Also known as: vocoder
A vocoder (voice coder) is a speech codec that compresses voice into a few kilobits per second by modelling how speech is produced — pitch, voicing, and spectral shape — rather than recording the waveform.1 It is what makes digital voice radio possible.
How it works
Many times a second the vocoder extracts compact parameters of a short speech segment and transmits only those; the receiver re-synthesises an audible voice from them. This is why digital voice can sound slightly robotic, especially on a weak signal.
Relevance to SDR
Decoding digital voice requires running the matching vocoder — IMBE for P25 Phase 1, AMBE+2 for DMR and P25 Phase 2, or Codec 2 for M17.