply is varied at an audio rate to follow the program. This system was pioneered by Hilmer Swanson and has a number of variations, all of which achieve high efficiency and sound quality. Digital methods The Harris Corporation obtained a patent for synthesizing a modulated high-power carrier wave from a set of digitally selected low-power amplifiers, running in phase at the same carrier frequency. The input signal is sampled by a conventional audio analog-to-digital converter (ADC), and fed to a digital exciter, which modulates overall transmitter output power by switching a series of low-power solid-state RF amplifiers on and off. The combined output drives the antenna system. Demodulation methods The simplest form of AM demodulator consists of a diode which is configured to act as envelope detector. Another type of demodulator, the product detector, can provide better-quality demodulation with additional circuit complexity. See also AM stereo Shortwave radio Amplitude modulation signalling system (AMSS) Modulation sphere Types of radio emissions Airband DSB-SC References Bibliography Newkirk, David and Karlquist, Rick (2004). Mixers, modulators and demodulators. In D. G. Reed (ed.), The ARRL Handbook for Radio Communications (81st ed.), pp. 15.1–15.36. Newington: ARRL. . External links Amplitude Modulation by Jakub Serych, Wolfram Demonstrations Project. Amplitude Modulation, by S Sastry. Amplitude Modulation, an introduction by Federation of American Scientists. Amplitude Modulation tutorial including related topics of modulators, demodulators, etc... Analog Modulation online interactive demonstration using Python in Google Colab Platform, by C Foh. Radio