The modulation index can be increased by using distorted wave forms that contain only triplen (multiples of three) harmonics.
通过使用只含有三次谐波的波形作为载波可以提高这个调节指数。
英语百科
Modulation index
The modulation index (or modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme. See: