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Thread: Weird noise audio shield

  1. #1
    Junior Member
    Join Date
    Dec 2015

    Weird noise audio shield


    I'm building a device around the recording code that is available at the teensy audio Github, but all of a sudden this wired glitchy started to happen on my board...
    This only happens when i record, if i run the filter code or the sample player code this noise is not present...

    Attached is an MP3 sample of this glitch noise, and i'm running the example code without any modification.

    Any ideia if my board's are broken (i have 3)?

    Attached Files Attached Files

  2. #2
    Senior Member
    Join Date
    Nov 2012
    The quiet high-pitched noise at the beginning sounds like digital noise being picked up from the processor. The louder noise sounds like hum caused by a ground loop. A grounding/wiring problem could cause both noises.
    If you're recording sound directly from a PC line output, that can be the problem. You may need to use an audio transformer to isolate the grounds - but I'll leave that to the more technically savvy guys here.


  3. #3
    Senior Member
    Join Date
    Feb 2016
    Hi Guys,
    Even I'm facing the same problem, I have to record the sound through electret mic and before feeding to teensy 3.2 I have to amplify it with max 3.7v preamplifier, as I am using 3.7V Li-ion battery.
    Maybe the issue is because of ground loop, so kindly suggest what to do to get rid of this ground loop.
    In my case the noise is extreme.
    I used 2N3904 to the preamp, Even I tried with different preamps but no expected output.

    Also, let me know after preamp it's required to use the audio-in circuit mention in audio GitHub or not?.

  4. #4

    Line In Tonal Noise Caused by floating Mic input

    If recording with line inputs, you should ground the microphone input.

    I also had to add this line to the Teensy audio library in control_sgtl5000.cpp in the AudioControlSGTL5000::enable(void) function.

    after line 515: write(CHIP_REF_CTRL, 0x01F2); // VAG=1.575, normal ramp, +12.5% bias current

    Add the following to turn off the microphone bias and set the microphone amplifier gain to its lowest setting:
    chipWrite(CHIP_MIC_CTRL, 0x00); // Mic bias off; Mic Amplifier gain 0 dB

    See thread here on NXP forum:

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