LYNN – There once was a time when just about every community on the North Shore had a drum and bugle corps.Lynn had its Continentals and its Jeannettes. Revere had its Reveries and its 27th Lancers. Saugus had its Socialites.And Lynn in the 1960s and ?70s was the de facto capital of drum corps on the East Coast. The hulking concrete of the former Manning Bowl, its acoustics well matched to the horns and snares that echoed within, served as the site of the World Open championships from 1966 to 1982, when corps from all over the country came to Lynn to compete to be called the best on the planet in the blaring of the bugles, the staccato of the snare, the twirling flags and the spinning of the rifles.Maybe you were a kid who wanted something to do in the summer and you weren’t into traditional sports. You showed up at the local Roman Catholic church or VFW hall, said you wanted to join, and someone handed you a bugle or a drum – sometimes a whole set of drums – and you practiced marching and music for hours a week at a local field or an empty, after-hours corporate parking lot. On the weekends you went to competitions, you’d spend hours on buses, then hours watching the competing corps, waiting for your 11 minutes in the football field spotlight.At the end of the show, you’d wait for the scores to come in, as eight judges collaborated on a complex formula that judged your corps on music, including level of difficulty and execution; marching, including the appeal of the precisely-choreographed routine; and a criterion called “general effect,” or the overall presentation.Then the scores would be announced, when you might cheer the judges’ infinite wisdom or scorn their obvious foolishness.View photos of past drum linesMichael Merrett, of Lynn, used to be one of those judges. The writer and songwriter who’s retired from insurance work pursued his passion for drum corps and percussion from the mid-1960s to the 1990s as a member (the Immaculate Conception Reveries of Revere), as an instructor (The Ambassadors/Diplomats of Malden), as a judge with the Massachusetts Judges Association and as an instructor again (East Coast Jazz).All those groups are gone now, the victim of a variety of economic and social forces from within and outside the drum corps community.”Kids today are more interested in video games or have different ways of spending their summers,” Merrett said. “They don’t gravitate to the discipline. It’s become more of an exclusive activity.”The evolution of drum corpsExclusive is an understatement. Elite is more like it. For example, the corps that will be competing in Friday’s Beantown Invitational take the unique drum corps art to levels that were unthinkable back in a more populist heyday.Frank Raffa runs the Beanpot Invitational in his role with the Beverly Cardinals Alumni Association. Raffa, who was a member of the Cardinals corps from 1960 to 1972, said the process of picking drum corps members is much more involved.Watch the 1971 Beverly Cardinals, in which Raffa was a member, perform“In wintertime the corps have camps throughout the country, and each particular unit will have auditions. Kids come in for three of four times,” Raffa explained.”Today there are audition videos and fees, and the talent level is much higher,” said Merrett, and corps members are now selected from throughout the country. For example, the Boston Crusaders, which will also be appearing in Friday’s competition, has members from Georgia and the Carolinas, and corps now include college music majors and those who might have be members of their