Encode / Decode refers to a two-part symmetrical process used to overcome the limitations of a storage medium or transmission channel. The signal is pre-processed (Encoded) to optimize it for the medium—whether to protect it from noise, reduce its file size, or encrypt it—and then reverse-processed (Decoded) at the destination to retrieve a close approximation or an exact replica of the original.

In analog audio, this is famously seen in "companding" noise reduction systems. The encoder boosts quiet high-frequency signals so they are recorded louder than the tape's inherent hiss. The decoder then attenuates these frequencies by the exact same amount on playback, pushing the tape hiss down while restoring the music to its correct level. In digital audio, this term defines the software algorithms (Codecs) that compress raw audio data into manageable streams (like streaming services use) and then reconstruct the audio waveform on your device.