I guess it is because of the use of amplitude modulation [1], where the envelope (&quot;strength&quot;) of the carrier (that is some higher frequency signal) follows the signal that is modulated onto it (speech, in this video). With say FM it wouldn&#x27;t work, I presume.<p>[1]: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Amplitude_modulation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Amplitude_modulation</a>