In BBC detective drama Ripper Street there is an American character, captain Homer Jackson (played by American actor Adam Rothenberg): former U.S. Army surgeon, ex-Pinkerton agent etc. For the story set in late XIX century, his accent appeared to me as modern. Mayhaps, more articulate, but still average American speech.
I am by no means a dialectologist, but it seems to me that century ago American accent was a lot closer to British. Today, perhaps, the dialects like Bostonian survive, that in some ways similar, and so it was earlier, as evidenced in MIT calculus videos form the 50s or 60s that I have seen. But listening to some examples of speech from first half of the XX century, such as F. D. Roosevelt speeches, it appears that today’s American accent is different from what it used to be: less articulate, a bit slurred, and slower, and ultimately easier to learn (I can attest, as a foreigner, that it takes less mental effort to speak American, even though I learned Queen’s English in the grade school).
So, is Jackson’s accent authentic, or they made it less such whether by accident (negligence), or in a deliberate attempt to more clearly differentiate that American from the rest of the Brits in the film?