When there is capacitor filtering, because the capacitor can hinder the voltage change, although the power supply voltage changes periodically, the voltage at both ends of the capacitor will not follow the change in real time, and its value will slowly decrease. Therefore, only when the power supply voltage is greater than the DC value of the current, charging will start from the current, and when the power supply voltage is less than the DC voltage, there will be no current and distortion will occur, and the larger the capacitance value, the more serious the distortion will be. You can simulate it and see the waveform. It's simple.