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PCM2906B 1kHz Noise during recording

Mariano Dupraz
Prodigy 45 points
Other Parts Discussed in Thread: PCM2904, PCM2906B

We developed a device for I/O USB Audio. When recording, we have a 1kHz noise. We have this problem with PCM2904. When replacing with 2906B the problem is still there

  • 0
    TI__Guru**** 189401 points

    Mariano,

    Can you describe the problem in more detail? 

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 45 points

    We developed an Audio Converter using PCM2904. Our circuit is like Bus Powered Typical Circuit from PCM2904 Datasheet.

    Our converter has balanced analog inputs and ouputs.

    When we use the converter for playback from Windows XP, using Windows Media Player or any other software, every works fine. We have great quality.

    When we use the converter for capture balanced analog audio, using Windows Recorder or any other software, it records, but with a high noise level. When I record "silence", looking at the waveform, we found a 1KHz noise.

    Reading some Errata and forum from TI, we saw that this is a documented problem with PCM2904, that is supposed to be fixed in PCM2906B. We replaced PCM2904 with PCM2906B in our circuit, and the problem is still there. We recorded in 44.1KHz, 22KHz, 11KHz, mono, stereo, 8 bits, 16 bits. Always we have this noise. We also test with TI's drivers, and the noise is still there.

     

  • 0 in reply to Mariano Dupraz
    TI__Guru**** 189401 points

    Mariano,

    OK, thanks! I just recently started working on these parts, so I'm coming up to speed, too. Let us look into this and get back to you.

    -d2

  • 0 in reply to Don Dapkus
    TI__Genius 16270 points

    Mariano,

    There will be a difference between the 1kHz noise at 16kHz mono sampling mode, which was fixed for the PCM2906B, and the general noise associated with USB operation.  The USB will be running 12MHz clock with a 1kHz interval, thus generating some noise at 1kHz.  Because the USB transmitter is being employed during recording, but not during playback, there will be more sensitivity to this during recording.  DAC output data is averaged by post circuits/tools where ADC capture data is taken at the moment of sampling, thus again this data will be more sensitive to noise. Also, the grounded source will be more sensitive because of the extra formed ground loop. 

    To minimize this, one thing to do is to ensure the current flow between the USB controller and the USB transceiver of the PCM2906B is minimized in the signal and GND path between the analog source and the analog input of the PCM2906B.  Second, connect all the GND pins to each other under the PCM2906B.  If the probelm persists, common mode choke coils on the data lines and power lines may help to reduce common mode noise and reduce clock disturbance, improving performance.