Broadcast television from the infancy of the medium well into the late 1960’s dominated the landscape and the news the average American saw.  Over the next three decades, thanks in good measure to Richard Nixon’s antipathy toward his portrayal on television, cable television was given the opportunity to grow and evolve into a powerful entertainment and news force in the country.  The problem has become how much the two have intersected eventually leading to an eminently watchable,though unreliable, source in services like Fox News.  It became the most disruptive force in modern American politics, as a twin to conservative talk radio.  The fascinating and little reported history of cable television, first as a retransmitter of broadcast television, and then as a programming juggernaut with services like HBO, ESPN, MTV, CNN, and C-Span.  At this moment when cable’s narrowcasting has provided a roadmap for digital broadband streaming, which is resulting in cord cutting that will effectively end cable as we know it, it is a good time to revisit that history to understand where we are headed.  To ably do that is Kathryn Cramer Brownell, an associate professor of history at Purdue University, and author of “24/7 Politics: Cable Television and the Fragmenting of America from Watergate to Fox News.”