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    • STM32F4 DAC Audio configuration

    Posted by ravi on 2016-04-14 01:09

    Hello Everyone,

    I have a custom board with STM32F4xx chipset DAC_OUT pin connected to audio amplifier.
    I am configuring DAC with DMA and TIM6 trigger for Audio playback.
    With the attached configuration code, I hear a initial click sound as I start playing audio file.
    After the initial click sound, the audio file is getting played fine.

    Below are the observations:
    • The click is heard every time we initiate playback.
    • The click is independent of the file being played.
    • The click is not caused by noise on the line.
    • The line is noise free before and after playback.
    • The click waveform does not look analog in nature.
    • The click is observable at the DAC output.
    • The click is observable across multiple boards, multiple MCUs (405/446), and multiple generations of boards and electrical layouts.

    Can anyone help me figuring out if there is any configuration issue or is this a known DAC issue ?

    Thanks,
    Ravi

    dac_dma_tim.c (3.00 Kb)
    dac_dma_tim.c (3.00 Kb)
    Reads: 17633
    Posted by dautrevaux on 2016-04-14 06:27 France

    Hi Ravi,

    The click has nothing to do with the MCU or the DAC; it is inherent to your way to start playing sound. The only configuration issue could be the position of the "idle" output: it must be at the middle of the output range (thus corresponding to "no signal"); otherwise you will get a steep front (thus a click) when switching even to no-signal waveform.

    However even in this case you will still hear a click if you start playing sound at full volume immediatly as going from no-signal to full-signal (even a sinus waveform), when converted in frequency space will have a wide spectrum at the beginning, heard as a click by your ear. Note that this artefact can be heard even on some high quality devices (like Bose Quiet-Comfort noise reducing headphones...)

    The cleanest way to suppress the click is thus to start playing your waveform at 0 volume, then raise it progressively to full volume in a few hudrends of milliseconds.

    Bernard (Ac6)