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TLV320AIC3206EVM-U: TI AIC3206 Codec: Why is headphone audio appearing in microphone recordings when analog bypass is enabled?

Cong Nguyen
Prodigy 10 points
Part Number: TLV320AIC3206EVM-U

Tool/software: AIC3206EVM GUI 

I'm working with the AIC3206 EVM board Rev A.When I enable analog bypass from IN3_L/R to MAL/R to HPL/R – with a TRS microphone connected to J3-MIC IN and headphone connected to J5-HP OUT,  I'm encountering an issue where my microphone recording contains muffled audio from the headphone output. This issue persists when microphone and headphones are disconnected. I have tried with different headsets and microphones. The issue persists.

If I remove analog bypass, then this problem goes away: default playback/recording works as is. I've gone through the datasheet and appnote (SLAA463) a few times but I still couldn't figure out why this feedback is present. I'm not sure if this is because of a board issue or misconfiguration – hoping it's not a hardware issue since this is an EVM.

I recently asked the question on Electronics StackExchange, and one of the commenters stating this is a normal behaviour (comment [1]). If it is, how can I disable it? It would be pretty confusing for people on a video conference to hear their voice again from the other person's line.

This is my configuration code: Configuration code [2]

[1]: https://electronics.stackexchange.com/questions/743940/ti-aic3206-codec-why-is-headphone-audio-appearing-in-microphone-recordings-when#comment1979809_743940

[2]: https://gist.github.com/rampadc/20f44485f70093d39fbdbf3eb5112f95

  • 0
    TI__Mastermind 46555 points

    Hi Cong,

    To make sure I understand,

    When you use default playback/recording, everything works normally. There is no headphone signal being recorded on the mic path.

    But once the mic signal is fed into the sidetone path, you hear the loop get closed: headphone output appears in the recording path.

    Is my understanding correct?

    Cong Nguyen said:
    This issue persists when microphone and headphones are disconnected.

    Can you give more details to this statement? How are you confirming that the issue exists even when there is no mic source or headphone load? Are you measuring the muffled sound through the ADC recording path as well or is this just an audible effect only?

    Thanks,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 10 points

    Yes, that is correct. When playback and recording are enabled separately and together, without sidetone, there is no headphone signal being recorded on the mic path. I also unplugged the headphones from the J5 while microphone is still plugged in, to make sure there is no noise leaking out from headset - issue persists then too.

    I test this by playing music on the computer with the EVM selected as the audio output, and record audio with Audacity using the EVM microphone path as input. The recording played back contains muffled audio from the headphones. I tested this on different computers as well, same issue.

  • 0 in reply to Cong Nguyen
    TI__Mastermind 46555 points

    Hi Cong,

    One test that will help narrow down the cause, can you try the reverse test of what you described:

    Disconnect the microphone but leave the headphones connected. Enable the side tone and measure the recording. Do this for both microphone input floating and shorted to GND. Let me know if there's any different results.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 10 points

    1. Disconnect the microphone but leave the headphones connected, with IN3_L/R, CM1L/R set with 10K in the GUI: yes issue persists.

    2. Disconnect the microphone but leave the headphones connected, with IN3_L/R disconnected, CM1L/R set with 10K in the GUI: no issue.

    3. Disconnect the microphone but leave the headphones connected, with IN3_L/R disconnected, CM1/R disconnected in the GUI: no issue.

  • 0 in reply to Cong Nguyen
    TI__Mastermind 46555 points

    Hi Cong,

    I have seen in the past noise coupling on the ADC path before due to questionable layout and I think that's what we're seeing now. Proof being that by disconnecting IN3_L/R causes the issue to go away.

    Unfortunately I think this is a consequence of the layout of the EVM. My apologies.

    Best regards,
    Jeff McPherson

  • 0 in reply to Jeff McPherson
    Prodigy 10 points

    Hey Jeff, thanks for the clarification. Before unfortunately marking this as resolved, have you got a reference on how to better layout to prevent this kind of issues? I was hoping to base my design off the EVM.

    Edit: If cross talk is because of questionable layout, then why wouldn't not enabling sidetone while enabling playback & recording generate this kind of feedback? Crosstalk between the lines was my first thought too, but then I tested that scenario and it points more towards a chip issue.

  • 0 in reply to Cong Nguyen
    TI__Mastermind 46555 points

    Hi Cong,

    That is a good point on there is no cross talk with sidetone disabled.. There are two paths you can use to achieve side-tone and maybe the result is different. Right now I believe you're taking the path out of the PGA, but you can also bypass that and take a true analog bypass. Maybe switching paths changes the result (or vice versa).

    For reference, keeping the ground return of the playback and recording paths separate with strong grounds in between is the best way to prevent this kind of crosstalk.