Finally I know where the speakers of the schools broadcasts and educational films of my youth did learn their trade. We had to listen to schools broadcast recordings or watch educational films whenever a teacher wasn't prepared, the topic was too difficult for the teacher, or the teacher didn't dare to prepare his own material.
The later was e.g. the case for sex education. Imagine getting a lecture about the facts of life in the style of this DoD videos. After that you are halfway ready to become a monk :-)