This application note describes how to implement the Speex software codec on the STM32F101xx and STM32F103xx microcontrollers to build a vocoder application. Speex is a free audio codec dedicated to speech encoding and decoding. It provides a high level of compression with a good quality of sound. That makes it a high-performance solution for any application using message playback or a voice recorder, like answering machines, building and home safety systems, intercoms, smart appliances, voice recorders or walkie-talkies.