What is meant by the term “flat-topping,” when referring to an amplitude-modulated phone signal?

The term 'flat-topping' comes from the visual appearance of a distorted radio signal when viewed on an oscilloscope. In a healthy amplitude-modulated (or SSB) signal, the peaks of the waveform are smooth and rounded, representing the natural human voice. However, if the microphone gain or speech levels are set too high, the amplifier is pushed beyond its maximum output capability. It simply cannot reproduce the highest peaks, causing the top and bottom of the waveform to be cut off, or 'flattened.' This distortion creates harsh harmonics and adjacent-channel splatter. A properly adjusted Automatic Level Control (ALC) prevents flat-topping by limiting the drive signal before it overloads the amplifier, which is why flat-topping indicates a failure or misadjustment, not proper ALC operation.