In what emission mode does the instantaneous amplitude (envelope) of the RF signal vary with the modulating audio?

When you view an Amplitude Modulated (AM) signal on an oscilloscope, you can see the overall outline of the wave fluctuating up and down. This outline is called the 'envelope,' and it directly matches the waveform of the modulating audio signal. Because the instantaneous amplitude of the RF carrier changes constantly to represent the voice or data, this is the defining characteristic of Amplitude Modulation. In contrast, Frequency Modulation (FM) maintains a constant amplitude (a flat envelope) and varies the carrier's frequency. Pulse modulation and frequency shift keying do not continuously vary the instantaneous amplitude to match an audio signal.