SAT语法求解during television's early years,many stations broadca

SAT语法求解
during television's early years,many stations broadcast programming only until midnight,after which television screens (displayed) a static image known as a test pattern.
这里为什么是displayed而不是had displayed?broadcast是过去式又在screens displayed image之后（after),所以screen后面的动词不是应该为过去的过去变成完成吗?
什么时候可以用到完成?