Question

What is Perfect Modulation?

Answer 1

To avoid distortion in the modulated signal, the message signal's maximum amplitude must be smaller than (or equal to) the carrier signal's maximum amplitude. If the carrier signal has an amplitude of $5$ volts, the message signal must have an amplitude of less than (or equal to) $5$ volts. As a result, when $Am{\text{ }} = {\text{ }}Ac$, the modulation index's maximum value will be less than or equal to one ($Mi < = 1$). The modulation index will have a minimum value of zero.
Based on this, there are $3$ types of modulation:
Perfect-Modulation
Under-Modulation
Over-Modulation

Complete step-by-step solution:
Perfect modulation:
When the message signal or modulating signal, maximum amplitude is approximately equal to the carrier signal's maximum amplitude ($Am{\text{ }} = {\text{ }}Ac$), perfect-modulation occurs.
The modulation index is the ratio of the message signal's maximum amplitude to the carrier signal's maximum amplitude. For example, if the maximum amplitude of the message signal is $4$ volts and the maximum amplitude of the carrier signal is also $4$ volts, the modulating signal amplitude ($4$ volts) to the carrier signal amplitude ($4$ volts) ratio is$1$. As a result, with perfect-modulation, the modulation index is one ($Mi{\text{ }} = {\text{ }}1$).
The modulation depth is another name for the modulation index. The modulation depth of the perfect-modulation is$100\%$. The carrier level in perfect-modulation drops to $0$. There is no distortion with perfect modulation.

Note: Modulation benefits:
The size of the antenna was reduced.
There is no signal mixing.
The range of communication expands.
The transmissions multiplexed.
It is possible to change the bandwidth.
The quality of the reception improves.