= Overmodulation
{title2=$m>1$}
{wiki}
For this single-tone <AM> model, overmodulation means $m>1$. The signed factor $q(t)$ becomes negative whenever $\cos(\Omega t)<-1/m$. On such intervals,
$$
Aq(t)\cos(\omega_ct)=A|q(t)|\cos(\omega_ct+\pi).
$$
The sign reversal is a $\pi$ <radian> shift of the carrier's <wave phase>. At each zero of $q$, the <wave envelope> vanishes and that phase is undefined. The waveform itself is continuous: the formula has not clipped it or created extra spectral lines.
At $m=1.4=7/5$, sign changes occur at message phases $\Omega t=\arccos(-5/7)$ and $2\pi-\arccos(-5/7)$ within each cycle. At the trough $\Omega t=\pi$, the desired signed factor is $-0.4$, whereas its nonnegative <wave envelope> is $0.4A$. Ideal envelope recovery followed by subtraction of $A$ would give $-0.6A$ there, rather than the desired message voltage $-1.4A$. This is loss of polarity in the detector, not loss of the message from the full modulated signal. Actual transmitter clipping is a separate <signal distortion> that can generate additional spectral components.
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