Here's an example of real-time audio DSP on the STM32F4 Discovery platform.

I have a set of DTMF tones generated in Sony Sound Forge playing in a loop and the headphone output of my Mac Pro is fed into a preamp I made to bring the audio signal into a range that the STM32F4's onboard ADC can handle. It's a dual op-amp split rail configuration with a virtual ground at VCC/2, this puts the audio in the 0-3v range required. The signal is then sampled into a circular buffer, an FFT is taken and the frequency spectrum and waveform are rendered to a Noritake VFD display, then the FFT is analyzed for DTMF tones. If one is found it's displayed in the top right of the VFD.

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