I don't see, how "detect the change in amplitude" should be related to synchronization. Assuming an analog PWM modulator, there's no restriction in representing infinitesimal voltage changes.
In an exact analysis, you'll however realize, that the simple "natural sampling" PWM modulator comprised of a triangle wave and a comparator introduces some signal distortions. To overcome it, you have to change to a "regular sampling" modulator, that uses an additional sample and hold stage.