If I generate a 20Khz digital sine wave with a microcontroller, outputing each sample at a rate of 44.100Khz, I don't understand how I can get a sine wave since I'm going to have only 2 samples for each period. How can a sine wave be reconstructed with only 2 samples by period ? With an LP reconstruction filter ? But if it is not a sine but a triangle, a square, a saw... I think the LP filter will distord the signal.

The sample rate of a CD is 44.100KHz and we can ear sound of 20Khz in this CD. Same question that above, how can we ear a perfect sine (or triangle,etc) in this CD since there are only 2 samples every period ?