At its center Christmas is the simplest of celebrations. Its god is created not in a magnificence, but in poverty attended by animals. Except you become as a little child…. I am thinking now of three powerful and simple beauties I have heard this Yuletide, full of integrity and without dilution. The first was the marvelous performance given by Anne Azema in the Boston Camerata’s Medieval Christmas at the Union College Chapel. Like all great singing, hers comes at you directly, no mediation, no hesitation. Her sound, her knowledge, even her appearance, are all part of one thing, and that thing is honest. Like all great artists she makes you know that her voice is the right instrument for the music. She sings an old cantiga with as much passion as another kind of soprano might sing Norma. An update of the Camerata’s first medieval Christmas program, this one was sparely accompanied, most often unaccompanied. The chant and monophonic songs held full sway. They were sung with a sharp and soaring energy which was always interesting, often riveting. This repertoire in a performance like this easily held the attention of a full house for over two hours. This was a performance of early music which was straight out, in no way manufactured. The highest compliment I can give it is that it was simple. And the model for this was the singing of Anne Azema.