What specific part of the Fairness Doctrine supposedly limited Free Speech?

I actually see it more as a restriction of free trade. I understand the argument that the airwaves are public. However using the Fairness Doctrine is a way to limit specific content. By doing so you are saying to station owners that they cannot exclusively provide content that sells ad time. Instead if they offer political shows then they must balance those shows with equal opposition content whether it sells or not. I don't believe that anyone is saying that it limited viewpoints in the past, but there is a difference. The rise of conservative based talk radio is a relatively recent phenomenon. There can be no doubt when you examine ratings shares that liberal programs do not fair as well as those featuring conservative content. So it easy to draw the conclusion that the goal of re-instituting the Fairness Doctrine is an attempt to remove some of this content. I don't think the average supporter believes that this will lead to more liberal content though. I think they believe and with good reason that rather than bow to the pressures of presenting equal time that many station operators will just drop their conservative programming in favor of different non political formats. In reality there is no need for the Fairness Doctrine anymore. It existed in a time when which we were much more limited in our choices. Today you can get news and commentary tailored to whatever particular viewpoint you desire with little effort.