What is meant by the term "flat-topping," when referring to an amplitude-modulated phone signal?
Why In an AM or SSB phone transmitter the RF envelope should follow the audio waveform. If the drive or microphone/speech gain is too high, the final amplifier runs out of headroom and the peaks of the envelope are clipped off, giving the envelope a flattened top. That clipping creates harmonic and intermodulation distortion products, which show up on the air as splatter in adjacent channels and poor audio quality. The cure is to back off the mic gain or drive so that ALC action is light and the envelope stays undistorted.
Watch out The choice about ALC being properly adjusted is the opposite situation: correct ALC keeps the envelope out of clipping, and flat-topping is what happens when it is overdriven. Insufficient collector current would cause low output or other distortion, not the classic clipped-envelope pattern.
Flat top = too much drive. Squash the peaks, splatter the neighbors.