While Bob was in the US, Ska music had evolved to Rock Steady. Because of this evolution the Wailing Wailers were forced off their music label. The producer did not want Rock Steady music, only Ska. They ran out of money so they looked to the aid of Lee Perry, who got them back on track. Lee is a producer, and his strongest influence is in the dub version of reggae music.
Dub is when the music is only instrumental, and there are barely any words.
When there are words they are mixed in to give the song a special effect, which is particular to dub. You can hear an example of this in Bobs' song Sun Is Shining . It is interesting to listen to his earlier works and compare them to his older work. The beats are all originated from the same place, but the style is slightly altered. At his point the Wailers were known throught the Caribbean, but still unknown internationally.