The trial was brought by Marcus Gray, better known as Christian rapper Flame, against the Grammy-nominated singer.
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Katy Perry arrives in Los Angeles courthouse for copyright trial
He alleges she plagiarized his gospel song "Joyful Noise'' in her 2013 smash hit "Dark Horse.''
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Gray is expected to bring in a musicologist expert, who says the underlying beat in both songs is nearly identical.
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Meanwhile, Perry's experts have submitted reports stating there is no similarity beyond generic elements.
"Joyful Noise'' hit YouTube in 2012 and has since been viewed nearly 2 million times.
The trial is expected to last about two weeks.