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TPA3221: HEAD modulation output waveform

Part Number: TPA3221


My customer are using HEAD modulation. They use AP to measure the THD+N is 0.1%. 

However when they use Scope to probe the output to ground waveform they saw a distortion waveform as below


When change to AD modulation, they distortion disappear. 

May i know why HEAD modulation see such distorion waveform ? and is there any problem with this waveform?