If we want to record very low frequency or DC signal on an audio recording system, we have to convert the DC/low frequency signal into the spectrum of audible sound. The figure of DC response data recording system is shown below.
We can record low-frequency analog data (DC to 10kHz) anywhere, store, and then reproduced. The frequency spectrum of the original data can be shifted up or down by varying the playback speed. [System’s diagram block source: Microchip Application Note]