Johnny Cash and June Carter Cash, New York City, 1975. (Sony Music Archives)
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Ken Burns’ PBS documentaries are always classy affairs, linking a single topic to the vast rubric of American history. Still, there are greater and lesser works within that filmography, and “Country Music” – an eight-part, 16-hour epic about a lot more than done-me-wrong songs – distinctly falls into the latter scale.

Over the last 30 years, Burns’ signature war trilogy – “The Civil War,” “The War” (about World War II) and “The Vietnam War” – provides the spine of his work for public television. Yet the collaboration also has devoted admirable time to “The National Parks,” “Baseball” and “Prohibition,” albeit with less operatic heft.

“Country Music” is at its best, not surprisingly, in the early going. Burns goes back to the 1920s, charting the birth of the genre, the way different groups placed their stamp on it, and its impact during the Depression.

Moving assiduously through history, the documentary connects individual artists to the development of the form, which evolves into subgenres like bluegrass and rockabilly. As writer Dayton Duncan observes in the press release, the music “sprang from diverse roots, and it sprouted many branches.”

Some of those stories are fascinating, and as always, Burns’ work is peppered with meticulously researched anecdotes. Those tend to be most engaging when charting rags-to-riches stories associated with country stars from Johnny Cash to Elvis Presley – like Willie Nelson, after receiving his first royalty check, planting a kiss on the singer who made his song a hit.

Burns produced with Duncan, also the author of a companion book, and Julie Dunfey. All told, it’s a massive undertaking, one that consists of more than 100 interviews and rare photos and footage, including a young Cash doing his best hip-swinging impersonation of Presley.

Too much of the 16 hours, however, reads like individual profiles of and mash notes to country stars. That tendency becomes more pronounced, perhaps inevitably, in the later chapters, when more of the talent is still around to discuss their exploits and laud contemporaries. In those stages, the documentary feels less connected to the big picture and more focused on its up-close-and-personal stories.

Admittedly, this appraisal comes from somebody who doesn’t include many country records on a must-have list. But then again, part of Burns’ knack has been introducing material to audiences in a way that makes even those with a passing interest in the subject matter feel more connected to it.

That’s not to say “Country Music” hits any real false notes. But compared to the best of the Burns-PBS collaborations, it drones on in a way that isn’t equal to the high expectations and fanfare associated with Burns’ epic made-for-public-TV ballads.

“Country Music” will air Sept. 15-18 and Sept. 22-25 at 8 p.m. on PBS.