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PCM2912A High pitched warbling sound when rec enabled (rec LED should be on)

Richard Clayton
Prodigy 45 points
Other Parts Discussed in Thread: PCM2912A

Hi

We have built a soundcard  for our training systems based on the PCM2912A and its reference design.

On the prototype we had a high pitched warbling sound when ever the record option is enabled in windows. i.e. whenever the rec led should be lit.

As we have no need for the record and play leds these were dropped on the second build and the 2 lines on the chip (pins 31 and 32 left disconnected) .

This has not fixed the problem and the noise is still there. When the led is not let the sound quality is perfect.

Can anyone point me toward the cause of this noise and should the unused lines be tied to ground?

  • 0
    Prodigy 45 points

    I have done some more testing and the noise is also present when the play led should be on as well.

    So either led illuminated caused the high frequency warble.

    I have tried adding another decoupling cap in the V+ line to the leds and I have also tried removing the leds and leaving the lines play and rec lines disconnected or tied to gnd and nothing makes any difference to the noise.

    help!!!

    Thanks

    Richard 

  • 0 in reply to Richard Clayton
    TI__Mastermind 27850 points

    Hi Richard,

    Could you send me more info about your setup, are you able to look at the signals on a scope? Do you have an EVM to hook up and test in the same fashion?

    Justin

  • 0 in reply to Richard Clayton
    Prodigy 45 points

    Hi

    I did post a reply a few days ago but it appears that it did not post correctly.

    The design is basically the reference design but setup so that daughter boards can be connected if additional features such as balanced audio input is required.

    If you can give me an email address I can send you a copy of the Schematic and layout. Unfortunately company policies prevent me posting them here.

    Thanks

  • 0 in reply to Richard Clayton
    Prodigy 45 points
    I have done a lot more investigation on this issue. I am pretty sure that the noise is the USB packets. I can scope the noise on the output and the USB data lines and they match exactly.
    What I don't understand is how the USB data is crosstalking with the mic input and more importantly why does the noise not appear on the sidetone if the record is not enabled.

    Surely mic input lines to the chip are exactly the same for either record or for sidetone. If you enable the sidetone in windows, you can hear the mic in the headphones clear as a bell, if you open the windows setup for the soundcard and click on the rec tab or open windows sound recorder (or anything that enables the record on the card) the 1KHz warble is superimposed on the mic input.