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.

DC RESPONSE DATA RECORDING SYSTEM circuit schematic

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]