The Analog to Digital conversion process
(Coursera's Introduction to Music Production homework)
Below is an interactive illustration of a simple analog signal being sampled (A/D), and then the sampled signal being reconstructed for playback (D/A). The samples and playback are shown in green when the input signal frequency is below half the sampling rate (known as the Nyquist frequency). They are both shown in red otherwise, and this is when aliasing is occurring. Aliasing is a loss and distortion of the frequency information of the input signal, which makes it so that the original signal cannot be reconstructed accurately after having been sampled. This is why most digital audio recording is done at a sampling rate of at least 44.1KHz, being roughly twice the highest frequency of normal human hearing.